* Re: [PATCH] kcm: Fix socket use-after-free in kcm_unattach()
2026-09-17 16:36 [PATCH] kcm: Fix socket use-after-free in kcm_unattach() Wentao Liang
@ 2026-09-21 17:24 ` netdev-bot+sashiko
0 siblings, 0 replies; 2+ messages in thread
From: netdev-bot+sashiko @ 2026-09-21 17:24 UTC (permalink / raw)
To: vulab
Cc: davem, edumazet, horms, kuba, linux-kernel, netdev, pabeni, tom, stable
Thank you for your contribution! Sashiko AI review found 2 potential
issue(s) to consider.
Critical: 0 · High: 1 · Medium: 1 · Low: 0 (1)
If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
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- [High] The patch fixes only one of the two sites that release the
psock's socket references, leaving the identical (and strictly worse)…
- [Medium] The commit message's premise is not supported by the code, so
the `Fixes:`/`Cc: stable` tags are unjustified.
Pre-existing issues:
- [Low] Stale/incorrect comment at the call site of the function this
patch modifies: kcm_unattach_ioctl() has '/* Lower socket lock should…
--
kcm: Fix socket use-after-free in kcm_unattach()
This reorders the teardown in the unreserved (no_reserved) branch of
kcm_unattach() so that fput(csk->sk_socket->file) and release_sock(csk)
happen before the psock's sock_put(csk), and returns early from that
branch. It carries a Fixes: tag for the original KCM commit and
Cc: stable.
> diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> index 71af69d442f21..079b73af91559 100644
> --- a/net/kcm/kcmsock.c
> +++ b/net/kcm/kcmsock.c
> @@ -1458,9 +1458,11 @@ static void kcm_unattach(struct kcm_psock *psock)
> mux->psocks_cnt--;
> spin_unlock_bh(&mux->lock);
>
> - sock_put(csk);
> fput(csk->sk_socket->file);
> kmem_cache_free(kcm_psockp, psock);
> + release_sock(csk);
> + sock_put(csk);
> + return;
[Severity: Medium]
The changelog says:
"the reference held by the psock on the underlying TCP socket is
dropped with sock_put() before the socket file is released with
fput(csk->sk_socket->file) and before release_sock() is called at
the end of the function. If that was the last reference on the
socket, both uses access freed memory."
Can that sock_put() actually be the last reference? kcm_attach() only
accepts inet TCP sockets:
if ((csk->sk_family != AF_INET && csk->sk_family != AF_INET6) ||
csk->sk_protocol != IPPROTO_TCP) {
and the psock keeps the socket file reference for its whole lifetime.
kcm_attach_ioctl() returns success without sockfd_put(csock):
/* Keep reference on file also */
return 0;
so while the psock still holds its sock_hold() reference, the struct
socket also still owns the sk_refcnt reference taken in sock_init_data(),
which is only dropped via inet_release() -> tcp_close() -> sock_put().
That makes the refcount at this point >= 2.
On top of that, does fput() ever free the socket synchronously here? For
an FMODE_OPENED file, fput() -> __fput_deferred() always defers:
if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
init_task_work(&file->f_task_work, ____fput);
if (!task_work_add(task, &file->f_task_work, TWA_RESUME))
return;
so the old trailing release_sock(csk) would not have touched freed memory
either. The new ordering looks fine on its own, but could the impact
statement, the Fixes: tag and the Cc: stable be reconsidered if no
use-after-free is reachable?
[Severity: High]
Should the second release site be changed too? unreserve_psock() has the
same pair in its deferred-free block, still in the order this patch calls
unsafe:
if (unlikely(psock->tx_stopped)) {
if (psock->done) {
/* Deferred free */
list_del(&psock->psock_list);
mux->psocks_cnt--;
sock_put(psock->sk);
fput(psock->sk->sk_socket->file);
kmem_cache_free(kcm_psockp, psock);
}
Here psock->sk is dereferenced twice (->sk_socket, ->file) after the
reference has been dropped, and the whole block runs under
spin_lock_bh(&mux->lock), so under the changelog's premise the final
sock_put() would also run sk_free()/sk_destruct() with BH disabled.
This is the continuation of the path this patch touches. When the psock
is reserved, kcm_unattach() does:
kcm_abort_tx_psock(psock, EPIPE, false); /* sets tx_stopped */
...
psock->done = 1;
smp_mb();
queue_work(kcm_wq, &psock->tx_kcm->tx_work);
and the tx work reaches kcm_write_msgs():
if (unlikely(psock && psock->tx_stopped)) {
/* A reserved psock was aborted asynchronously. Unreserve
* it and we'll retry the message.
*/
unreserve_psock(kcm);
which takes the psock->done deferred-free branch above. So for every
psock unattached while reserved (SIOCKCMUNATTACH or release_mux()), the
exact sequence declared unsafe still executes.
Would it make sense to factor the release sequence into one helper used
by both sites, so the intended ordering invariant is unambiguous?
> }
>
> release_sock(csk);
[Severity: Low]
This is a pre-existing issue and not introduced by this patch, but while
the exit paths of kcm_unattach() are being changed, is the comment at the
kcm_unattach_ioctl() call site still accurate?
/* Lower socket lock should already be held */
kcm_unattach(psock);
kcm_unattach() takes the lower socket lock itself on entry:
struct sock *csk = psock->sk;
struct kcm_mux *mux = psock->mux;
lock_sock(csk);
and now releases it on all three exits, including the early return added
here. If the caller really held that lock, lock_sock() would deadlock;
kcm_unattach_ioctl() only holds a sockfd_lookup() reference on csock.
Could that comment be dropped or corrected?
--
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260917163656.2162621-1-vulab%40iscas.ac.cn
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