[PATCH 4.18 309/350] Cramfs: fix abad comparison when wrap-arounds occur

[Greg Kroah-Hartman]

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

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

From: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>

commit 672ca9dd13f1aca0c17516f76fc5b0e8344b3e46 upstream.

It is possible for corrupted filesystem images to produce very large
block offsets that may wrap when a length is added, and wrongly pass
the buffer size test.

Reported-by: Anatoly Trosinenko <anatoly.trosinenko@xxxxxxxxx>
Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/cramfs/inode.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -202,7 +202,8 @@ static void *cramfs_blkdev_read(struct s
 			continue;
 		blk_offset = (blocknr - buffer_blocknr[i]) << PAGE_SHIFT;
 		blk_offset += offset;
-		if (blk_offset + len > BUFFER_SIZE)
+		if (blk_offset > BUFFER_SIZE ||
+		    blk_offset + len > BUFFER_SIZE)
 			continue;
 		return read_buffers[i] + blk_offset;
 	}