395 lines
16 KiB
Plaintext
395 lines
16 KiB
Plaintext
=========
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Livepatch
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=========
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This document outlines basic information about kernel livepatching.
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Table of Contents:
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1. Motivation
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2. Kprobes, Ftrace, Livepatching
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3. Consistency model
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4. Livepatch module
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4.1. New functions
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4.2. Metadata
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4.3. Livepatch module handling
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5. Livepatch life-cycle
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5.1. Registration
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5.2. Enabling
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5.3. Disabling
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5.4. Unregistration
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6. Sysfs
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7. Limitations
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1. Motivation
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=============
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There are many situations where users are reluctant to reboot a system. It may
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be because their system is performing complex scientific computations or under
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heavy load during peak usage. In addition to keeping systems up and running,
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users want to also have a stable and secure system. Livepatching gives users
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both by allowing for function calls to be redirected; thus, fixing critical
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functions without a system reboot.
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2. Kprobes, Ftrace, Livepatching
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================================
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There are multiple mechanisms in the Linux kernel that are directly related
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to redirection of code execution; namely: kernel probes, function tracing,
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and livepatching:
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+ The kernel probes are the most generic. The code can be redirected by
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putting a breakpoint instruction instead of any instruction.
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+ The function tracer calls the code from a predefined location that is
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close to the function entry point. This location is generated by the
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compiler using the '-pg' gcc option.
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+ Livepatching typically needs to redirect the code at the very beginning
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of the function entry before the function parameters or the stack
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are in any way modified.
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All three approaches need to modify the existing code at runtime. Therefore
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they need to be aware of each other and not step over each other's toes.
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Most of these problems are solved by using the dynamic ftrace framework as
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a base. A Kprobe is registered as a ftrace handler when the function entry
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is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
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a live patch is called with the help of a custom ftrace handler. But there are
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some limitations, see below.
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3. Consistency model
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====================
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Functions are there for a reason. They take some input parameters, get or
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release locks, read, process, and even write some data in a defined way,
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have return values. In other words, each function has a defined semantic.
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Many fixes do not change the semantic of the modified functions. For
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example, they add a NULL pointer or a boundary check, fix a race by adding
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a missing memory barrier, or add some locking around a critical section.
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Most of these changes are self contained and the function presents itself
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the same way to the rest of the system. In this case, the functions might
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be updated independently one by one.
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But there are more complex fixes. For example, a patch might change
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ordering of locking in multiple functions at the same time. Or a patch
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might exchange meaning of some temporary structures and update
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all the relevant functions. In this case, the affected unit
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(thread, whole kernel) need to start using all new versions of
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the functions at the same time. Also the switch must happen only
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when it is safe to do so, e.g. when the affected locks are released
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or no data are stored in the modified structures at the moment.
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The theory about how to apply functions a safe way is rather complex.
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The aim is to define a so-called consistency model. It attempts to define
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conditions when the new implementation could be used so that the system
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stays consistent. The theory is not yet finished. See the discussion at
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https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz
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The current consistency model is very simple. It guarantees that either
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the old or the new function is called. But various functions get redirected
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one by one without any synchronization.
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In other words, the current implementation _never_ modifies the behavior
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in the middle of the call. It is because it does _not_ rewrite the entire
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function in the memory. Instead, the function gets redirected at the
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very beginning. But this redirection is used immediately even when
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some other functions from the same patch have not been redirected yet.
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See also the section "Limitations" below.
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4. Livepatch module
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===================
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Livepatches are distributed using kernel modules, see
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samples/livepatch/livepatch-sample.c.
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The module includes a new implementation of functions that we want
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to replace. In addition, it defines some structures describing the
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relation between the original and the new implementation. Then there
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is code that makes the kernel start using the new code when the livepatch
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module is loaded. Also there is code that cleans up before the
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livepatch module is removed. All this is explained in more details in
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the next sections.
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4.1. New functions
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------------------
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New versions of functions are typically just copied from the original
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sources. A good practice is to add a prefix to the names so that they
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can be distinguished from the original ones, e.g. in a backtrace. Also
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they can be declared as static because they are not called directly
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and do not need the global visibility.
