[PATCH 5.7 115/163] ovl: fix out of bounds access warning in ovl_check_fb_len()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Amir Goldstein <amir73il@gmail.com>

commit 522f6e6cba6880a038e2bd88e10390b84cd3febd upstream.

syzbot reported out of bounds memory access from open_by_handle_at()
with a crafted file handle that looks like this:

  { .handle_bytes = 2, .handle_type = OVL_FILEID_V1 }

handle_bytes gets rounded down to 0 and we end up calling:
  ovl_check_fh_len(fh, 0) => ovl_check_fb_len(fh + 3, -3)

But fh buffer is only 2 bytes long, so accessing struct ovl_fb at
fh + 3 is illegal.

Fixes: cbe7fba8edfc ("ovl: make sure that real fid is 32bit aligned in memory")
Reported-and-tested-by: syzbot+61958888b1c60361a791@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org> # v5.5
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/overlayfs/overlayfs.h |    3 +++
 1 file changed, 3 insertions(+)

--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -355,6 +355,9 @@ int ovl_check_fb_len(struct ovl_fb *fb,
 
 static inline int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
 {
+	if (fh_len < sizeof(struct ovl_fh))
+		return -EINVAL;
+
 	return ovl_check_fb_len(&fh->fb, fh_len - OVL_FH_WIRE_OFFSET);
 }