fuse: do not use iocb after it may have been freed

There's a race in fuse_direct_IO(), whereby is_sync_kiocb() is called on an
iocb that could have been freed if async io has already completed.  The fix
in this case is simple and obvious: cache the result before starting io.

It was discovered by KASan:

kernel: ==================================================================
kernel: BUG: KASan: use after free in fuse_direct_IO+0xb1a/0xcc0 at addr ffff88036c414390

Signed-off-by: Robert Doebbelin <robert@quobyte.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: bcba24ccdc82 ("fuse: enable asynchronous processing direct IO")
Cc: <stable@vger.kernel.org> # 3.10+

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b03d253ece15..34a23df361ca 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2843,6 +2843,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter, loff_t offset)
         loff_t i_size;
         size_t count = iov_iter_count(iter);
         struct fuse_io_priv *io;
+        bool is_sync = is_sync_kiocb(iocb);
 
         pos = offset;
         inode = file->f_mapping->host;
@@ -2882,11 +2883,11 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter, loff_t offset)
          * to wait on real async I/O requests, so we must submit this request
          * synchronously.
          */
-        if (!is_sync_kiocb(iocb) && (offset + count > i_size) &&
+        if (!is_sync && (offset + count > i_size) &&
             iov_iter_rw(iter) == WRITE)
                 io->async = false;
 
-        if (io->async && is_sync_kiocb(iocb))
+        if (io->async && is_sync)
                 io->done = &wait;
 
         if (iov_iter_rw(iter) == WRITE) {
@@ -2900,7 +2901,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter, loff_t offset)
                 fuse_aio_complete(io, ret < 0 ? ret : 0, -1);
 
                 /* we have a non-extending, async request, so return */
-                if (!is_sync_kiocb(iocb))
+                if (!is_sync)
                         return -EIOCBQUEUED;
 
                 wait_for_completion(&wait);