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net: kcm: Fix race condition in kcm_unattach()

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syzbot found a race condition when kcm_unattach(psock)
and kcm_release(kcm) are executed at the same time.

kcm_unattach() is missing a check of the flag
kcm->tx_stopped before calling queue_work().

If the kcm has a reserved psock, kcm_unattach() might get executed
between cancel_work_sync() and unreserve_psock() in kcm_release(),
requeuing kcm->tx_work right before kcm gets freed in kcm_done().

Remove kcm->tx_stopped and replace it by the less
error-prone disable_work_sync().

Fixes: ab7ac4eb98 ("kcm: Kernel Connection Multiplexor module")
Reported-by: syzbot+e62c9db591c30e174662@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e62c9db591c30e174662
Reported-by: syzbot+d199b52665b6c3069b94@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d199b52665b6c3069b94
Reported-by: syzbot+be6b1fdfeae512726b4e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=be6b1fdfeae512726b4e
Signed-off-by: Sven Stegemann <sven@stegemann.de>
Link: https://patch.msgid.link/20250812191810.27777-1-sven@stegemann.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Sven Stegemann 2025-08-12 21:18:03 +02:00 committed by Jakub Kicinski
parent 1c756093cd
commit 52565a9352
2 changed files with 2 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/net/kcm.h
View File

@ -71,7 +71,6 @@ struct kcm_sock {
struct list_head wait_psock_list;
struct sk_buff *seq_skb;
struct mutex tx_mutex;
u32 tx_stopped : 1;
/* Don't use bit fields here, these are set under different locks */
bool tx_wait;

10
net/kcm/kcmsock.c
View File

@ -430,7 +430,7 @@ static void psock_write_space(struct sock *sk)
/* Check if the socket is reserved so someone is waiting for sending. */
kcm = psock->tx_kcm;
if (kcm && !unlikely(kcm->tx_stopped))
if (kcm)
queue_work(kcm_wq, &kcm->tx_work);
spin_unlock_bh(&mux->lock);
@ -1693,12 +1693,6 @@ static int kcm_release(struct socket *sock)
*/
__skb_queue_purge(&sk->sk_write_queue);
/* Set tx_stopped. This is checked when psock is bound to a kcm and we
* get a writespace callback. This prevents further work being queued
* from the callback (unbinding the psock occurs after canceling work.
*/
kcm->tx_stopped = 1;
release_sock(sk);
spin_lock_bh(&mux->lock);
@ -1714,7 +1708,7 @@ static int kcm_release(struct socket *sock)
/* Cancel work. After this point there should be no outside references
* to the kcm socket.
*/
cancel_work_sync(&kcm->tx_work);
disable_work_sync(&kcm->tx_work);
lock_sock(sk);
psock = kcm->tx_psock;