Re: [PATCH v8] seccomp, ptrace: add support for dumping seccomp filters

[Daniel Borkmann]

On 10/21/2015 10:12 PM, Kees Cook wrote:
> On Wed, Oct 21, 2015 at 12:15 PM, Tycho Andersen
> <tycho.andersen@xxxxxxxxxxxxx> wrote:
> > Hi Oleg,
> >
> > On Wed, Oct 21, 2015 at 08:51:46PM +0200, Oleg Nesterov wrote:
> > > On 10/20, Tycho Andersen wrote:
> > > > Hi Kees, Oleg,
> > > >
> > > > On Tue, Oct 20, 2015 at 10:20:24PM +0200, Oleg Nesterov wrote:
> > > > > No, you can't do copy_to_user() from atomic context. You need to pin this
> > > > > filter, drop the lock/irq, then copy_to_user().
> > > >
> > > > Attached is a patch which addresses this.
> > >
> > > Looks good to me, feel free to add my reviewed-by.
> > >
> > >
> > > a couple of questions, I am just curious...
> > >
> > > > +long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> > > > +                   void __user *data)
> > > > +{
> > > > +   struct seccomp_filter *filter;
> > > > +   struct sock_fprog_kern *fprog;
> > > > +   long ret;
> > > > +   unsigned long count = 0;
> > > > +
> > > > +   if (!capable(CAP_SYS_ADMIN) ||
> > > > +       current->seccomp.mode != SECCOMP_MODE_DISABLED) {
> > > > +           return -EACCES;
> > > > +   }
> > > > +
> > > > +   spin_lock_irq(&task->sighand->siglock);
> > > > +   if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
> > > > +           ret = -EINVAL;
> > > > +           goto out;
> > > > +   }
> > > > +
> > > > +   filter = task->seccomp.filter;
> > > > +   while (filter) {
> > > > +           filter = filter->prev;
> > > > +           count++;
> > > > +   }
> > > > +
> > > > +   if (filter_off >= count) {
> > > > +           ret = -ENOENT;
> > > > +           goto out;
> > > > +   }
> > > > +   count -= filter_off;
> > > > +
> > > > +   filter = task->seccomp.filter;
> > > > +   while (filter && count > 1) {
> > > > +           filter = filter->prev;
> > > > +           count--;
> > > > +   }
> > > > +
> > > > +   if (WARN_ON(count != 1)) {
> > > > +           /* The filter tree shouldn't shrink while we're using it. */
> > > > +           ret = -ENOENT;
> > >
> > > Yes. but this looks a bit confusing. If we want this WARN_ON() check
> > > because we are paranoid, then we should do
> > >
> > >        WARN_ON(count != 1 || filter);
> >
> > I guess you mean !filter here? We want filter to be non-null, because
> > we use it later.
> >
> > > And "while we're using it" look misleading, we rely on ->siglock.
> > >
> > > Plus if we could be shrinked the additional check can't help anyway,
> > > we can used the free filter. So I don't really understand this check
> > > and "filter != NULL" in the previous "while (filter && count > 1)".
> > > Nevermind...
> >
> > Just paranoia. You're right that we could get rid of WARN_ON and the
> > null check. I can send an updated patch to drop these bits if
> > necessary. Kees?
>
> I like being really paranoid when dealing with the filters. Let's keep
> the WARN_ON (with the "|| !filter" added) but maybe wrap it in
> "unlikely"?

Btw, the conditions inside the WARN_ON() macro would already resolve
to unlikely().

Best,
Daniel
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