In the Linux kernel, the following vulnerability has been resolved: net/9p: fix infinite loop in p9_client_rpc on fatal signal When p9_client_rpc() is called with type P9_TFLUSH and the transport has no peer (e.g. fd transport backed by pipes with no 9p server), a fatal signal causes an infinite loop: again: err = io_wait_event_killable(req->wq, ...) /* SIGKILL wakes the task, returns -ERESTARTSYS */ if (err == -ERESTARTSYS && c->status == Connected && type == P9_TFLUSH) { sigpending = 1; clear_thread_flag(TIF_SIGPENDING); goto again; } clear_thread_flag() clears TIF_SIGPENDING before jumping back to io_wait_event_killable(). signal_pending_state() checks TIF_SIGPENDING, finds it zero, and the task goes to sleep again. The task can only wake on the next signal delivery that calls signal_wake_up() and sets TIF_SIGPENDING again. When that happens the loop repeats, clears TIF_SIGPENDING, and sleeps again indefinitely. This is triggered in practice by coredump_wait(): when a thread in a multi-threaded process causes a coredump (e.g. via SIGSYS from Syscall User Dispatch), coredump_wait() sends SIGKILL to all other threads and waits for them to call mm_release(). If one of those threads is blocked in p9_client_rpc() over an fd transport with no peer, it enters the P9_TFLUSH loop and never calls mm_release(), so coredump_wait() stalls forever: INFO: task syz.0.18:676 blocked for more than 143 seconds. Not tainted 6.12.77+ #1 task:syz.0.18 state:D stack:27600 pid:676 tgid:673 ppid:630 flags:0x00000004 Call Trace: <TASK> context_switch kernel/sched/core.c:5344 [inline] __schedule+0xcb4/0x5d50 kernel/sched/core.c:6724 __schedule_loop kernel/sched/core.c:6801 [inline] schedule+0xe5/0x350 kernel/sched/core.c:6816 schedule_timeout+0x253/0x290 kernel/time/timer.c:2593 do_wait_for_common kernel/sched/completion.c:95 [inline] __wait_for_common+0x409/0x600 kernel/sched/completion.c:116 wait_for_common kernel/sched/completion.c:127 [inline] wait_for_completion_state+0x1d/0x40 kernel/sched/completion.c:264 coredump_wait fs/coredump.c:448 [inline] do_coredump+0x854/0x4350 fs/coredump.c:629 get_signal+0x1425/0x2730 kernel/signal.c:2903 arch_do_signal_or_restart+0x81/0x880 arch/x86/kernel/signal.c:337 exit_to_user_mode_loop kernel/entry/common.c:111 [inline] exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline] __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline] syscall_exit_to_user_mode+0xf9/0x160 kernel/entry/common.c:218 do_syscall_64+0x102/0x220 arch/x86/entry/common.c:84 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Fix: check fatal_signal_pending() before clearing TIF_SIGPENDING in the P9_TFLUSH retry loop. At that point TIF_SIGPENDING is still set, so fatal_signal_pending() works correctly. If a fatal signal is pending, jump to recalc_sigpending to restore TIF_SIGPENDING and return -ERESTARTSYS to the caller. The same defect is present in stable kernels back to 5.4. On those kernels the infinite loop is broken earlier by a second SIGKILL from the parent process (e.g. kill_and_wait() retrying after a timeout), resulting in a zombie process and a shutdown delay rather than a permanent D-state hang, but the underlying flaw is the same. Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Quoted source text, attributed separately from HOL analysis.

In the Linux kernel, the following vulnerability has been resolved: net/9p: fix infinite loop in p9_client_rpc on fatal signal When p9_client_rpc() is called with type P9_TFLUSH and the transport has no peer (e.g. fd transport backed by pipes with no 9p server), a fatal signal causes an infinite loop: again: err = io_wait_event_killable(req->wq, ...) /* SIGKILL wakes the task, returns -ERESTARTSYS */ if (err == -ERESTARTSYS && c->status == Connected && type == P9_TFLUSH) { sigpending = 1; clear_thread_flag(TIF_SIGPENDING); goto again; } clear_thread_flag() clears TIF_SIGPENDING before jumping back to io_wait_event_killable(). signal_pending_state() checks TIF_SIGPENDING, finds it zero, and the task goes to sleep again. The task can only wake on the next signal delivery that calls signal_wake_up() and sets TIF_SIGPENDING again. When that happens the loop repeats, clears TIF_SIGPENDING, and sleeps again indefinitely. This is triggered in practice by coredump_wait(): when a thread in a multi-threaded process causes a coredump (e.g. via SIGSYS from Syscall User Dispatch), coredump_wait() sends SIGKILL to all other threads and waits for them to call mm_release(). If one of those threads is blocked in p9_client_rpc() over an fd transport with no peer, it enters the P9_TFLUSH loop and never calls mm_release(), so coredump_wait() stalls forever: INFO: task syz.0.18:676 blocked for more than 143 seconds. Not tainted 6.12.77+ #1 task:syz.0.18 state:D stack:27600 pid:676 tgid:673 ppid:630 flags:0x00000004 Call Trace: <TASK> context_switch kernel/sched/core.c:5344 [inline] __schedule+0xcb4/0x5d50 kernel/sched/core.c:6724 __schedule_loop kernel/sched/core.c:6801 [inline] schedule+0xe5/0x350 kernel/sched/core.c:6816 schedule_timeout+0x253/0x290 kernel/time/timer.c:2593 do_wait_for_common kernel/sched/completion.c:95 [inline] __wait_for_common+0x409/0x600 kernel/sched/completion.c:116 wait_for_common kernel/sched/completion.c:127 [inline] wait_for_completion_state+0x1d/0x40 kernel/sched/completion.c:264 coredump_wait fs/coredump.c:448 [inline] do_coredump+0x854/0x4350 fs/coredump.c:629 get_signal+0x1425/0x2730 kernel/signal.c:2903 arch_do_signal_or_restart+0x81/0x880 arch/x86/kernel/signal.c:337 exit_to_user_mode_loop kernel/entry/common.c:111 [inline] exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline] __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline] syscall_exit_to_user_mode+0xf9/0x160 kernel/entry/common.c:218 do_syscall_64+0x102/0x220 arch/x86/entry/common.c:84 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> Fix: check fatal_signal_pending() before clearing TIF_SIGPENDING in the P9_TFLUSH retry loop. At that point TIF_SIGPENDING is still set, so fatal_signal_pending() works correctly. If a fatal signal is pending, jump to recalc_sigpending to restore TIF_SIGPENDING and return -ERESTARTSYS to the caller. The same defect is present in stable kernels back to 5.4. On those kernels the infinite loop is broken earlier by a second SIGKILL from the parent process (e.g. kill_and_wait() retrying after a timeout), resulting in a zombie process and a shutdown delay rather than a permanent D-state hang, but the underlying flaw is the same. Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Quoted source text, attributed separately from HOL analysis.