Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

From: Matthew Wilcox <willy@infradead.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Casey Schaufler <casey@schaufler-ca.com>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	linux-security-module@vger.kernel.org,
	Paul Moore <paul@paul-moore.com>,
	Eric Paris <eparis@parisplace.org>,
	selinux@vger.kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU
Date: Sun, 13 Dec 2020 16:29:41 +0000	[thread overview]
Message-ID: <20201213162941.GG2443@casper.infradead.org> (raw)
In-Reply-To: <877dpln5uf.fsf@x220.int.ebiederm.org>

On Sun, Dec 13, 2020 at 08:22:32AM -0600, Eric W. Biederman wrote:
> Matthew Wilcox <willy@infradead.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.