Re: [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req().

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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;
>   }
>