Re: [PATCH V5] ext4: check hash version and filesystem casefolded consistent

[Eric Biggers]

On Tue, Jun 04, 2024 at 03:06:32PM -0400, Gabriel Krisman Bertazi wrote:
> Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx> writes:
> 
> > On Mon, 03 Jun 2024 10:50:51 -0400, Gabriel Krisman Bertazi wrote:
> >> > When mounting the ext4 filesystem, if the hash version and casefolded are not
> >> > consistent, exit the mounting.
> >> >
> >> > Reported-by: syzbot+340581ba9dceb7e06fb3@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> > Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> >> > ---
> >> >  fs/ext4/super.c | 5 +++++
> >> >  1 file changed, 5 insertions(+)
> >> >
> >> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> >> > index c682fb927b64..0ad326504c50 100644
> >> > --- a/fs/ext4/super.c
> >> > +++ b/fs/ext4/super.c
> >> > @@ -5262,6 +5262,11 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
> >> >  		goto failed_mount;
> >> >  
> >> >  	ext4_hash_info_init(sb);
> >> > +	if (es->s_def_hash_version == DX_HASH_SIPHASH && 
> >> > +	    !ext4_has_feature_casefold(sb)) {
> >> 
> >> Can we ever have DX_HASH_SIPHASH set up in the super block?  I thought
> >> it was used solely for directories where ext4_hash_in_dirent(inode) is
> >> true.
> > The value of s'def_hash_version is obtained by reading the super block from the
> > buffer cache of the block device in ext4_load_super().
> 
> Yes, I know.  My point is whether this check should just be:
> 
> if (es->s_def_hash_version == DX_HASH_SIPHASH)
> 	goto failed_mount;
> 
> Since, IIUC, DX_HASH_SIPHASH is done per-directory and not written to
> the sb.
> 

That seems right to me.  SipHash can never be the default because it's only used
on directories that are both encrypted and casefolded.

- Eric