linux-kernel - Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP

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Message-ID: <20201213162941.GG2443@casper.infradead.org>
Date:   Sun, 13 Dec 2020 16:29:41 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>, selinux@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

On Sun, Dec 13, 2020 at 08:22:32AM -0600, Eric W. Biederman wrote:
> Matthew Wilcox <willy@...radead.org> writes:
> 
> > On Thu, Dec 03, 2020 at 04:02:12PM -0800, Stephen Brennan wrote:
> >> -void pid_update_inode(struct task_struct *task, struct inode *inode)
> >> +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
> >> +			       unsigned int flags)
> >
> > I'm really nitpicking here, but this function only _updates_ the inode
> > if flags says it should.  So I was thinking something like this
> > (compile tested only).
> >
> > I'd really appreocate feedback from someone like Casey or Stephen on
> > what they need for their security modules.
> 
> Just so we don't have security module questions confusing things
> can we please make this a 2 patch series?  With the first
> patch removing security_task_to_inode?
> 
> The justification for the removal is that all security_task_to_inode
> appears to care about is the file type bits in inode->i_mode.  Something
> that never changes.  Having this in a separate patch would make that
> logical change easier to verify.

I don't think that's right, which is why I keep asking Stephen & Casey
for their thoughts.  For example,

 * Sets the smack pointer in the inode security blob
 */
static void smack_task_to_inode(struct task_struct *p, struct inode *inode)
{
        struct inode_smack *isp = smack_inode(inode);
        struct smack_known *skp = smk_of_task_struct(p);

        isp->smk_inode = skp;
        isp->smk_flags |= SMK_INODE_INSTANT;
}

That seems to do rather more than checking the file type bits.