linux-ext4 - Re: [PATCH] ext4: Verify fast symlink length

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Message-ID: <20250206152419.GB1130956@mit.edu>
Date: Thu, 6 Feb 2025 10:24:19 -0500
From: "Theodore Ts'o" <tytso@....edu>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org,
        syzbot+48a99e426f29859818c0@...kaller.appspotmail.com,
        "Darrick J. Wong" <djwong@...nel.org>
Subject: Re: [PATCH] ext4: Verify fast symlink length

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@...kaller.appspotmail.com
> Tested-by: syzbot+48a99e426f29859818c0@...kaller.appspotmail.com
> Fixes: bae80473f7b0 ("ext4: use inode_set_cached_link()")
> Suggested-by: "Darrick J. Wong" <djwong@...nel.org>
> Signed-off-by: Jan Kara <jack@...e.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);


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