linux-kernel - Re: [PATCH] ext4: add casefolded file check

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Date: Fri, 31 May 2024 09:47:47 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <ebiggers@...nel.org>
CC: <coreteam@...filter.org>, <davem@...emloft.net>, <fw@...len.de>,
        <jaegeuk@...nel.org>, <kadlec@...filter.org>, <kuba@...nel.org>,
        <linux-fscrypt@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <lizhi.xu@...driver.com>, <netdev@...r.kernel.org>,
        <netfilter-devel@...r.kernel.org>, <pablo@...filter.org>,
        <syzbot+340581ba9dceb7e06fb3@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>, <tytso@....edu>
Subject: Re: [PATCH] ext4: add casefolded file check

On Thu, 30 May 2024 18:05:13 -0700, Eric Biggers wrote:
> > The file name that needs to calculate the siphash must have both flags casefolded
> > and dir at the same time, so before calculating it, confirm that the flag meets
> > the conditions.
> >
> > Reported-by: syzbot+340581ba9dceb7e06fb3@...kaller.appspotmail.com
> > Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> > ---
> >  fs/ext4/hash.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/fs/ext4/hash.c b/fs/ext4/hash.c
> > index deabe29da7fb..c8840cfc01dd 100644
> > --- a/fs/ext4/hash.c
> > +++ b/fs/ext4/hash.c
> > @@ -265,6 +265,10 @@ static int __ext4fs_dirhash(const struct inode *dir, const char *name, int len,
> >  		__u64	combined_hash;
> >
> >  		if (fscrypt_has_encryption_key(dir)) {
> > +			if (!IS_CASEFOLDED(dir)) {
> > +				ext4_warning_inode(dir, "Siphash requires Casefolded file");
> > +				return -2;
> > +			}
> >  			combined_hash = fscrypt_fname_siphash(dir, &qname);
> >  		} else {
> >  			ext4_warning_inode(dir, "Siphash requires key");
> 
> First, this needs to be sent to the ext4 mailing list (and not to irrelevant
> mailing lists such as netdev).  Please use ./scripts/get_maintainer.pl, as is
> recommended by Documentation/process/submitting-patches.rst.
> 
> Second, ext4 already checks for the directory being casefolded before allowing
> siphash.  This is done by dx_probe().  Evidently syzbot found some way around
> that, so what needs to be done is figure out why that happened and what is the
> best fix to prevent it.  This is not necessarily the patch you've proposed, as
> the real issue might actually be a missing check at some earlier time like when
> reading the inode from disk or when mounting the filesystem.
I have confirmed that there is no casefolded feature when creating the directory.
I agree with your statement that it should be checked for casefold features when
mounting or reading from disk.

Lizhi