erofs: handle corrupted images whose decompressed size less than it'd be

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>
Fixes: 7fc45dbc938a ("staging: erofs: introduce generic decompression backend")
[ Gao Xiang: v5.3+, I will manually backport this to stable later. ]
Link: https://lore.kernel.org/r/20200226081008.86348-3-gaoxiang25@huawei.com
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index cd978af6bdb9c98f0f1ffee3fbec871022addd9f..5d2d819406794a4e55708caa956095accdc88c67 100644 (file)
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -166,14 +166,18 @@ 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;
        }