On Thu, Feb 06, 2025 at 10:44:55AM +0100, Jan Kara wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: syzbot+48a99e426f29859818c0@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
> Suggested-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> 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);
I don't think this will do the right thing if the fast symlink is
encrypted. See ext4_encrypted_get_link() in fs/ext4/symlink.c in the
kernel sources, and also look at how e2fsck_pass1_check_symlink()
handles checking the size of an encrypted, fast symlink.
Thanks,
- Ted
