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

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@quobyte.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: bcba24ccdc82 ("fuse: enable asynchronous processing direct IO")
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index e2a2c14a90ee7a0a1758d279f39ccb15f92fc915..a6442041ab64db202b18cf45bd31cdf904ac1924 100644 (file)
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2876,6 +2876,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,
        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 *iocb, struct iov_iter *iter,
         * 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 *iocb, struct iov_iter *iter,
                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);