Re: [PATCH net-next] net/rds: Fix circular locking dependency in rds_tcp_tune
From: Eric Dumazet <edumazet@google.com>
Date: 2026-02-26 23:25:42
Also in:
linux-kselftest, linux-rdma
On Thu, Feb 26, 2026 at 10:34 PM Allison Henderson [off-list ref] wrote:
syzbot reported a circular locking dependency in rds_tcp_tune() where
sk_net_refcnt_upgrade() is called while holding the socket lock:
======================================================
WARNING: possible circular locking dependency detected
------------------------------------------------------
kworker/u10:8/15040 is trying to acquire lock:
ffffffff8e9aaf80 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc_cache_noprof+0x4b/0x6f0
but task is already holding lock:
ffff88805a3c1ce0 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: rds_tcp_tune+0xd7/0x930
The issue occurs because sk_net_refcnt_upgrade() performs memory allocation
(via get_net_track() -> ref_tracker_alloc()) while the socket lock is held,
creating a circular dependency with fs_reclaim.
Fix this by moving sk_net_refcnt_upgrade() outside the socket lock critical
section. Since the fresh socket is not yet exposed to other threads, no
locks are needed at this time.
Reported-by: syzbot+2e2cf5331207053b8106@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2e2cf5331207053b8106
Fixes: 5c70eb5c593d ("net: better track kernel sockets lifetime")
Are you sure this is the right Fixes: tag ?
Before this patch we had a GFP_KERNEL allocation already ?
This might instead come from
commit 3a58f13a881ed351198ffab4cf9953cf19d2ab3a
net: rds: acquire refcount on TCP sockets
quoted hunk ↗ jump to hunk
Signed-off-by: Allison Henderson <achender@kernel.org> --- net/rds/tcp.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-)diff --git a/net/rds/tcp.c b/net/rds/tcp.c index 04f310255692..da22b3dfdbf0 100644 --- a/net/rds/tcp.c +++ b/net/rds/tcp.c@@ -490,18 +490,24 @@ bool rds_tcp_tune(struct socket *sock) struct rds_tcp_net *rtn; tcp_sock_set_nodelay(sock->sk); - lock_sock(sk); /* TCP timer functions might access net namespace even after * a process which created this net namespace terminated. */ if (!sk->sk_net_refcnt) { - if (!maybe_get_net(net)) { - release_sock(sk); + if (!maybe_get_net(net)) return false; - } + /* + * We call sk_net_refcnt_upgrade before the lock_sock since it is + * not yet shared, no lock is needed at this time. Further, + * because sk_net_refcnt_upgrade does a GFP_KERNEL allocation, + * this can trigger an fs_reclaim in other systems which creates + * a circular lock dependancy. Avoid this by upgrading the + * refcnt before the locking the socket. + */ sk_net_refcnt_upgrade(sk); put_net(net); } + lock_sock(sk); rtn = net_generic(net, rds_tcp_netid); if (rtn->sndbuf_size > 0) { sk->sk_sndbuf = rtn->sndbuf_size; --2.43.0