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+

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b03d253ece159c4340765e46993127a53354b21b..34a23df361ca2c6211c7661cab8eba481387a332 100644 (file)
--- 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);