From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp153-141.sina.com.cn (smtp153-141.sina.com.cn [61.135.153.141]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 32A3E486639 for ; Wed, 26 Aug 2026 22:50:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=61.135.153.141 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787784617; cv=none; b=I32RrcMBsoIj/eFIBTRqwK6V4RGjSYqLVNTKOVKY0S7uR4GXaQEDyOi9dBKAy55mTk+xsNfg+1HTBHV4MJd5VL8JQJzMFqNVNmuhWmTOmzjl1ZG47e8xqr/tjtuHSVRkhANmQol3M3QOkMNL/NCQzyITReICbNv4sxfukCC4+Jw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787784617; c=relaxed/simple; bh=30+cNEQGWxKaZ47JOrBpMF5BmlWYIDN/QUJU99aQOGM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aUedqex9PxI1wCohWOw3DFtbokm0UVWVAyuD2+AJaHoAccAJ45uhgOLrh3l/yWfZkAkx8THKRQvGNbUEUASFzvOuZ/tn7zlF/LPWeMQ1C3dx3SU4IgGCvYmxrTY44r1WVXYTQKDZOFNgL+uofNDEuZnobvMSkX8uRqVZiE/RyFQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com; spf=pass smtp.mailfrom=sina.com; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b=JfUnWyKm; arc=none smtp.client-ip=61.135.153.141 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b="JfUnWyKm" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sina.com; s=201208; t=1787784608; bh=CoOH3iJ4VHTGu3S3hWgkalHbFFVL4BPGTHkMrJL4Bac=; h=From:Subject:Date:Message-ID; b=JfUnWyKmRB+WOpSf5lmYLXM/EcPWieYoXwWNjDU/MnNr+yNEtR+u6l0apmzfG2pM5 8ee94c3ILQYw34TMJwgQWi05iTwAmuVBi0EF7RD8AtZKGytryyk0i4Ou7XRmrGfr3L NpgSut0g0Gm8GhvlsTSA8nAOs2NIqebeAM0wR1JA= X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([114.249.62.194]) by sina.com (10.54.253.31) with ESMTP id 6A8F6D92000037C3; Wed, 27 Aug 2026 06:49:56 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 9555476816356 X-SMAIL-UIID: D39102DEBA3F47688D93274A14CC1A2F-20260827-064956-1 From: Hillf Danton To: syzbot Cc: linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot+2d770620059281e225a4@syzkaller.appspotmail.com, Eric Dumazet Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_link_sibling (2) Date: Thu, 27 Aug 2026 06:49:43 +0800 Message-ID: <20260826224944.1592-1-hdanton@sina.com> In-Reply-To: <6a8f3f8b.50659fcc.60cd.0005.GAE@google.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit > Date: Wed, 26 Aug 2026 12:33:31 -0700 [thread overview] > syzbot has found a reproducer for the following issue on: > > HEAD commit: 818bebeb63dd drm/xe: Don't hand out the flat CCS storage a.. > git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git > console output: https://syzkaller.appspot.com/x/log.txt?x=10059625580000 > kernel config: https://syzkaller.appspot.com/x/.config?x=ccca94d2c01b9e78 > dashboard link: https://syzkaller.appspot.com/bug?extid=dd764a450d93885d847f > compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44 > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13de8d49580000 #syz test syzbot reported a lockdep splat hitting DEBUG_LOCKS_WARN_ON(1) in hlock_class() due to an invalid class_idx: WARNING: kernel/locking/lockdep.c:238 at __lock_acquire+0x382/0x2cf0 kernel/locking/lockdep.c:5203 Workqueue: wg-crypt-wg0 wg_packet_tx_worker RIP: 0010:hlock_class kernel/locking/lockdep.c:238 [inline] RIP: 0010:check_wait_context kernel/locking/lockdep.c:4870 [inline] RIP: 0010:__lock_acquire+0x389/0x2cf0 kernel/locking/lockdep.c:5203 Call Trace: lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5886 _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:173 tcp_tsq_handler+0x29/0x200 net/ipv4/tcp_output.c:1291 tcp_tsq_workfn+0x384/0x410 net/ipv4/tcp_output.c:1325 ... When a lock class is zapped (e.g. during module unload or key unregistration), zap_class() clears the class's bit in lock_classes_in_use and removes it from the class hash table. However, existing lockdep_map instances embedded in data structures may still retain a pointer to the zapped class in their class_cache[] array. When __lock_acquire() subsequently runs on such a lock, it finds lock->class_cache[subclass] != NULL, skipping register_lock_class() and assigning hlock->class_idx to the index of the zapped class. When check_wait_context() or hlock_class() inspects the held_lock, it finds !test_bit(class_idx, lock_classes_in_use) and warns. Furthermore, if