[syzbot] [kernel?] WARNING in futex_pivot_pending

[syzbot] [kernel?] WARNING in futex_pivot_pending

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    f9054278acf4 Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=11ee7879580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ccf4bea59f67007
dashboard link: https://syzkaller.appspot.com/bug?extid=9c28ada89c468ad30713
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
userspace arch: arm64
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=163b3132580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14d52a9e580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/dbe6df6b5108/disk-f9054278.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c3892df8a641/vmlinux-f9054278.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a12ddc085ab/Image-f9054278.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com

------------[ cut here ]------------
do not call blocking ops when !TASK_RUNNING; state=2 set at [<000000003d62a0c3>] prepare_to_wait_event+0x2e4/0x3ac kernel/sched/wait.c:-1
WARNING: kernel/sched/core.c:9124 at __might_sleep+0xc0/0xdc kernel/sched/core.c:9120, CPU#0: syz.0.17/4920
Modules linked in:
CPU: 0 UID: 0 PID: 4920 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/02/2026
pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : __might_sleep+0xc0/0xdc kernel/sched/core.c:9120
lr : __might_sleep+0xc0/0xdc kernel/sched/core.c:9120
sp : ffff8000954d7930
x29: ffff8000954d7930 x28: ffff8000805a9ec8 x27: ffff8000954d7a00
x26: 0000000000000000 x25: ffff8000954d7a08 x24: 0000000000000000
x23: ffff700012a9af3c x22: ffff0000c4c716d0 x21: 0000000000000002
x20: ffff8000869ca160 x19: 000000000000026f x18: 00000000ffffffff
x17: ffff8000801682f0 x16: ffff800080168534 x15: ffff80008034b12c
x14: 0000000000000000 x13: 0000000000000001 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000002 x9 : dd2fee083d5bf500
x8 : dd2fee083d5bf500 x7 : ffff80008048ea5c x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffff8000802f8870
x2 : 0000000100000000 x1 : ffff0000c4c70000 x0 : 0000000000000000
Call trace:
 __might_sleep+0xc0/0xdc kernel/sched/core.c:9120 (P)
 __mutex_lock_common kernel/locking/mutex.c:623 [inline]
 __mutex_lock+0xf8/0xf14 kernel/locking/mutex.c:821
 mutex_lock_nested+0x24/0x30 kernel/locking/mutex.c:874
 class_mutex_constructor include/linux/mutex.h:253 [inline]
 futex_pivot_pending+0x30/0xa4 kernel/futex/core.c:1789
 futex_hash_allocate+0x73c/0xc20 kernel/futex/core.c:1872
 futex_hash_prctl+0xd0/0x324 kernel/futex/core.c:2027
 __do_sys_prctl kernel/sys.c:2885 [inline]
 __se_sys_prctl kernel/sys.c:2534 [inline]
 __arm64_sys_prctl+0x9c0/0x14e8 kernel/sys.c:2534
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0xec/0x23c arch/arm64/kernel/syscall.c:121
 do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
 el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:758
 el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:777
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:590
irq event stamp: 78
hardirqs last  enabled at (77): [<ffff800080492020>] vprintk_store+0x924/0xb94 kernel/printk/printk.c:2385
hardirqs last disabled at (78): [<ffff8000869055a8>] el1_brk64+0x20/0x54 arch/arm64/kernel/entry-common.c:445
softirqs last  enabled at (54): [<ffff80008013c050>] local_bh_enable include/linux/bottom_half.h:33 [inline]
softirqs last  enabled at (54): [<ffff80008013c050>] put_cpu_fpsimd_context arch/arm64/kernel/fpsimd.c:251 [inline]
softirqs last  enabled at (54): [<ffff80008013c050>] fpsimd_restore_current_state+0x39c/0x9c8 arch/arm64/kernel/fpsimd.c:1797
softirqs last disabled at (52): [<ffff80008013bcf8>] local_bh_disable include/linux/bottom_half.h:20 [inline]
softirqs last disabled at (52): [<ffff80008013bcf8>] get_cpu_fpsimd_context arch/arm64/kernel/fpsimd.c:234 [inline]
softirqs last disabled at (52): [<ffff80008013bcf8>] fpsimd_restore_current_state+0x44/0x9c8 arch/arm64/kernel/fpsimd.c:1790
---[ end trace 0000000000000000 ]---


---
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Re: [syzbot] [kernel?] WARNING in futex_pivot_pending

[Edward Adam Davis]

#syz test

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f225..154140fb6081 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1786,6 +1786,7 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
 
+	sched_annotate_sleep();
 	guard(mutex)(&mmph->lock);
 
 	if (!mmph->hash_new)

Re: [syzbot] [kernel?] WARNING in futex_pivot_pending

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com
Tested-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com

Tested on:

commit:         21d6ac05 Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=11b2c6c6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=95ac36a2cf7bfd28
dashboard link: https://syzkaller.appspot.com/bug?extid=9c28ada89c468ad30713
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
userspace arch: arm64
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1585e949580000

Note: testing is done by a robot and is best-effort only.

