Cramfs: fix abad comparison when wrap-arounds occur

BugLink: https://bugs.launchpad.net/bugs/1810947

commit 672ca9dd 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@gmail.com>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/cramfs/inode.c

@@ -185,7 +185,8 @@ static void *cramfs_read(struct super_block *sb, unsigned int offset, unsigned i
 			continue;
 		blk_offset = (blocknr - buffer_blocknr[i]) << PAGE_CACHE_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;
 	}