1 .. SPDX-License-Identifier: GPL-2.0 2 3 ===================== 4 Syscall User Dispatch 5 ===================== 6 7 Background 8 ---------- 9 10 Compatibility layers like Wine need a way to efficiently emulate system 11 calls of only a part of their process - the part that has the 12 incompatible code - while being able to execute native syscalls without 13 a high performance penalty on the native part of the process. Seccomp 14 falls short on this task, since it has limited support to efficiently 15 filter syscalls based on memory regions, and it doesn't support removing 16 filters. Therefore a new mechanism is necessary. 17 18 Syscall User Dispatch brings the filtering of the syscall dispatcher 19 address back to userspace. The application is in control of a flip 20 switch, indicating the current personality of the process. A 21 multiple-personality application can then flip the switch without 22 invoking the kernel, when crossing the compatibility layer API 23 boundaries, to enable/disable the syscall redirection and execute 24 syscalls directly (disabled) or send them to be emulated in userspace 25 through a SIGSYS. 26 27 The goal of this design is to provide very quick compatibility layer 28 boundary crosses, which is achieved by not executing a syscall to change 29 personality every time the compatibility layer executes. Instead, a 30 userspace memory region exposed to the kernel indicates the current 31 personality, and the application simply modifies that variable to 32 configure the mechanism. 33 34 There is a relatively high cost associated with handling signals on most 35 architectures, like x86, but at least for Wine, syscalls issued by 36 native Windows code are currently not known to be a performance problem, 37 since they are quite rare, at least for modern gaming applications. 38 39 Since this mechanism is designed to capture syscalls issued by 40 non-native applications, it must function on syscalls whose invocation 41 ABI is completely unexpected to Linux. Syscall User Dispatch, therefore 42 doesn't rely on any of the syscall ABI to make the filtering. It uses 43 only the syscall dispatcher address and the userspace key. 44 45 As the ABI of these intercepted syscalls is unknown to Linux, these 46 syscalls are not instrumentable via ptrace or the syscall tracepoints. 47 48 Interface 49 --------- 50 51 A thread can setup this mechanism on supported kernels by executing the 52 following prctl: 53 54 prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <offset>, <length>, [selector]) 55 56 <op> is either PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF, to enable and 57 disable the mechanism globally for that thread. When 58 PR_SYS_DISPATCH_OFF is used, the other fields must be zero. 59 60 [<offset>, <offset>+<length>) delimit a memory region interval 61 from which syscalls are always executed directly, regardless of the 62 userspace selector. This provides a fast path for the C library, which 63 includes the most common syscall dispatchers in the native code 64 applications, and also provides a way for the signal handler to return 65 without triggering a nested SIGSYS on (rt\_)sigreturn. Users of this 66 interface should make sure that at least the signal trampoline code is 67 included in this region. In addition, for syscalls that implement the 68 trampoline code on the vDSO, that trampoline is never intercepted. 69 70 [selector] is a pointer to a char-sized region in the process memory 71 region, that provides a quick way to enable disable syscall redirection 72 thread-wide, without the need to invoke the kernel directly. selector 73 can be set to SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK. 74 Any other value should terminate the program with a SIGSYS. 75 76 Additionally, a tasks syscall user dispatch configuration can be peeked 77 and poked via the PTRACE_(GET|SET)_SYSCALL_USER_DISPATCH_CONFIG ptrace 78 requests. This is useful for checkpoint/restart software. 79 80 Security Notes 81 -------------- 82 83 Syscall User Dispatch provides functionality for compatibility layers to 84 quickly capture system calls issued by a non-native part of the 85 application, while not impacting the Linux native regions of the 86 process. It is not a mechanism for sandboxing system calls, and it 87 should not be seen as a security mechanism, since it is trivial for a 88 malicious application to subvert the mechanism by jumping to an allowed 89 dispatcher region prior to executing the syscall, or to discover the 90 address and modify the selector value. If the use case requires any 91 kind of security sandboxing, Seccomp should be used instead. 92 93 Any fork or exec of the existing process resets the mechanism to 94 PR_SYS_DISPATCH_OFF.
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