91 lines
4.2 KiB
ReStructuredText
91 lines
4.2 KiB
ReStructuredText
|
.. 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 SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK.
|
||
|
|
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.
|