Re: [syzbot] [kernel?] WARNING in futex_pivot_pending

[Edward Adam Davis]

#syz test

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f225..a0b17d2f41da 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1785,14 +1785,24 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 {
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
+	unsigned int state;
+	bool ret;
+
+	state = current->__state;
+	__set_current_state(TASK_RUNNING);
+	scoped_guard(mutex, &mmph->lock) {
+		if (!mmph->hash_new) {
+			ret = true;
+			goto out;
+		}
 
-	guard(mutex)(&mmph->lock);
-
-	if (!mmph->hash_new)
-		return true;
+		fph = rcu_dereference_raw(mmph->hash);
+		ret = futex_ref_is_dead(fph);
+	}
+out:
+	__set_current_state(state);
+	return ret;
 
-	fph = rcu_dereference_raw(mmph->hash);
-	return futex_ref_is_dead(fph);
 }
 
 static bool futex_hash_less(struct futex_private_hash *a,

Re: [syzbot] [kernel?] WARNING in futex_pivot_pending

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com
Tested-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com

Tested on:

commit:         21d6ac05 Merge branch 'for-next/core' into for-kernelci
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=12d1ea25580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=95ac36a2cf7bfd28
dashboard link: https://syzkaller.appspot.com/bug?extid=9c28ada89c468ad30713
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
userspace arch: arm64
patch:          https://syzkaller.appspot.com/x/patch.diff?x=13f1da79580000

Note: testing is done by a robot and is best-effort only.

[PATCH] futex: Temporarily set the task state to running for pivot pending

[Edward Adam Davis]

There is a report of futex_pivot_pending() locking a mutex while not
TASK_RUNNING, which is due to wait_var_event(mm, futex_pivot_pending(mm))
calls __wait_var_event(), which sets the current task state to 2 before
the condition function `futex_pivot_pending()` executes, which triggers
the warning in [1].

Temporarily set the current task state to 0 within futex_pivot_pending()
to allow `mmph->lock` to complete its lifecycle normally.

[1]
do not call blocking ops when !TASK_RUNNING; state=2 set at [<000000003d62a0c3>] prepare_to_wait_event+0x2e4/0x3ac kernel/sched/wait.c:-1
WARNING: kernel/sched/core.c:9124 at __might_sleep+0xc0/0xdc kernel/sched/core.c:9120, CPU#0: syz.0.17/4920
Call trace:
 class_mutex_constructor include/linux/mutex.h:253 [inline]
 futex_pivot_pending+0x30/0xa4 kernel/futex/core.c:1789
 futex_hash_allocate+0x73c/0xc20 kernel/futex/core.c:1872
 futex_hash_prctl+0xd0/0x324 kernel/futex/core.c:2027

Fixes: 8e7ff730dd96 ("futex: Fix race in futex_pivot_pending() during private hash resize")
Reported-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9c28ada89c468ad30713
Tested-by: syzbot+9c28ada89c468ad30713@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 kernel/futex/core.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f225..a0b17d2f41da 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1785,14 +1785,24 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 {
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
+	unsigned int state;
+	bool ret;
+
+	state = current->__state;
+	__set_current_state(TASK_RUNNING);
+	scoped_guard(mutex, &mmph->lock) {
+		if (!mmph->hash_new) {
+			ret = true;
+			goto out;
+		}
 
-	guard(mutex)(&mmph->lock);
-
-	if (!mmph->hash_new)
-		return true;
+		fph = rcu_dereference_raw(mmph->hash);
+		ret = futex_ref_is_dead(fph);
+	}
+out:
+	__set_current_state(state);
+	return ret;
 
-	fph = rcu_dereference_raw(mmph->hash);
-	return futex_ref_is_dead(fph);
 }
 
 static bool futex_hash_less(struct futex_private_hash *a,
-- 
2.43.0