[PATCH] nbd: don't warn when reclassifying a busy socket lock

[PATCH] nbd: don't warn when reclassifying a busy socket lock

[Deepanshu Kartikey]

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 <kartikey406@gmail.com>
---
 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

Re: [PATCH] nbd: don't warn when reclassifying a busy socket lock

[Hillf Danton]

On Mon, 22 Jun 2026 05:22:55 +0530 Deepanshu Kartikey wrote:
> 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.
> 
Given the syzbot report, if you are right (I suspect) then Eric delivered
another half-baked croissant, and feel free to cut it off instead to make
room for correct fix.

> 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 <kartikey406@gmail.com>
> ---
>  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

Re: [PATCH] nbd: don't warn when reclassifying a busy socket lock

[Eric Dumazet]

On Sun, Jun 21, 2026 at 6:43 PM Hillf Danton <hdanton@sina.com> wrote:
>
> On Mon, 22 Jun 2026 05:22:55 +0530 Deepanshu Kartikey wrote:
> > 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.
> >
> Given the syzbot report, if you are right (I suspect) then Eric delivered
> another half-baked croissant, and feel free to cut it off instead to make
> room for correct fix.

Nobody (including you) caught this.difference between nbd and other
sock_allow_reclassification()
callers.

What was the "correct fix" you envisioned exactly?