[PATCH v2 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[PATCH v2 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[Kuniyuki Iwashima]

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>
---
v2: Move preempt_disable_nested() just before mod_timer()
    to avoid unnecessary latency as much as possible.
    (tcp_timeout_init() calls SOCK_OPS bpf prog)

v1: https://lore.kernel.org/netdev/20260530055907.280160-1-kuniyu@google.com/
---
 net/ipv4/inet_connection_sock.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index dbcd37dfdc15..5b934ce8d98a 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1148,6 +1148,9 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
 	/* 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);
+
+	preempt_disable_nested();
+
 	mod_timer(&req->rsk_timer, jiffies + req->timeout);
 
 	/* before letting lookups find us, make sure all req fields
@@ -1155,6 +1158,9 @@ static bool reqsk_queue_hash_req(struct request_sock *req)
 	 */
 	smp_wmb();
 	refcount_set(&req->rsk_refcnt, 2 + 1);
+
+	preempt_enable_nested();
+
 	return true;
 }
 
-- 
2.54.0.929.g9b7fa37559-goog

Re: [PATCH v2 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[Eric Dumazet]

On Mon, Jun 1, 2026 at 11:21 AM Kuniyuki Iwashima <kuniyu@google.com> 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().
>
> 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: Eric Dumazet <edumazet@google.com>

Re: [PATCH v2 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[Sebastian Andrzej Siewior]

On 2026-06-01 18:20:55 [+0000], Kuniyuki Iwashima wrote:
> syzbot reported a weird reqsk->rsk_refcnt underflow in
> __inet_csk_reqsk_queue_drop().
…
> 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>
> ---
> v2: Move preempt_disable_nested() just before mod_timer()
>     to avoid unnecessary latency as much as possible.
>     (tcp_timeout_init() calls SOCK_OPS bpf prog)

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

Re: [PATCH v2 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon,  1 Jun 2026 18:20:55 +0000 you 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.
> 
> [...]

Here is the summary with links:
  - [v2,net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().
    https://git.kernel.org/netdev/net/c/e10902df2448

You are awesome, thank you!
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