io_uring: fail io_uring_register(2) on a dying io_uring instance

If we have multiple threads doing io_uring_register(2) on an io_uring
fd, then we can potentially try and kill the percpu reference while
someone else has already killed it.

Prevent this race by failing io_uring_register(2) if the ref is marked
dying. This is safe since we're inside the io_uring mutex.

Fixes: b19062a56726 ("io_uring: fix possible deadlock between io_uring_{enter,register}")
Reported-by: syzbot <syzbot+10d25e23199614b7721f@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f65f85d..a2f39fa 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2934,6 +2934,14 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 {
        int ret;
 
+       /*
+        * We're inside the ring mutex, if the ref is already dying, then
+        * someone else killed the ctx or is already going through
+        * io_uring_register().
+        */
+       if (percpu_ref_is_dying(&ctx->refs))
+               return -ENXIO;
+
        percpu_ref_kill(&ctx->refs);
 
        /*