91 lines
4.2 KiB
ReStructuredText
91 lines
4.2 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=====================
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Syscall User Dispatch
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=====================
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Background
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----------
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Compatibility layers like Wine need a way to efficiently emulate system
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calls of only a part of their process - the part that has the
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incompatible code - while being able to execute native syscalls without
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a high performance penalty on the native part of the process. Seccomp
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falls short on this task, since it has limited support to efficiently
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filter syscalls based on memory regions, and it doesn't support removing
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filters. Therefore a new mechanism is necessary.
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Syscall User Dispatch brings the filtering of the syscall dispatcher
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address back to userspace. The application is in control of a flip
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switch, indicating the current personality of the process. A
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multiple-personality application can then flip the switch without
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invoking the kernel, when crossing the compatibility layer API
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boundaries, to enable/disable the syscall redirection and execute
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syscalls directly (disabled) or send them to be emulated in userspace
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through a SIGSYS.
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The goal of this design is to provide very quick compatibility layer
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boundary crosses, which is achieved by not executing a syscall to change
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personality every time the compatibility layer executes. Instead, a
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userspace memory region exposed to the kernel indicates the current
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personality, and the application simply modifies that variable to
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configure the mechanism.
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There is a relatively high cost associated with handling signals on most
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architectures, like x86, but at least for Wine, syscalls issued by
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native Windows code are currently not known to be a performance problem,
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since they are quite rare, at least for modern gaming applications.
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Since this mechanism is designed to capture syscalls issued by
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non-native applications, it must function on syscalls whose invocation
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ABI is completely unexpected to Linux. Syscall User Dispatch, therefore
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doesn't rely on any of the syscall ABI to make the filtering. It uses
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only the syscall dispatcher address and the userspace key.
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As the ABI of these intercepted syscalls is unknown to Linux, these
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syscalls are not instrumentable via ptrace or the syscall tracepoints.
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Interface
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---------
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A thread can setup this mechanism on supported kernels by executing the
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following prctl:
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prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector])
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<op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and
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disable the mechanism globally for that thread. When
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PR_SYS_DISPATCH_OFF is used, the other fields must be zero.
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[<offset>, <offset>+<length>) delimit a memory region interval
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from which syscalls are always executed directly, regardless of the
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userspace selector. This provides a fast path for the C library, which
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includes the most common syscall dispatchers in the native code
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applications, and also provides a way for the signal handler to return
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without triggering a nested SIGSYS on (rt\_)sigreturn. Users of this
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interface should make sure that at least the signal trampoline code is
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included in this region. In addition, for syscalls that implement the
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trampoline code on the vDSO, that trampoline is never intercepted.
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[selector] is a pointer to a char-sized region in the process memory
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region, that provides a quick way to enable disable syscall redirection
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thread-wide, without the need to invoke the kernel directly. selector
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can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF. Any other
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value should terminate the program with a SIGSYS.
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Security Notes
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--------------
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Syscall User Dispatch provides functionality for compatibility layers to
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quickly capture system calls issued by a non-native part of the
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application, while not impacting the Linux native regions of the
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process. It is not a mechanism for sandboxing system calls, and it
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should not be seen as a security mechanism, since it is trivial for a
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malicious application to subvert the mechanism by jumping to an allowed
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dispatcher region prior to executing the syscall, or to discover the
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address and modify the selector value. If the use case requires any
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kind of security sandboxing, Seccomp should be used instead.
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Any fork or exec of the existing process resets the mechanism to
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PR_SYS_DISPATCH_OFF.
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