Re: [RFC PATCH v3 16/16] ceph: create symlinks with encrypted and base64-encoded targets

From: Jeff Layton <jlayton@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: ceph-devel@vger.kernel.org, linux-fscrypt@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH v3 16/16] ceph: create symlinks with encrypted and base64-encoded targets
Date: Wed, 16 Sep 2020 08:15:50 -0400	[thread overview]
Message-ID: <db27ff786e38effda896df738f58e1755a573a3a.camel@kernel.org> (raw)
In-Reply-To: <20200915204953.GB3999121@gmail.com>

On Tue, 2020-09-15 at 13:49 -0700, Eric Biggers wrote:
> On Tue, Sep 15, 2020 at 10:05:53AM -0400, Jeff Layton wrote:
> > > > +static const char *ceph_encrypted_get_link(struct dentry *dentry, struct inode *inode,
> > > > +					   struct delayed_call *done)
> > > > +{
> > > > +	struct ceph_inode_info *ci = ceph_inode(inode);
> > > > +
> > > > +	if (!dentry)
> > > > +		return ERR_PTR(-ECHILD);
> > > > +
> > > > +	return fscrypt_get_symlink(inode, ci->i_symlink, ksize(ci->i_symlink), done);
> > > 
> > > Using ksize() seems wrong here, since that would allow fscrypt_get_symlink() to
> > > read beyond the part of the buffer that is actually initialized.
> > > 
> > 
> > Is that actually a problem? I did have an earlier patch that carried
> > around the length, but it didn't seem to be necessary.
> > 
> > ISTM that that might end up decrypting more data than is actually
> > needed, but eventually there will be a NULL terminator in the data and
> > the rest would be ignored.
> > 
> 
> Yes it's a problem.  The code that decrypts the symlink adds the null terminator
> at the end.  So if the stated buffer size is wrong, then decrypted uninitialized
> memory can be included into the symlink target that userspace then sees.
> 
> > If it is a problem, then we should probably change the comment header
> > over fscrypt_get_symlink. It currently says:
> > 
> >    * @max_size: size of @caddr buffer
> > 
> > ...which is another reason why I figured using ksize there was OK.
> 
> ksize() is rarely used, as it should be.  (For one, it disables KASAN on the
> buffer...)  I think that when people see "buffer size" they almost always think
> the actual allocated size of the buffer, not ksize().  But we could change it to
> say "allocated size" if that would make it clearer...
> 

Ok, I'll rework it to carry around the length too. That should take care of the problem.

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>