fuse: handle zero sized retrieve correctly

BugLink: https://bugs.launchpad.net/bugs/1824553
Dereferencing req->page_descs[0] will Oops if req->max_pages is zero.

Reported-by: syzbot+c1e36d30ee3416289cc0@syzkaller.appspotmail.com
Tested-by: syzbot+c1e36d30ee3416289cc0@syzkaller.appspotmail.com
Fixes: b2430d7567a3 ("fuse: add per-page descriptor <offset, length> to fuse_req")
Cc: <stable@vger.kernel.org> # v3.9
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
(cherry picked from commit 97e1532ef81acb31c30f9e75bf00306c33a77812)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index b8721498411858af7b703b9bf0a26e91e3bd8c56..cfae27c7a828b5b080eae0663c20386ecf67afc8 100644 (file)
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1702,7 +1702,6 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
        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;
@@ -1717,6 +1716,7 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
 
                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++;