io_uring: don't touch ctx in setup after ring fd install

syzkaller reported an issue where it looks like a malicious app can
trigger a use-after-free of reading the ctx ->sq_array and ->rings
value right after having installed the ring fd in the process file
table.

Defer ring fd installation until after we're done reading those
values.

Fixes: 75b28affdd6a ("io_uring: allocate the two rings together")
Reported-by: syzbot+6f03d895a6cd0d06187f@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ba1431046c98..c11c4157a4c2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3829,10 +3829,6 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
         if (ret)
                 goto err;
 
-        ret = io_uring_get_fd(ctx);
-        if (ret < 0)
-                goto err;
-
         memset(&p->sq_off, 0, sizeof(p->sq_off));
         p->sq_off.head = offsetof(struct io_rings, sq.head);
         p->sq_off.tail = offsetof(struct io_rings, sq.tail);
@@ -3850,6 +3846,14 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
         p->cq_off.overflow = offsetof(struct io_rings, cq_overflow);
         p->cq_off.cqes = offsetof(struct io_rings, cqes);
 
+        /*
+         * Install ring fd as the very last thing, so we don't risk someone
+         * having closed it before we finish setup
+         */
+        ret = io_uring_get_fd(ctx);
+        if (ret < 0)
+                goto err;
+
         p->features = IORING_FEAT_SINGLE_MMAP;
         return ret;
 err: