Hi Joanne,
On 3/14/25 20:13, Joanne Koong wrote:
> There is a race condition leading to a kernel crash from a null
> dereference when attemping to access fc->lock in
> fuse_uring_create_queue(). fc may be NULL in the case where another
> thread is creating the uring in fuse_uring_create() and has set
> fc->ring but has not yet set ring->fc when fuse_uring_create_queue()
> reads ring->fc.
>
> This fix sets fc->ring only after ring->fc has been set, which
> guarantees now that ring->fc is a proper pointer when any queues are
> created.
>
> Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> Fixes: 24fe962c86f5 ("fuse: {io-uring} Handle SQEs - register commands")
> ---
> fs/fuse/dev_uring.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
> index ab8c26042aa8..618a413ef400 100644
> --- a/fs/fuse/dev_uring.c
> +++ b/fs/fuse/dev_uring.c
> @@ -235,9 +235,9 @@ static struct fuse_ring *fuse_uring_create(struct fuse_conn *fc)
>
> init_waitqueue_head(&ring->stop_waitq);
>
> - fc->ring = ring;
> ring->nr_queues = nr_queues;
> ring->fc = fc;
> + fc->ring = ring;
> ring->max_payload_sz = max_payload_size;
> atomic_set(&ring->queue_refs, 0);
>
oh, I didn't get that and even KCSAN didn't complain. But I see that it
would be possible. I'm just a bit scared that the compiler might
re-order things on its own.
What about this?
diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index 9d78c9f29a09..f33a7e6f5ec3 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -241,11 +241,12 @@ static struct fuse_ring *fuse_uring_create(struct fuse_conn *fc)
init_waitqueue_head(&ring->stop_waitq);
- fc->ring = ring;
ring->nr_queues = nr_queues;
ring->fc = fc;
ring->max_payload_sz = max_payload_size;
atomic_set(&ring->queue_refs, 0);
+ /* Ensures initialization is visible before ring pointer */
+ smp_store_release(&fc->ring, ring);
spin_unlock(&fc->lock);
return ring;
Thanks,
Bernd