the zapped slot is subsequently re-allocated to an unrelated lock key, the stale class_cache entry would erroneously match the unrelated class (ABA issue). Add lock_class_cache_is_valid() to validate that the cached class is within lock_classes bounds, still allocated in lock_classes_in_use (using uninstrumented arch_test_bit() in __always_inline context so it is safe in noinstr contexts like match_held_lock()), and that class->key matches the expected subkey (taking lockdep_set_subclass() overrides into account). Also use READ_ONCE()/WRITE_ONCE() when accessing class_cache[]. If the entry is invalid or stale, fall back to register_lock_class() / look_up_lock_class(). Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") Reported-by: syzbot+2d770620059281e225a4@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/6a8c66dc.4d75e56a.c9a88.0050.GAE@google.com/T/#u Assisted-by: Gemini:gemini-3.1-pro Signed-off-by: Eric Dumazet --- kernel/locking/lockdep.c | 50 +++++++++++++++++++++++++++++++++------- 1 file changed, 42 insertions(+), 8 deletions(-) diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 25d77d4a1061..763f79806bd8 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -963,6 +963,34 @@ look_up_lock_class(const struct lockdep_map *lock, unsigned int subclass) return NULL; } +static __always_inline bool lock_class_cache_is_valid(const struct lockdep_map *lock, + const struct lock_class *class, + unsigned int subclass) +{ + unsigned int class_subclass; + + if (!class) + return false; + + if (unlikely(class < lock_classes || class >= lock_classes + MAX_LOCKDEP_KEYS)) + return false; + + if (unlikely(!arch_test_bit(class - lock_classes, lock_classes_in_use))) + return false; + + if (unlikely(!lock->key)) + return false; + + class_subclass = subclass ? subclass : class->subclass; + if (unlikely(class_subclass >= MAX_LOCKDEP_SUBCLASSES)) + return false; + + if (unlikely(READ_ONCE(class->key) != lock->key->subkeys + class_subclass)) + return false; + + return true; +} + /* * Static locks do not have their class-keys yet - for them the key is * the lock object itself. If the lock is in the per cpu area, the @@ -1395,9 +1423,9 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force) out_set_class_cache: if (!subclass || force) - lock->class_cache[0] = class; + WRITE_ONCE(lock->class_cache[0], class); else if (subclass < NR_LOCKDEP_CACHING_CLASSES) - lock->class_cache[subclass] = class; + WRITE_ONCE(lock->class_cache[subclass], class); /* * Hash collision, did we smoke some? We found a class with a matching @@ -4957,7 +4985,7 @@ void lockdep_init_map_type(struct lockdep_map *lock, const char *name, int i; for (i = 0; i < NR_LOCKDEP_CACHING_CLASSES; i++) - lock->class_cache[i] = NULL; + WRITE_ONCE(lock->class_cache[i], NULL); #ifdef CONFIG_LOCK_STAT lock->cpu = raw_smp_processor_id(); @@ -5022,12 +5050,15 @@ EXPORT_SYMBOL_GPL(__lockdep_no_track__); void lockdep_set_lock_cmp_fn(struct lockdep_map *lock, lock_cmp_fn cmp_fn, lock_print_fn print_fn) { - struct lock_class *class = lock->class_cache[0]; + struct lock_class *class = READ_ONCE(lock->class_cache[0]); unsigned long flags; raw_local_irq_save(flags); lockdep_recursion_inc(); + if (!lock_class_cache_is_valid(lock, class, 0)) + class = NULL; + if (!class) class = register_lock_class(lock, 0, 0); @@ -5119,8 +5150,11 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass, if (DEBUG_LOCKS_WARN_ON(subclass >= MAX_LOCKDEP_SUBCLASSES)) return 0; - if (subclass < NR_LOCKDEP_CACHING_CLASSES) - class = lock->class_cache[subclass]; + if (subclass < NR_LOCKDEP_CACHING_CLASSES) { + class = READ_ONCE(lock->class_cache[subclass]); + if (!lock_class_cache_is_valid(lock, class, subclass)) + class = NULL; + } /* * Not cached? */ @@ -5323,9 +5357,9 @@ static noinstr int match_held_lock(const struct held_lock *hlock, return 1; if (hlock->references) { - const struct lock_class *class = lock->class_cache[0]; + const struct lock_class *class = READ_ONCE(lock->class_cache[0]); - if (!class) + if (!lock_class_cache_is_valid(lock, class, 0)) class = look_up_lock_class(lock, 0); /* base-commit: 0a0d1d55dad570724bf8c7ea83409639cfb4be9b -- 2.55.0.766.g2966f0265a-goog