* [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().
@ 2026-05-30 5:59 Kuniyuki Iwashima
2026-05-30 10:11 ` Jiayuan Chen
0 siblings, 1 reply; 2+ messages in thread
From: Kuniyuki Iwashima @ 2026-05-30 5:59 UTC (permalink / raw)
To: Eric Dumazet, Neal Cardwell, David S. Miller, Jakub Kicinski,
Paolo Abeni
Cc: Simon Horman, Sebastian Andrzej Siewior, Kuniyuki Iwashima,
Kuniyuki Iwashima, netdev, syzbot+e809069bc15f26300526
syzbot reported a weird reqsk->rsk_refcnt underflow in
__inet_csk_reqsk_queue_drop().
The captured reqsk_put() in __inet_csk_reqsk_queue_drop()
is called only when it successfully removes reqsk from ehash.
Moreover, reqsk_timer_handler() calls another reqsk_put()
after that.
This indicates that the reqsk was missing both refcnts for
ehash and the timer itself.
Since all the syzbot reports had PREEMPT_RT enabled, the only
possible scenario is that reqsk_queue_hash_req() is preempted
after mod_timer() and before refcount_set(), and then the timer
triggered after 1s aborts the reqsk due to its listener's close().
Let's wrap mod_timer() and refcount_set() with
preempt_disable_nested() and preempt_enable_nested().
Note that inet_ehash_insert() holds the normal spin_lock()
(mutex in PREEMPT_RT), so it must be called outside of
preempt_disable_nested(), but this is fine.
The lookup path just ignores 0 sk_refcnt entries in ehash
and tries to create another reqsk, but this will fail at
inet_ehash_insert().
[0]:
refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28
Code: e4 7d d1 0a 67 48 0f b9 3a eb 4a e8 38 3d 23 fd 48 8d 3d e1 7d d1 0a 67 48 0f b9 3a eb 37 e8 25 3d 23 fd 48 8d 3d de 7d d1 0a <67> 48 0f b9 3a eb 24 e8 12 3d 23 fd 48 8d 3d db 7d d1 0a 67 48 0f
RSP: 0000:ffffc90000157948 EFLAGS: 00010246
RAX: ffffffff84a1301b RBX: 0000000000000003 RCX: ffff88801ca98000
RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffffff8f72ae00
RBP: ffffffff99ae3b01 R08: ffff88801ca98000 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880425ef568
R13: ffff8880425ef4f8 R14: ffff8880425ef578 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff888126386000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7b46710e9c CR3: 000000000dbb6000 CR4: 00000000003526f0
Call Trace:
<TASK>
__refcount_sub_and_test include/linux/refcount.h:400 [inline]
__refcount_dec_and_test include/linux/refcount.h:432 [inline]
refcount_dec_and_test include/linux/refcount.h:450 [inline]
reqsk_put include/net/request_sock.h:136 [inline]
__inet_csk_reqsk_queue_drop+0x3ce/0x440 net/ipv4/inet_connection_sock.c:1007
reqsk_timer_handler+0x651/0xdf0 net/ipv4/inet_connection_sock.c:1137
call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748
expire_timers kernel/time/timer.c:1799 [inline]
__run_timers kernel/time/timer.c:2374 [inline]
__run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386
run_timer_base kernel/time/timer.c:2395 [inline]
run_timer_softirq+0x67/0x170 kernel/time/timer.c:2403
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:622
__do_softirq kernel/softirq.c:656 [inline]
run_ktimerd+0x69/0x100 kernel/softirq.c:1151
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
Reported-by: syzbot+e809069bc15f26300526@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6a1a7bcf.0a9e871e.332604.000b.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
net/ipv4/inet_connection_sock.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index dbcd37dfdc15..b546ce15ee03 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1145,6 +1145,8 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
if (!inet_ehash_insert(req_to_sk(req), NULL, &found_dup_sk))
return false;
+ preempt_disable_nested();
+
/* The timer needs to be setup after a successful insertion. */
req->timeout = tcp_timeout_init((struct sock *)req);
timer_setup(&req->rsk_timer, reqsk_timer_handler, TIMER_PINNED);
@@ -1155,6 +1157,9 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
*/
smp_wmb();
refcount_set(&req->rsk_refcnt, 2 + 1);
+
+ preempt_enable_nested();
+
return true;
}
--
2.54.0.823.g6e5bcc1fc9-goog
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().
