[PATCH 3/3] erofs: handle corrupted images whose decompressed size less than it'd be

[Gao Xiang <gaoxiang25@huawei.com>]

As Lasse pointed out, "Looking at fs/erofs/decompress.c,
the return value from LZ4_decompress_safe_partial is only
checked for negative value to catch errors. ... So if
I understood it correctly, if there is bad data whose
uncompressed size is much less than it should be, it can
leave part of the output buffer untouched and expose the
previous data as the file content. "

Let's fix it now.

Cc: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
 fs/erofs/decompressor.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index c77cec4327fa..be8d9adef236 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -165,14 +165,18 @@ static int z_erofs_lz4_decompress(struct z_erofs_decompress_req *rq, u8 *out)
 		ret = LZ4_decompress_safe(src + inputmargin, out,
 					  inlen, rq->outputsize);
 
-	if (ret < 0) {
-		erofs_err(rq->sb, "failed to decompress, in[%u, %u] out[%u]",
-			  inlen, inputmargin, rq->outputsize);
+	if (ret != rq->outputsize) {
+		erofs_err(rq->sb, "failed to decompress %d in[%u, %u] out[%u]",
+			  ret, inlen, inputmargin, rq->outputsize);
+
 		WARN_ON(1);
 		print_hex_dump(KERN_DEBUG, "[ in]: ", DUMP_PREFIX_OFFSET,
 			       16, 1, src + inputmargin, inlen, true);
 		print_hex_dump(KERN_DEBUG, "[out]: ", DUMP_PREFIX_OFFSET,
 			       16, 1, out, rq->outputsize, true);
+
+		if (ret >= 0)
+			memset(out + ret, 0, rq->outputsize - ret);
 		ret = -EIO;
 	}
 
-- 
2.17.1