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Tue, 10 Nov 2026 13:57:41 -0800 (PST) Received: from fedora ([187.120.157.132]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8b3933d365fsm29877687b3.20.2026.11.10.13.57.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Nov 2026 13:57:41 -0800 (PST) From: Guilherme Giacomo Simoes To: josef@toxicpanda.com, axboe@kernel.dk Cc: linux-block@vger.kernel.org, nbd@other.debian.org, linux-kernel@vger.kernel.org, Guilherme Giacomo Simoes , syzbot+d7d72f3d467710442af0@syzkaller.appspotmail.com Subject: [PATCH] nbd: fix circular lock dependency Date: Tue, 10 Nov 2026 18:57:23 -0300 Message-ID: <20261110215723.199948-1-trintaeoitogc@gmail.com> X-Mailer: git-send-email 2.52.0 X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit nbd_genl_reconnect_sock_cb() replaces a dead socket with a new one obtained through nbd_get_socket(), but unlike nbd_add_socket() it never runs the socket through nbd_reclassify_socket(). The replacement socket therefore keeps the generic sk_lock-AF_INET/AF_INET6/AF_UNIX lock class instead of the NBD-specific class leading to a possible lock dependency. Move the reclassification into nbd_get_socket(), the single place where sockets are acquired for NBD, so both the connect and reconnect paths reclassify the socket. nbd_add_socket() no longer needs to do it. Reported-by: syzbot+d7d72f3d467710442af0@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?extid=d7d72f3d467710442af0 Fixes: d532cddb6c60 ("nbd: Reclassify sockets to avoid lockdep circular dependency") Signed-off-by: Guilherme Giacomo Simoes --- drivers/block/nbd.c | 66 +++++++++++++++++++++++++-------------------- 1 file changed, 37 insertions(+), 29 deletions(-) diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index ffce519bf008..bc74ddb1d9d1 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c @@ -1210,34 +1210,6 @@ static blk_status_t nbd_queue_rq(struct blk_mq_hw_ctx *hctx, return ret; } -static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd, - int *err) -{ - struct socket *sock; - - *err = 0; - sock = sockfd_lookup(fd, err); - if (!sock) - return NULL; - - if (!sk_is_tcp(sock->sk) && - !sk_is_stream_unix(sock->sk)) { - dev_err(disk_to_dev(nbd->disk), "Unsupported socket: should be TCP or UNIX.\n"); - *err = -EINVAL; - sockfd_put(sock); - return NULL; - } - - if (sock->ops->shutdown == sock_no_shutdown) { - dev_err(disk_to_dev(nbd->disk), "Unsupported socket: shutdown callout must be supported.\n"); - *err = -EINVAL; - sockfd_put(sock); - return NULL; - } - - return sock; -} - #ifdef CONFIG_DEBUG_LOCK_ALLOC static struct lock_class_key nbd_key[3]; static struct lock_class_key nbd_slock_key[3]; @@ -1274,6 +1246,43 @@ static void nbd_reclassify_socket(struct socket *sock) static inline void nbd_reclassify_socket(struct socket *sock) {} #endif +static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd, + int *err) +{ + struct socket *sock; + + *err = 0; + sock = sockfd_lookup(fd, err); + if (!sock) + return NULL; + + if (!sk_is_tcp(sock->sk) && + !sk_is_stream_unix(sock->sk)) { + dev_err(disk_to_dev(nbd->disk), "Unsupported socket: should be TCP or UNIX.\n"); + *err = -EINVAL; + sockfd_put(sock); + return NULL; + } + + if (sock->ops->shutdown == sock_no_shutdown) { + dev_err(disk_to_dev(nbd->disk), "Unsupported socket: shutdown callout must be supported.\n"); + *err = -EINVAL; + sockfd_put(sock); + return NULL; + } + + /* + * NBD sockets are used from block/swap/writeback context, which can + * already hold locks that fs_reclaim depends on. Give them their own + * lockdep class so that a page fault on an unrelated userspace socket + * (sk_lock -> mmap_lock) is not chained to the NBD socket lock. + */ + if (sock) + nbd_reclassify_socket(sock); + + return sock; +} + static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg, bool netlink) { @@ -1296,7 +1305,6 @@ static int nbd_add_socket(struct nbd_device *nbd, unsigned long arg, sock = nbd_get_socket(nbd, arg, &err); if (!sock) return err; - nbd_reclassify_socket(sock); if (!netlink && !nbd->task_setup && !test_bit(NBD_RT_BOUND, &config->runtime_flags)) -- 2.52.0