From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta0.migadu.com (out-172.mta0.migadu.com [91.218.175.172]) (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 84D11399036 for ; Sat, 30 May 2026 10:12:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780135926; cv=none; b=NYF3opdshQIf6iAkvZrp+moibRJpcT3IJ9PLNk6zK6hDxvhlCDKF6uAHwQu4mY757DCIr41MsH0fOr+U/55IlxYqIyUecrcoDTOLAm/qq1kQIQoMpNt+HzfooXrzvo4uMlp020RCfhBcB9eLGKslNpuMYoVqGN1+3TF7AjDJVH4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780135926; c=relaxed/simple; bh=ZVpw9b7iySBIlBvNdq3UByr/PyhAr7hc2TVJArzH5MU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hraPxy+LV7tinxd9rFSGlb572ehqkB/BJ6jZRdolGREWDpmZgx7ZQF36YERAGT30AvOVFTYPj24SG741E9U/xFVdetkyxiubUzTLWFAVw7w8VFYTko1WnLv5+VvVzWiViYOu2w5YUmzUq/gz4buT9g8nFNE8qgxaUGLsO5zywpU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=az2nvJFl; arc=none smtp.client-ip=91.218.175.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="az2nvJFl" Message-ID: <2e23fe05-6152-41c1-9a37-afae35bc0d6c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780135921; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=cg3WMgctz68U2WVhip1+prh7ZcPQYzZ+5kUxfiB4kgA=; b=az2nvJFlzkFdwDc4/jcOliG2hgOiTT19Xvy+uusovUgFOyV5ySBnIlva94hvwEgd/oSEKt eSc67/KUNNuKOdWyWc9aT13uciNmDKo/GvGq7F3B5+P55qgscGXv6FpblNyJVRRbEUSEnS Yz8cqeoa21k+20Nbtw3zzWwO+ajFEAk= Date: Sat, 30 May 2026 18:11:51 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 net] tcp: Add preempt_{disable,enable}_nested() in reqsk_queue_hash_req(). To: Kuniyuki Iwashima , Eric Dumazet , Neal Cardwell , "David S. Miller" , Jakub Kicinski , Paolo Abeni Cc: Simon Horman , Sebastian Andrzej Siewior , Kuniyuki Iwashima , netdev@vger.kernel.org, syzbot+e809069bc15f26300526@syzkaller.appspotmail.com References: <20260530055907.280160-1-kuniyu@google.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Jiayuan Chen In-Reply-To: <20260530055907.280160-1-kuniyu@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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: > > __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 > > > 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 Reviewed-by: Jiayuan Chen > --- > 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; > } >