aio: fix io_setup/io_destroy race

Have ioctx_alloc() return an extra reference, so that caller would drop it
on success and not bother with re-grabbing it on failure exit.  The current
code is obviously broken - io_destroy() from another thread that managed
to guess the address io_setup() would've returned would free ioctx right
under us; gets especially interesting if aio_context_t * we pass to
io_setup() points to PROT_READ mapping, so put_user() fails and we end
up doing io_destroy() on kioctx another thread has just got freed...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 67e4b9047cc9..f6578cb22d00 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -273,7 +273,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
         mm = ctx->mm = current->mm;
         atomic_inc(&mm->mm_count);
 
-        atomic_set(&ctx->users, 1);
+        atomic_set(&ctx->users, 2);
         spin_lock_init(&ctx->ctx_lock);
         spin_lock_init(&ctx->ring_info.ring_lock);
         init_waitqueue_head(&ctx->wait);
@@ -1338,10 +1338,10 @@ SYSCALL_DEFINE2(io_setup, unsigned, nr_events, aio_context_t __user *, ctxp)
         ret = PTR_ERR(ioctx);
         if (!IS_ERR(ioctx)) {
                 ret = put_user(ioctx->user_id, ctxp);
-                if (!ret)
+                if (!ret) {
+                        put_ioctx(ioctx);
                         return 0;
-
-                get_ioctx(ioctx); /* io_destroy() expects us to hold a ref */
+                }
                 io_destroy(ioctx);
         }