The following commit has been merged into the core/entry branch of tip: Commit-ID: a4452e671c6770e1bb80764f39995934067f70a0 Gitweb: https://git.kernel.org/tip/a4452e671c6770e1bb80764f39995934067f70a0 Author: Gabriel Krisman Bertazi <kris...@collabora.com> AuthorDate: Fri, 27 Nov 2020 14:32:38 -05:00 Committer: Thomas Gleixner <t...@linutronix.de> CommitterDate: Wed, 02 Dec 2020 15:07:57 +01:00
docs: Document Syscall User Dispatch Explain the interface, provide some background and security notes. [ tglx: Add note about non-visibility, add it to the index and fix the kerneldoc warning ] Signed-off-by: Gabriel Krisman Bertazi <kris...@collabora.com> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Reviewed-by: Kees Cook <keesc...@chromium.org> Reviewed-by: Andy Lutomirski <l...@kernel.org> Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org> Link: https://lore.kernel.org/r/20201127193238.821364-8-kris...@collabora.com --- Documentation/admin-guide/index.rst | 1 +- Documentation/admin-guide/syscall-user-dispatch.rst | 90 ++++++++++++- 2 files changed, 91 insertions(+) create mode 100644 Documentation/admin-guide/syscall-user-dispatch.rst diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst index 4e0c4ae..b29d3c1 100644 --- a/Documentation/admin-guide/index.rst +++ b/Documentation/admin-guide/index.rst @@ -111,6 +111,7 @@ configure specific aspects of kernel behavior to your liking. rtc serial-console svga + syscall-user-dispatch sysrq thunderbolt ufs diff --git a/Documentation/admin-guide/syscall-user-dispatch.rst b/Documentation/admin-guide/syscall-user-dispatch.rst new file mode 100644 index 0000000..a380d65 --- /dev/null +++ b/Documentation/admin-guide/syscall-user-dispatch.rst @@ -0,0 +1,90 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Syscall User Dispatch +===================== + +Background +---------- + +Compatibility layers like Wine need a way to efficiently emulate system +calls of only a part of their process - the part that has the +incompatible code - while being able to execute native syscalls without +a high performance penalty on the native part of the process. Seccomp +falls short on this task, since it has limited support to efficiently +filter syscalls based on memory regions, and it doesn't support removing +filters. Therefore a new mechanism is necessary. + +Syscall User Dispatch brings the filtering of the syscall dispatcher +address back to userspace. The application is in control of a flip +switch, indicating the current personality of the process. A +multiple-personality application can then flip the switch without +invoking the kernel, when crossing the compatibility layer API +boundaries, to enable/disable the syscall redirection and execute +syscalls directly (disabled) or send them to be emulated in userspace +through a SIGSYS. + +The goal of this design is to provide very quick compatibility layer +boundary crosses, which is achieved by not executing a syscall to change +personality every time the compatibility layer executes. Instead, a +userspace memory region exposed to the kernel indicates the current +personality, and the application simply modifies that variable to +configure the mechanism. + +There is a relatively high cost associated with handling signals on most +architectures, like x86, but at least for Wine, syscalls issued by +native Windows code are currently not known to be a performance problem, +since they are quite rare, at least for modern gaming applications. + +Since this mechanism is designed to capture syscalls issued by +non-native applications, it must function on syscalls whose invocation +ABI is completely unexpected to Linux. Syscall User Dispatch, therefore +doesn't rely on any of the syscall ABI to make the filtering. It uses +only the syscall dispatcher address and the userspace key. + +As the ABI of these intercepted syscalls is unknown to Linux, these +syscalls are not instrumentable via ptrace or the syscall tracepoints. + +Interface +--------- + +A thread can setup this mechanism on supported kernels by executing the +following prctl: + + prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector]) + +<op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and +disable the mechanism globally for that thread. When +PR_SYS_DISPATCH_OFF is used, the other fields must be zero. + +[<offset>, <offset>+<length>) delimit a memory region interval +from which syscalls are always executed directly, regardless of the +userspace selector. This provides a fast path for the C library, which +includes the most common syscall dispatchers in the native code +applications, and also provides a way for the signal handler to return +without triggering a nested SIGSYS on (rt\_)sigreturn. Users of this +interface should make sure that at least the signal trampoline code is +included in this region. In addition, for syscalls that implement the +trampoline code on the vDSO, that trampoline is never intercepted. + +[selector] is a pointer to a char-sized region in the process memory +region, that provides a quick way to enable disable syscall redirection +thread-wide, without the need to invoke the kernel directly. selector +can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF. Any other +value should terminate the program with a SIGSYS. + +Security Notes +-------------- + +Syscall User Dispatch provides functionality for compatibility layers to +quickly capture system calls issued by a non-native part of the +application, while not impacting the Linux native regions of the +process. It is not a mechanism for sandboxing system calls, and it +should not be seen as a security mechanism, since it is trivial for a +malicious application to subvert the mechanism by jumping to an allowed +dispatcher region prior to executing the syscall, or to discover the +address and modify the selector value. If the use case requires any +kind of security sandboxing, Seccomp should be used instead. + +Any fork or exec of the existing process resets the mechanism to +PR_SYS_DISPATCH_OFF.