From: Yao Kai Date: Tue, 4 Aug 2026 12:55:30 +0000 (+0800) Subject: futex: Fix race in futex_pivot_pending() during private hash resize X-Git-Tag: v7.2-rc7~4^2 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=8e7ff730dd96519a333d1570edf1c3fabb6d3629;p=thirdparty%2Flinux.git futex: Fix race in futex_pivot_pending() during private hash resize A task performing a custom private hash resize can remain blocked in uninterruptible sleep indefinitely. The hung-task detector reports: INFO: task futex-resizer:314 blocked for more than 10 seconds. task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311 Call Trace: __schedule+0x521/0xf30 schedule+0x22/0xa0 futex_hash_allocate+0x3db/0x490 __do_sys_prctl+0x6f5/0xbd0 do_syscall_64+0xf9/0x530 entry_SYSCALL_64_after_hwframe+0x77/0x7f Kernel panic - not syncing: hung_task: blocked tasks futex_pivot_pending() allows the resize request to continue when either no replacement hash is pending (hash_new == NULL) or the current hash reference count has reached zero. After the final-reference wake, another futex task can complete the pivot between the two observations: T1 T2 futex_hash_allocate() wait_var_event(mm, ...) futex_pivot_pending(mm) hash_new != NULL futex_hash() futex_ref_get(old) -> false futex_pivot_hash(mm) hash_new = NULL __futex_pivot_hash(mm, new) rcu_assign_pointer(hash, new) fph = rcu_dereference(hash) /* new */ futex_ref_is_dead(fph) -> false schedule() The pivot changes the state from hash_new != NULL with a dead current hash to hash_new == NULL with a live current hash. Because futex_pivot_pending() reads hash_new and hash without serialization, the resize task can observe hash_new in the pre-pivot state and hash in the post-pivot state, causing futex_pivot_pending() to return false even though the pivot has completed. The task then goes to sleep after the wakeup has already been consumed. Serialize state reads in futex_pivot_pending() using futex_mm_phash::lock. This guarantees that futex_pivot_pending() observes hash_new and hash atomically, eliminating the race condition. Fixes: bd54df5ea7ca ("futex: Allow to resize the private local hash") Suggested-by: Peter Zijlstra Signed-off-by: Yao Kai Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260804125530.3933754-1-yaokai34@huawei.com --- diff --git a/kernel/futex/core.c b/kernel/futex/core.c index 2650d1e52803..128c5752f225 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -1783,14 +1783,15 @@ void futex_hash_free(struct mm_struct *mm) static bool futex_pivot_pending(struct mm_struct *mm) { + struct futex_mm_phash *mmph = &mm->futex.phash; struct futex_private_hash *fph; - guard(rcu)(); + guard(mutex)(&mmph->lock); - if (!mm->futex.phash.hash_new) + if (!mmph->hash_new) return true; - fph = rcu_dereference(mm->futex.phash.hash); + fph = rcu_dereference_raw(mmph->hash); return futex_ref_is_dead(fph); }