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glibc/sysdeps/nptl/cancellation-pc-check.h

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nptl: Fix Race conditions in pthread cancellation [BZ#12683] The current racy approach is to enable asynchronous cancellation before making the syscall and restore the previous cancellation type once the syscall returns, and check if cancellation has happen during the cancellation entrypoint. As described in BZ#12683, this approach shows 2 problems: 1. Cancellation can act after the syscall has returned from the kernel, but before userspace saves the return value. It might result in a resource leak if the syscall allocated a resource or a side effect (partial read/write), and there is no way to program handle it with cancellation handlers. 2. If a signal is handled while the thread is blocked at a cancellable syscall, the entire signal handler runs with asynchronous cancellation enabled. This can lead to issues if the signal handler call functions which are async-signal-safe but not async-cancel-safe. For the cancellation to work correctly, there are 5 points at which the cancellation signal could arrive: [ ... )[ ... )[ syscall ]( ... 1 2 3 4 5 1. Before initial testcancel, e.g. [*... testcancel) 2. Between testcancel and syscall start, e.g. [testcancel...syscall start) 3. While syscall is blocked and no side effects have yet taken place, e.g. [ syscall ] 4. Same as 3 but with side-effects having occurred (e.g. a partial read or write). 5. After syscall end e.g. (syscall end...*] And libc wants to act on cancellation in cases 1, 2, and 3 but not in cases 4 or 5. For the 4 and 5 cases, the cancellation will eventually happen in the next cancellable entrypoint without any further external event. The proposed solution for each case is: 1. Do a conditional branch based on whether the thread has received a cancellation request; 2. It can be caught by the signal handler determining that the saved program counter (from the ucontext_t) is in some address range beginning just before the "testcancel" and ending with the syscall instruction. 3. SIGCANCEL can be caught by the signal handler and determine that the saved program counter (from the ucontext_t) is in the address range beginning just before "testcancel" and ending with the first uninterruptable (via a signal) syscall instruction that enters the kernel. 4. In this case, except for certain syscalls that ALWAYS fail with EINTR even for non-interrupting signals, the kernel will reset the program counter to point at the syscall instruction during signal handling, so that the syscall is restarted when the signal handler returns. So, from the signal handler's standpoint, this looks the same as case 2, and thus it's taken care of. 5. For syscalls with side-effects, the kernel cannot restart the syscall; when it's interrupted by a signal, the kernel must cause the syscall to return with whatever partial result is obtained (e.g. partial read or write). 6. The saved program counter points just after the syscall instruction, so the signal handler won't act on cancellation. This is similar to 4. since the program counter is past the syscall instruction. So The proposed fixes are: 1. Remove the enable_asynccancel/disable_asynccancel function usage in cancellable syscall definition and instead make them call a common symbol that will check if cancellation is enabled (__syscall_cancel at nptl/cancellation.c), call the arch-specific cancellable entry-point (__syscall_cancel_arch), and cancel the thread when required. 2. Provide an arch-specific generic system call wrapper function that contains global markers. These markers will be used in SIGCANCEL signal handler to check if the interruption has been called in a valid syscall and if the syscalls has side-effects. A reference implementation sysdeps/unix/sysv/linux/syscall_cancel.c is provided. However, the markers may not be set on correct expected places depending on how INTERNAL_SYSCALL_NCS is implemented by the architecture. It is expected that all architectures add an arch-specific implementation. 3. Rewrite SIGCANCEL asynchronous handler to check for both canceling type and if current IP from signal handler falls between the global markers and act accordingly. 4. Adjust libc code to replace LIBC_CANCEL_ASYNC/LIBC_CANCEL_RESET to use the appropriate cancelable syscalls. 5. Adjust 'lowlevellock-futex.h' arch-specific implementations to provide cancelable futex calls. Some architectures require specific support on syscall handling: * On i386 the syscall cancel bridge needs to use the old int80 instruction because the optimized vDSO symbol the resulting PC value for an interrupted syscall points to an address outside the expected markers in __syscall_cancel_arch. It has been discussed in LKML [1] on how kernel could help userland to accomplish it, but afaik discussion has stalled. Also, sysenter should not be used directly by libc since its calling convention is set by the kernel depending of the underlying x86 chip (check kernel commit 30bfa7b3488bfb1bb75c9f50a5fcac1832970c60). * mips o32 is the only kABI that requires 7 argument syscall, and to avoid add a requirement on all architectures to support it, mips support is added with extra internal defines. Checked on aarch64-linux-gnu, arm-linux-gnueabihf, powerpc-linux-gnu, powerpc64-linux-gnu, powerpc64le-linux-gnu, i686-linux-gnu, and x86_64-linux-gnu. [1] https://lkml.org/lkml/2016/3/8/1105 Reviewed-by: Carlos O'Donell <carlos@redhat.com>
2024-06-25 19:17:44 +00:00
/* Architecture specific code for pthread cancellation handling.
Copyright (C) 2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#ifndef _NPTL_CANCELLATION_PC_CHECK
#define _NPTL_CANCELLATION_PC_CHECK
#include <sigcontextinfo.h>
/* For syscalls with side-effects (e.g read that might return partial read),
the kernel cannot restart the syscall when interrupted by a signal, it must
return from the call with whatever partial result. In this case, the saved
program counter is set just after the syscall instruction, so the SIGCANCEL
handler should not act on cancellation.
The __syscall_cancel_arch function, used for all cancellable syscalls,
contains two extra markers, __syscall_cancel_arch_start and
__syscall_cancel_arch_end. The former points to just before the initial
conditional branch that checks if the thread has received a cancellation
request, while former points to the instruction after the one responsible
to issue the syscall.
The function check if the program counter (PC) from ucontext_t CTX is
within the start and then end boundary from the __syscall_cancel_arch
bridge. Return TRUE if the PC is within the boundary, meaning the
syscall does not have any side effects; or FALSE otherwise. */
static __always_inline bool
cancellation_pc_check (void *ctx)
{
/* Both are defined in syscall_cancel.S. */
extern const char __syscall_cancel_arch_start[1];
extern const char __syscall_cancel_arch_end[1];
uintptr_t pc = sigcontext_get_pc (ctx);
return pc >= (uintptr_t) __syscall_cancel_arch_start
&& pc < (uintptr_t) __syscall_cancel_arch_end;
}
#endif
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