CVE-2026-53352: signal: clear JOBCTL_PENDING_MASK for caller in zap_other_threads()

In the Linux kernel, the following vulnerability has been resolved:

signal: clear JOBCTLPENDINGMASK for caller in zapotherthreads()

When a multi-threaded process receives a stop signal (e.g., SIGSTOP), dosignalstop() sets JOBCTLSTOPPENDING and JOBCTLSTOPCONSUME on all threads and sets signal->groupstopcount to the number of threads. If one of the threads concurrently calls execve(), dethread() invokes zapotherthreads() to kill all other threads. zapotherthreads() aborts the pending group stop by resetting signal->groupstopcount to 0 and clears the JOBCTLPENDINGMASK for all other threads. However, it fails to clear the job control flags for the calling thread.

When execve() completes, the calling thread returns to user mode and checks for pending signals. Seeing the stale JOBCTLSTOPPENDING flag, it calls dosignalstop(), which invokes taskparticipategroupstop(). Since JOBCTLSTOPCONSUME is still set, it attempts to decrement the already-zero signal->groupstopcount, triggering a warning:

sig->groupstopcount == 0 WARNING: CPU: 1 PID: 6475 at kernel/signal.c:373 taskparticipategroupstop+0x215/0x2d0 Call Trace: <TASK> dosignalstop+0x3be/0x5c0 kernel/signal.c:2619 getsignal+0xa8c/0x1330 kernel/signal.c:2884 archdosignalorrestart+0xbc/0x840 arch/x86/kernel/signal.c:337 exittousermodeloop+0x8c/0x4d0 kernel/entry/common.c:98 dosyscall64+0x33e/0xf80 arch/x86/entry/syscall64.c:100 entrySYSCALL64afterhwframe+0x77/0x7f </TASK>

Fix this race condition by clearing the JOBCTLPENDINGMASK for the calling thread in zapotherthreads(), ensuring it does not retain any stale job control state after the thread group is destroyed. This aligns with other functions that tear down a thread group and abort group stops, such as zapprocess() and completesignal(), which correctly clear these flags for all threads including the current one.