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([2405:201:682f:383f:a10c:b74:f527:1266]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-84564ec335csm5449327b3a.49.2026.06.21.16.52.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 21 Jun 2026 16:53:01 -0700 (PDT) From: Deepanshu Kartikey To: josef@toxicpanda.com, axboe@kernel.dk, edumazet@google.com Cc: linux-block@vger.kernel.org, nbd@other.debian.org, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+6b85d1e39a5b8ed9a954@syzkaller.appspotmail.com Subject: [PATCH] nbd: don't warn when reclassifying a busy socket lock Date: Mon, 22 Jun 2026 05:22:55 +0530 Message-ID: <20260621235255.66015-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit nbd_reclassify_socket() warns via WARN_ON_ONCE() if the socket lock is held at the point of reclassification. That assertion was copied from nvme-tcp, where the socket is created internally by the kernel (sock_create_kern()) and is never visible to user space, so the lock is guaranteed to be free. NBD is different: the socket is looked up from a user-supplied fd in nbd_get_socket(), and user space retains that fd. A concurrent syscall on the same socket (or softirq processing taking bh_lock_sock() on a connected TCP socket) can legitimately hold the lock at the instant NBD reclassifies it. sock_allow_reclassification() then returns false and the WARN_ON_ONCE() fires, which turns into a crash under panic_on_warn. This is reachable by simply racing NBD_CMD_CONNECT against socket activity on the same fd, as reported by syzbot. Hitting a held lock here is expected for an externally owned socket and is not a kernel bug, so skip reclassification silently instead of warning. Reclassification is a lockdep-only annotation, so skipping it in the rare racing case is harmless. Reported-by: syzbot+6b85d1e39a5b8ed9a954@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=6b85d1e39a5b8ed9a954 Fixes: d532cddb6c60 ("nbd: Reclassify sockets to avoid lockdep circular dependency") Signed-off-by: Deepanshu Kartikey --- drivers/block/nbd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index 3a585a0c882a..8f10762e90ef 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c @@ -1246,7 +1246,7 @@ static void nbd_reclassify_socket(struct socket *sock) { struct sock *sk = sock->sk; - if (WARN_ON_ONCE(!sock_allow_reclassification(sk))) + if (!sock_allow_reclassification(sk)) return; switch (sk->sk_family) { -- 2.43.0