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The patch contains only functions that are really modified. But they
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might want to access functions or data from the original source file
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that may only be locally accessible. This can be solved by a special
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relocation section in the generated livepatch module, see
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Documentation/livepatch/module-elf-format.txt for more details.
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4.2. Metadata
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------------
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The patch is described by several structures that split the information
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into three levels:
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+ struct klp_func is defined for each patched function. It describes
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the relation between the original and the new implementation of a
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particular function.
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The structure includes the name, as a string, of the original function.
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The function address is found via kallsyms at runtime.
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Then it includes the address of the new function. It is defined
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directly by assigning the function pointer. Note that the new
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function is typically defined in the same source file.
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As an optional parameter, the symbol position in the kallsyms database can
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be used to disambiguate functions of the same name. This is not the
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absolute position in the database, but rather the order it has been found
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only for a particular object ( vmlinux or a kernel module ). Note that
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kallsyms allows for searching symbols according to the object name.
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+ struct klp_object defines an array of patched functions (struct
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klp_func) in the same object. Where the object is either vmlinux
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(NULL) or a module name.
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The structure helps to group and handle functions for each object
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together. Note that patched modules might be loaded later than
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the patch itself and the relevant functions might be patched
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only when they are available.
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+ struct klp_patch defines an array of patched objects (struct
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klp_object).
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This structure handles all patched functions consistently and eventually,
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synchronously. The whole patch is applied only when all patched
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symbols are found. The only exception are symbols from objects
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(kernel modules) that have not been loaded yet. Also if a more complex
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consistency model is supported then a selected unit (thread,
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kernel as a whole) will see the new code from the entire patch
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only when it is in a safe state.
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4.3. Livepatch module handling
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------------------------------
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The usual behavior is that the new functions will get used when
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the livepatch module is loaded. For this, the module init() function
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has to register the patch (struct klp_patch) and enable it. See the
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section "Livepatch life-cycle" below for more details about these
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two operations.
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Module removal is only safe when there are no users of the underlying
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functions. The immediate consistency model is not able to detect this;
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therefore livepatch modules cannot be removed. See "Limitations" below.
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5. Livepatch life-cycle
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=======================
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Livepatching defines four basic operations that define the life cycle of each
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live patch: registration, enabling, disabling and unregistration. There are
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several reasons why it is done this way.
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First, the patch is applied only when all patched symbols for already
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loaded objects are found. The error handling is much easier if this
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check is done before particular functions get redirected.
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Second, the immediate consistency model does not guarantee that anyone is not
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sleeping in the new code after the patch is reverted. This means that the new
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code needs to stay around "forever". If the code is there, one could apply it
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again. Therefore it makes sense to separate the operations that might be done
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once and those that need to be repeated when the patch is enabled (applied)
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again.
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Third, it might take some time until the entire system is migrated
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when a more complex consistency model is used. The patch revert might
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block the livepatch module removal for too long. Therefore it is useful
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to revert the patch using a separate operation that might be called
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explicitly. But it does not make sense to remove all information
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until the livepatch module is really removed.
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5.1. Registration
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-----------------
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Each patch first has to be registered using klp_register_patch(). This makes
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the patch known to the livepatch framework. Also it does some preliminary
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computing and checks.
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In particular, the patch is added into the list of known patches. The
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addresses of the patched functions are found according to their names.
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The special relocations, mentioned in the section "New functions", are
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applied. The relevant entries are created under
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/sys/kernel/livepatch/<name>. The patch is rejected when any operation
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fails.
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5.2. Enabling
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-------------
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Registered patches might be enabled either by calling klp_enable_patch() or
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by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
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start using the new implementation of the patched functions at this stage.
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In particular, if an original function is patched for the first time, a
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function specific struct klp_ops is created and an universal ftrace handler
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is registered.
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Functions might be patched multiple times. The ftrace handler is registered
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only once for the given function. Further patches just add an entry to the
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list (see field `func_stack`) of the struct klp_ops. The last added
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entry is chosen by the ftrace handler and becomes the active function
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replacement.
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Note that the patches might be enabled in a different order than they were
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registered.
