From 78004e9a87f240df03e2f73120d291763c32e0a7 Mon Sep 17 00:00:00 2001 From: Ali Ahmet Memis Date: Fri, 21 Aug 2026 01:45:27 +0000 Subject: openrisc: fix arbitrary kernel memory access via or1k_atomic syscall sys_or1k_atomic() (syscall 244 in the "or1k" ABI) takes two user pointers, v1 and v2, and swaps the words they point to in hand-written assembly. l.lwz r29,0(r4) l.lwz r27,0(r5) l.sw 0(r4),r27 l.sw 0(r5),r29 The pointers are not checked with access_ok(). The four memory accesses also have no exception table entries. A caller passes a kernel address as either pointer, and the syscall reads from and writes to it directly. This gives an unprivileged process a kernel read/write primitive. It overwrites kernel data such as the sys_call_table, gaining code execution in kernel context. Check both pointers before entering the critical section. Add fixups for the four memory accesses so faults on valid but unmapped user addresses return -EFAULT. [shorne@gmail.com: fix comment style] Fixes: 9d02a4283e9c ("OpenRISC: Boot code") Cc: stable@vger.kernel.org Signed-off-by: Ali Ahmet Memis Signed-off-by: Stafford Horne --- .../admin-guide/syscall-user-dispatch.rst | 99 ++++++++++++++++++++++ 1 file changed, 99 insertions(+) create mode 100644 Documentation/admin-guide/syscall-user-dispatch.rst (limited to 'Documentation/admin-guide/syscall-user-dispatch.rst') diff --git a/Documentation/admin-guide/syscall-user-dispatch.rst b/Documentation/admin-guide/syscall-user-dispatch.rst new file mode 100644 index 000000000..c1768d9e8 --- /dev/null +++ b/Documentation/admin-guide/syscall-user-dispatch.rst @@ -0,0 +1,99 @@ +.. 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, , , , [selector]) + + is either PR_SYS_DISPATCH_EXCLUSIVE_ON/PR_SYS_DISPATCH_INCLUSIVE_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. + +For PR_SYS_DISPATCH_EXCLUSIVE_ON [, +) 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. + +For PR_SYS_DISPATCH_INCLUSIVE_ON [, +) delimit +a memory region interval from which syscalls are dispatched based on +the userspace selector. Syscalls from outside of the range are always +executed directly. + +[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. + +Additionally, a tasks syscall user dispatch configuration can be peeked +and poked via the PTRACE_(GET|SET)_SYSCALL_USER_DISPATCH_CONFIG ptrace +requests. This is useful for checkpoint/restart software. + +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. -- cgit v1.3.1