185 lines
8.8 KiB
ReStructuredText
185 lines
8.8 KiB
ReStructuredText
Tainted kernels
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---------------
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The kernel will mark itself as 'tainted' when something occurs that might be
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relevant later when investigating problems. Don't worry too much about this,
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most of the time it's not a problem to run a tainted kernel; the information is
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mainly of interest once someone wants to investigate some problem, as its real
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cause might be the event that got the kernel tainted. That's why bug reports
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from tainted kernels will often be ignored by developers, hence try to reproduce
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problems with an untainted kernel.
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Note the kernel will remain tainted even after you undo what caused the taint
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(i.e. unload a proprietary kernel module), to indicate the kernel remains not
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trustworthy. That's also why the kernel will print the tainted state when it
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notices an internal problem (a 'kernel bug'), a recoverable error
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('kernel oops') or a non-recoverable error ('kernel panic') and writes debug
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information about this to the logs ``dmesg`` outputs. It's also possible to
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check the tainted state at runtime through a file in ``/proc/``.
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Tainted flag in bugs, oops or panics messages
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You find the tainted state near the top in a line starting with 'CPU:'; if or
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why the kernel was tainted is shown after the Process ID ('PID:') and a shortened
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name of the command ('Comm:') that triggered the event::
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BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
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Oops: 0002 [#1] SMP PTI
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CPU: 0 PID: 4424 Comm: insmod Tainted: P W O 4.20.0-0.rc6.fc30 #1
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Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
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RIP: 0010:my_oops_init+0x13/0x1000 [kpanic]
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[...]
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You'll find a 'Not tainted: ' there if the kernel was not tainted at the
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time of the event; if it was, then it will print 'Tainted: ' and characters
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either letters or blanks. In above example it looks like this::
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Tainted: P W O
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The meaning of those characters is explained in the table below. In this case
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the kernel got tainted earlier because a proprietary Module (``P``) was loaded,
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a warning occurred (``W``), and an externally-built module was loaded (``O``).
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To decode other letters use the table below.
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Decoding tainted state at runtime
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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At runtime, you can query the tainted state by reading
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``cat /proc/sys/kernel/tainted``. If that returns ``0``, the kernel is not
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tainted; any other number indicates the reasons why it is. The easiest way to
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decode that number is the script ``tools/debugging/kernel-chktaint``, which your
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distribution might ship as part of a package called ``linux-tools`` or
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``kernel-tools``; if it doesn't you can download the script from
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`git.kernel.org <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/tools/debugging/kernel-chktaint>`_
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and execute it with ``sh kernel-chktaint``, which would print something like
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this on the machine that had the statements in the logs that were quoted earlier::
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Kernel is Tainted for following reasons:
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* Proprietary module was loaded (#0)
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* Kernel issued warning (#9)
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* Externally-built ('out-of-tree') module was loaded (#12)
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See Documentation/admin-guide/tainted-kernels.rst in the Linux kernel or
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https://www.kernel.org/doc/html/latest/admin-guide/tainted-kernels.html for
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a more details explanation of the various taint flags.
