[PATCH] ext4: Verify fast symlink length

[Jan Kara <jack@suse.cz>]

Verify fast symlink length stored in inode->i_size matches the string
stored in the inode to avoid surprises from corrupted filesystems.

Reported-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
Tested-by: syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com
Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/inode.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7c54ae5fcbd4..64e280fed911 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5007,8 +5007,16 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 			inode->i_op = &ext4_encrypted_symlink_inode_operations;
 		} else if (ext4_inode_is_fast_symlink(inode)) {
 			inode->i_op = &ext4_fast_symlink_inode_operations;
-			nd_terminate_link(ei->i_data, inode->i_size,
-				sizeof(ei->i_data) - 1);
+			if (inode->i_size == 0 ||
+			    inode->i_size >= sizeof(ei->i_data) ||
+			    strnlen((char *)ei->i_data, inode->i_size + 1) !=
+								inode->i_size) {
+				ext4_error_inode(inode, function, line, 0,
+					"invalid fast symlink length %llu",
+					 (unsigned long long)inode->i_size);
+				ret = -EFSCORRUPTED;
+				goto bad_inode;
+			}
 			inode_set_cached_link(inode, (char *)ei->i_data,
 					      inode->i_size);
 		} else {
-- 
2.43.0