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

[Matthew Wilcox <willy@infradead.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.