linux-ext4 - Re: [PATCH v27 03/21] vfs: Add MAY_DELETE_SELF and MAY_DELETE

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Date:   Mon, 13 Feb 2017 16:42:27 +0100
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Dave Chinner <david@...morbit.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        linux-cifs@...r.kernel.org, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v27 03/21] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD
 permission flags

On Fri, Dec 2, 2016 at 10:57 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, Oct 11, 2016 at 2:50 PM, Andreas Gruenbacher
> <agruenba@...hat.com> wrote:
>> Normally, deleting a file requires MAY_WRITE access to the parent
>> directory.  With richacls, a file may be deleted with MAY_DELETE_CHILD access
>> to the parent directory or with MAY_DELETE_SELF access to the file.
>>
>> To support that, pass the MAY_DELETE_CHILD mask flag to inode_permission()
>> when checking for delete access inside a directory, and MAY_DELETE_SELF
>> when checking for delete access to a file itself.
>>
>> The MAY_DELETE_SELF permission overrides the sticky directory check.
>
> And MAY_DELETE_SELF seems totally inappropriate to any kind of rename,
> since from the point of view of the inode we are not doing anything at
> all.  The modifications are all in the parent(s), and that's where the
> permission checks need to be.
>
>> @@ -2780,14 +2780,20 @@ static int may_delete_or_replace(struct inode *dir, struct dentry *victim,
>>         BUG_ON(victim->d_parent->d_inode != dir);
>>         audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
>>
>> -       error = inode_permission(dir, mask);
>> +       error = inode_permission(dir, mask | MAY_WRITE | MAY_DELETE_CHILD);
>> +       if (!error && check_sticky(dir, inode))
>> +               error = -EPERM;
>> +       if (error && IS_RICHACL(inode) &&
>> +           inode_permission(inode, MAY_DELETE_SELF) == 0 &&
>> +           inode_permission(dir, mask) == 0)
>> +               error = 0;
>
> Why is MAY_WRITE missing here?  Everything not aware of
> MAY_DELETE_SELF (e.g. LSMs) will still need MAY_WRITE otherwise this
> is going to be a loophole.

Hmm, this has indeed slipped me. Should be fixed in the version I've
just posted.

Many thanks for the review.

Andreas