[PATCH 3.16 097/217] fuse: do not use iocb after it may have been freed

[Ben Hutchings]

3.16.35-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Robert Doebbelin <robert@xxxxxxxxxxx>

commit 7cabc61e01a0a8b663bd2b4c982aa53048218734 upstream.

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@xxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Fixes: bcba24ccdc82 ("fuse: enable asynchronous processing direct IO")
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/fuse/file.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2876,6 +2876,7 @@ fuse_direct_IO(int rw, struct kiocb *ioc
 	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;
@@ -2915,7 +2916,7 @@ fuse_direct_IO(int rw, struct kiocb *ioc
 	 * to wait on real async I/O requests, so we must submit this request
 	 * synchronously.
 	 */
-	if (!is_sync_kiocb(iocb) && (offset + count > i_size) && rw == WRITE)
+	if (!is_sync && (offset + count > i_size) && rw == WRITE)
 		io->async = false;
 
 	if (rw == WRITE)
@@ -2927,7 +2928,7 @@ fuse_direct_IO(int rw, struct kiocb *ioc
 		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;
 
 		ret = wait_on_sync_kiocb(iocb);