[PATCH 1/1] io_uring/zcrx: fix user_ref race between scrub and refill paths

[PATCH 1/1] io_uring/zcrx: fix user_ref race between scrub and refill paths

[Pavel Begunkov]

From: Kai Aizen <kai@snailsploit.com>

The io_zcrx_put_niov_uref() function uses a non-atomic
check-then-decrement pattern (atomic_read followed by separate
atomic_dec) to manipulate user_refs. This is serialized against other
callers by rq_lock, but io_zcrx_scrub() modifies the same counter with
atomic_xchg() WITHOUT holding rq_lock.

On SMP systems, the following race exists:

  CPU0 (refill, holds rq_lock)          CPU1 (scrub, no rq_lock)
  put_niov_uref:
    atomic_read(uref) - 1
    // window opens
                                        atomic_xchg(uref, 0) - 1
                                        return_niov_freelist(niov) [PUSH #1]
    // window closes
    atomic_dec(uref) - wraps to -1
    returns true
    return_niov(niov)
    return_niov_freelist(niov)           [PUSH #2: DOUBLE-FREE]

The same niov is pushed to the freelist twice, causing free_count to
exceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds
write (a u32 value) past the kvmalloc'd freelist array into the adjacent
slab object.

Fix this by replacing the non-atomic read-then-dec in
io_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically
tests and decrements user_refs. This makes the operation safe against
concurrent atomic_xchg from scrub without requiring scrub to acquire
rq_lock.

Fixes: 34a3e60821ab ("io_uring/zcrx: implement zerocopy receive pp memory provider")
Cc: stable@vger.kernel.org
Signed-off-by: Kai Aizen <kai@snailsploit.com>
[pavel: removed a warning and a comment]
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/zcrx.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 3d377523ff7e..0c9bf540b12b 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -341,10 +341,14 @@ static inline atomic_t *io_get_user_counter(struct net_iov *niov)
 static bool io_zcrx_put_niov_uref(struct net_iov *niov)
 {
 	atomic_t *uref = io_get_user_counter(niov);
+	int old;
+
+	old = atomic_read(uref);
+	do {
+		if (unlikely(old == 0))
+			return false;
+	} while (!atomic_try_cmpxchg(uref, &old, old - 1));
 
-	if (unlikely(!atomic_read(uref)))
-		return false;
-	atomic_dec(uref);
 	return true;
 }
 
-- 
2.52.0

Re: [PATCH 1/1] io_uring/zcrx: fix user_ref race between scrub and refill paths

[Jens Axboe]

On Wed, 18 Feb 2026 17:36:41 +0000, Pavel Begunkov wrote:
> The io_zcrx_put_niov_uref() function uses a non-atomic
> check-then-decrement pattern (atomic_read followed by separate
> atomic_dec) to manipulate user_refs. This is serialized against other
> callers by rq_lock, but io_zcrx_scrub() modifies the same counter with
> atomic_xchg() WITHOUT holding rq_lock.
> 
> On SMP systems, the following race exists:
> 
> [...]

Applied, thanks!

[1/1] io_uring/zcrx: fix user_ref race between scrub and refill paths
      commit: 003049b1c4fb8aabb93febb7d1e49004f6ad653b

Best regards,
-- 
Jens Axboe