[PATCH 4.14 179/205] fuse: handle zero sized retrieve correctly

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

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

From: Miklos Szeredi <mszeredi@xxxxxxxxxx>

commit 97e1532ef81acb31c30f9e75bf00306c33a77812 upstream.

Dereferencing req->page_descs[0] will Oops if req->max_pages is zero.

Reported-by: syzbot+c1e36d30ee3416289cc0@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+c1e36d30ee3416289cc0@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: b2430d7567a3 ("fuse: add per-page descriptor <offset, length> to fuse_req")
Cc: <stable@xxxxxxxxxxxxxxx> # v3.9
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/fuse/dev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1691,7 +1691,6 @@ static int fuse_retrieve(struct fuse_con
 	req->in.h.nodeid = outarg->nodeid;
 	req->in.numargs = 2;
 	req->in.argpages = 1;
-	req->page_descs[0].offset = offset;
 	req->end = fuse_retrieve_end;
 
 	index = outarg->offset >> PAGE_SHIFT;
@@ -1706,6 +1705,7 @@ static int fuse_retrieve(struct fuse_con
 
 		this_num = min_t(unsigned, num, PAGE_SIZE - offset);
 		req->pages[req->num_pages] = page;
+		req->page_descs[req->num_pages].offset = offset;
 		req->page_descs[req->num_pages].length = this_num;
 		req->num_pages++;