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Raw taint value as int/string: 4609/'P W O '
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You can try to decode the number yourself. That's easy if there was only one
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reason that got your kernel tainted, as in this case you can find the number
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with the table below. If there were multiple reasons you need to decode the
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number, as it is a bitfield, where each bit indicates the absence or presence of
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a particular type of taint. It's best to leave that to the aforementioned
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script, but if you need something quick you can use this shell command to check
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which bits are set::
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$ for i in $(seq 18); do echo $(($i-1)) $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done
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Table for decoding tainted state
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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=== === ====== ========================================================
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Bit Log Number Reason that got the kernel tainted
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=== === ====== ========================================================
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0 G/P 1 proprietary module was loaded
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1 _/F 2 module was force loaded
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2 _/S 4 kernel running on an out of specification system
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3 _/R 8 module was force unloaded
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4 _/M 16 processor reported a Machine Check Exception (MCE)
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5 _/B 32 bad page referenced or some unexpected page flags
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6 _/U 64 taint requested by userspace application
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7 _/D 128 kernel died recently, i.e. there was an OOPS or BUG
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8 _/A 256 ACPI table overridden by user
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9 _/W 512 kernel issued warning
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10 _/C 1024 staging driver was loaded
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11 _/I 2048 workaround for bug in platform firmware applied
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12 _/O 4096 externally-built ("out-of-tree") module was loaded
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13 _/E 8192 unsigned module was loaded
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14 _/L 16384 soft lockup occurred
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15 _/K 32768 kernel has been live patched
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16 _/X 65536 auxiliary taint, defined for and used by distros
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17 _/T 131072 kernel was built with the struct randomization plugin
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18 _/N 262144 an in-kernel test has been run
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=== === ====== ========================================================
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Note: The character ``_`` is representing a blank in this table to make reading
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easier.
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More detailed explanation for tainting
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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0) ``G`` if all modules loaded have a GPL or compatible license, ``P`` if
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any proprietary module has been loaded. Modules without a
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MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
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insmod as GPL compatible are assumed to be proprietary.
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1) ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all
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modules were loaded normally.
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2) ``S`` if the kernel is running on a processor or system that is out of
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specification: hardware has been put into an unsupported configuration,
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therefore proper execution cannot be guaranteed.
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Kernel will be tainted if, for example:
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- on x86: PAE is forced through forcepae on intel CPUs (such as Pentium M)
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which do not report PAE but may have a functional implementation, an SMP
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kernel is running on non officially capable SMP Athlon CPUs, MSRs are
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being poked at from userspace.
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- on arm: kernel running on certain CPUs (such as Keystone 2) without
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having certain kernel features enabled.
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- on arm64: there are mismatched hardware features between CPUs, the
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bootloader has booted CPUs in different modes.
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- certain drivers are being used on non supported architectures (such as
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scsi/snic on something else than x86_64, scsi/ips on non
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x86/x86_64/itanium, have broken firmware settings for the
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irqchip/irq-gic on arm64 ...).
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- x86/x86_64: Microcode late loading is dangerous and will result in
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tainting the kernel. It requires that all CPUs rendezvous to make sure
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the update happens when the system is as quiescent as possible. However,
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a higher priority MCE/SMI/NMI can move control flow away from that
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rendezvous and interrupt the update, which can be detrimental to the
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machine.
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3) ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all
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modules were unloaded normally.
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4) ``M`` if any processor has reported a Machine Check Exception,
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``' '`` if no Machine Check Exceptions have occurred.
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5) ``B`` If a page-release function has found a bad page reference or some
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unexpected page flags. This indicates a hardware problem or a kernel bug;
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there should be other information in the log indicating why this tainting
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occurred.
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6) ``U`` if a user or user application specifically requested that the
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Tainted flag be set, ``' '`` otherwise.
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7) ``D`` if the kernel has died recently, i.e. there was an OOPS or BUG.
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8) ``A`` if an ACPI table has been overridden.
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9) ``W`` if a warning has previously been issued by the kernel.
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(Though some warnings may set more specific taint flags.)
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10) ``C`` if a staging driver has been loaded.
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11) ``I`` if the kernel is working around a severe bug in the platform
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firmware (BIOS or similar).
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12) ``O`` if an externally-built ("out-of-tree") module has been loaded.
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13) ``E`` if an unsigned module has been loaded in a kernel supporting
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module signature.
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14) ``L`` if a soft lockup has previously occurred on the system.
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15) ``K`` if the kernel has been live patched.
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16) ``X`` Auxiliary taint, defined for and used by Linux distributors.
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17) ``T`` Kernel was build with the randstruct plugin, which can intentionally
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produce extremely unusual kernel structure layouts (even performance
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pathological ones), which is important to know when debugging. Set at
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build time.
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