2026-05-30 5:59 [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req() Kuniyuki Iwashima
@ 2026-05-30 10:11 ` Jiayuan Chen
0 siblings, 0 replies; 2+ messages in thread
From: Jiayuan Chen @ 2026-05-30 10:11 UTC (permalink / raw)
To: Kuniyuki Iwashima, Eric Dumazet, Neal Cardwell, David S. Miller,
Jakub Kicinski, Paolo Abeni
Cc: Simon Horman, Sebastian Andrzej Siewior, Kuniyuki Iwashima,
netdev, syzbot+e809069bc15f26300526
On 5/30/26 1:59 PM, Kuniyuki Iwashima wrote:
> syzbot reported a weird reqsk->rsk_refcnt underflow in
> __inet_csk_reqsk_queue_drop().
>
> The captured reqsk_put() in __inet_csk_reqsk_queue_drop()
> is called only when it successfully removes reqsk from ehash.
>
> Moreover, reqsk_timer_handler() calls another reqsk_put()
> after that.
>
> This indicates that the reqsk was missing both refcnts for
> ehash and the timer itself.
>
> Since all the syzbot reports had PREEMPT_RT enabled, the only
> possible scenario is that reqsk_queue_hash_req() is preempted
> after mod_timer() and before refcount_set(), and then the timer
> triggered after 1s aborts the reqsk due to its listener's close().
>
> Let's wrap mod_timer() and refcount_set() with
> preempt_disable_nested() and preempt_enable_nested().
>
> Note that inet_ehash_insert() holds the normal spin_lock()
> (mutex in PREEMPT_RT), so it must be called outside of
> preempt_disable_nested(), but this is fine.
>
> The lookup path just ignores 0 sk_refcnt entries in ehash
> and tries to create another reqsk, but this will fail at
> inet_ehash_insert().
>
> [0]:
> refcount_t: underflow; use-after-free.
> WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16
> Modules linked in:
> CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
> Tainted: [L]=SOFTLOCKUP
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
> RIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28
> Code: e4 7d d1 0a 67 48 0f b9 3a eb 4a e8 38 3d 23 fd 48 8d 3d e1 7d d1 0a 67 48 0f b9 3a eb 37 e8 25 3d 23 fd 48 8d 3d de 7d d1 0a <67> 48 0f b9 3a eb 24 e8 12 3d 23 fd 48 8d 3d db 7d d1 0a 67 48 0f
> RSP: 0000:ffffc90000157948 EFLAGS: 00010246
> RAX: ffffffff84a1301b RBX: 0000000000000003 RCX: ffff88801ca98000
> RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffffff8f72ae00
> RBP: ffffffff99ae3b01 R08: ffff88801ca98000 R09: 0000000000000005
> R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880425ef568
> R13: ffff8880425ef4f8 R14: ffff8880425ef578 R15: 0000000000000000
> FS: 0000000000000000(0000) GS:ffff888126386000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7b46710e9c CR3: 000000000dbb6000 CR4: 00000000003526f0
> Call Trace:
> <TASK>
> __refcount_sub_and_test include/linux/refcount.h:400 [inline]
> __refcount_dec_and_test include/linux/refcount.h:432 [inline]
> refcount_dec_and_test include/linux/refcount.h:450 [inline]
> reqsk_put include/net/request_sock.h:136 [inline]
> __inet_csk_reqsk_queue_drop+0x3ce/0x440 net/ipv4/inet_connection_sock.c:1007
> reqsk_timer_handler+0x651/0xdf0 net/ipv4/inet_connection_sock.c:1137
> call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748
> expire_timers kernel/time/timer.c:1799 [inline]
> __run_timers kernel/time/timer.c:2374 [inline]
> __run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386
> run_timer_base kernel/time/timer.c:2395 [inline]
> run_timer_softirq+0x67/0x170 kernel/time/timer.c:2403
> handle_softirqs+0x1de/0x6d0 kernel/softirq.c:622
> __do_softirq kernel/softirq.c:656 [inline]
> run_ktimerd+0x69/0x100 kernel/softirq.c:1151
> smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
> kthread+0x388/0x470 kernel/kthread.c:436
> ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
>
> Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
> Reported-by: syzbot+e809069bc15f26300526@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6a1a7bcf.0a9e871e.332604.000b.GAE@google.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
> net/ipv4/inet_connection_sock.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index dbcd37dfdc15..b546ce15ee03 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -1145,6 +1145,8 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
> if (!inet_ehash_insert(req_to_sk(req), NULL, &found_dup_sk))
> return false;
>
> + preempt_disable_nested();
> +
> /* The timer needs to be setup after a successful insertion. */
> req->timeout = tcp_timeout_init((struct sock *)req);
> timer_setup(&req->rsk_timer, reqsk_timer_handler, TIMER_PINNED);
> @@ -1155,6 +1157,9 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
> */
> smp_wmb();
> refcount_set(&req->rsk_refcnt, 2 + 1);
> +
> + preempt_enable_nested();
> +
> return true;
> }
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2026-05-30 5:59 [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req() Kuniyuki Iwashima
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