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5.3. Disabling
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--------------
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Enabled patches might get disabled either by calling klp_disable_patch() or
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by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
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either the code from the previously enabled patch or even the original
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code gets used.
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Here all the functions (struct klp_func) associated with the to-be-disabled
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patch are removed from the corresponding struct klp_ops. The ftrace handler
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is unregistered and the struct klp_ops is freed when the func_stack list
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becomes empty.
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Patches must be disabled in exactly the reverse order in which they were
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enabled. It makes the problem and the implementation much easier.
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5.4. Unregistration
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-------------------
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Disabled patches might be unregistered by calling klp_unregister_patch().
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This can be done only when the patch is disabled and the code is no longer
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used. It must be called before the livepatch module gets unloaded.
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At this stage, all the relevant sys-fs entries are removed and the patch
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is removed from the list of known patches.
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6. Sysfs
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========
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Information about the registered patches can be found under
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/sys/kernel/livepatch. The patches could be enabled and disabled
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by writing there.
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See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
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7. Limitations
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==============
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The current Livepatch implementation has several limitations:
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+ The patch must not change the semantic of the patched functions.
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The current implementation guarantees only that either the old
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or the new function is called. The functions are patched one
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by one. It means that the patch must _not_ change the semantic
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of the function.
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+ Data structures can not be patched.
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There is no support to version data structures or anyhow migrate
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one structure into another. Also the simple consistency model does
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not allow to switch more functions atomically.
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Once there is more complex consistency mode, it will be possible to
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use some workarounds. For example, it will be possible to use a hole
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for a new member because the data structure is aligned. Or it will
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be possible to use an existing member for something else.
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There are no plans to add more generic support for modified structures
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at the moment.
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+ Only functions that can be traced could be patched.
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Livepatch is based on the dynamic ftrace. In particular, functions
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implementing ftrace or the livepatch ftrace handler could not be
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patched. Otherwise, the code would end up in an infinite loop. A
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potential mistake is prevented by marking the problematic functions
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by "notrace".
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+ Anything inlined into __schedule() can not be patched.
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The switch_to macro is inlined into __schedule(). It switches the
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context between two processes in the middle of the macro. It does
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not save RIP in x86_64 version (contrary to 32-bit version). Instead,
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the currently used __schedule()/switch_to() handles both processes.
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Now, let's have two different tasks. One calls the original
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__schedule(), its registers are stored in a defined order and it
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goes to sleep in the switch_to macro and some other task is restored
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using the original __schedule(). Then there is the second task which
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calls patched__schedule(), it goes to sleep there and the first task
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is picked by the patched__schedule(). Its RSP is restored and now
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the registers should be restored as well. But the order is different
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in the new patched__schedule(), so...
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There is work in progress to remove this limitation.
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+ Livepatch modules can not be removed.
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The current implementation just redirects the functions at the very
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beginning. It does not check if the functions are in use. In other
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words, it knows when the functions get called but it does not
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know when the functions return. Therefore it can not decide when
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the livepatch module can be safely removed.
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This will get most likely solved once a more complex consistency model
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is supported. The idea is that a safe state for patching should also
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mean a safe state for removing the patch.
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Note that the patch itself might get disabled by writing zero
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to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
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code will not longer get called. But it does not guarantee
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that anyone is not sleeping anywhere in the new code.
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+ Livepatch works reliably only when the dynamic ftrace is located at
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the very beginning of the function.
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The function need to be redirected before the stack or the function
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parameters are modified in any way. For example, livepatch requires
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using -fentry gcc compiler option on x86_64.
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One exception is the PPC port. It uses relative addressing and TOC.
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Each function has to handle TOC and save LR before it could call
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the ftrace handler. This operation has to be reverted on return.
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Fortunately, the generic ftrace code has the same problem and all
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this is is handled on the ftrace level.
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+ Kretprobes using the ftrace framework conflict with the patched
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functions.
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Both kretprobes and livepatches use a ftrace handler that modifies
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the return address. The first user wins. Either the probe or the patch
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is rejected when the handler is already in use by the other.
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+ Kprobes in the original function are ignored when the code is
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redirected to the new implementation.
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There is a work in progress to add warnings about this situation.
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