[PATCH] fs: romfs: Fix potential uninitialized memory leak

[Fullway Wang]

romfs_dev_read() fetches a caller-supplied number of bytes from the
backing device in an all-or-nothing manner.

Commit bcf85fc fixed CVE-2020-29371, which exposed the bug when
the requested length crossed the filesystem size limit, the number
of bytes to requested was truncated. However, in romfs_readdir()
in super.c, the requested number is firstly truncated with
romfs_dev_strnlen() then passed to romfs_dev_read(), leaving
the bug exploitable still.

Fix this by returning an error code instead of changing the length
in romfs_dev_strnlen() when the read might go beyond the end of
the filesystem.

Signed-off-by: Fullway Wang <fullwaywang@xxxxxxxxxxx>
---
 fs/romfs/storage.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/romfs/storage.c b/fs/romfs/storage.c
index b57b3ffcbc32..e0e4ecca4e65 100644
--- a/fs/romfs/storage.c
+++ b/fs/romfs/storage.c
@@ -240,10 +240,8 @@ ssize_t romfs_dev_strnlen(struct super_block *sb,
 	size_t limit;
 
 	limit = romfs_maxsize(sb);
-	if (pos >= limit)
+	if (pos >= limit || buflen > limit - pos)
 		return -EIO;
-	if (maxlen > limit - pos)
-		maxlen = limit - pos;
 
 #ifdef CONFIG_ROMFS_ON_MTD
 	if (sb->s_mtd)
-- 
2.39.3 (Apple Git-145)