Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

[Kees Cook <keescook@chromium.org>]

On Tue, Oct 30, 2018 at 8:54 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> On Tue, Oct 30, 2018 at 04:02:54PM +0100, Oleg Nesterov wrote:
>> On 10/29, Tycho Andersen wrote:
>> >
>> > +static long seccomp_notify_recv(struct seccomp_filter *filter,
>> > +                           void __user *buf)
>> > +{
>> > +   struct seccomp_knotif *knotif = NULL, *cur;
>> > +   struct seccomp_notif unotif;
>> > +   ssize_t ret;
>> > +
>> > +   memset(&unotif, 0, sizeof(unotif));
>> > +
>> > +   ret = down_interruptible(&filter->notif->request);
>> > +   if (ret < 0)
>> > +           return ret;
>> > +
>> > +   mutex_lock(&filter->notify_lock);
>> > +   list_for_each_entry(cur, &filter->notif->notifications, list) {
>> > +           if (cur->state == SECCOMP_NOTIFY_INIT) {
>> > +                   knotif = cur;
>> > +                   break;
>> > +           }
>> > +   }
>> > +
>> > +   /*
>> > +    * If we didn't find a notification, it could be that the task was
>> > +    * interrupted by a fatal signal between the time we were woken and
>> > +    * when we were able to acquire the rw lock.
>> > +    *
>> > +    * This is the place where we handle the extra high semaphore count
>> > +    * mentioned in seccomp_do_user_notification().
>> > +    */
>> > +   if (!knotif) {
>> > +           ret = -ENOENT;
>> > +           goto out;
>> > +   }
>> > +
>> > +   unotif.id = knotif->id;
>> > +   unotif.pid = task_pid_vnr(knotif->task);
>> > +   if (knotif->signaled)
>> > +           unotif.flags |= SECCOMP_NOTIF_FLAG_SIGNALED;
>> > +   unotif.data = *(knotif->data);
>>
>> Tycho, I forgot everything about seccomp, most probably I am wrong but let me
>> ask anyway.
>>
>> __seccomp_filter(SECCOMP_RET_TRACE) does
>>
>>               /*
>>                * Recheck the syscall, since it may have changed. This
>>                * intentionally uses a NULL struct seccomp_data to force
>>                * a reload of all registers. This does not goto skip since
>>                * a skip would have already been reported.
>>                */
>>               if (__seccomp_filter(this_syscall, NULL, true))
>>                       return -1;
>>
>> and the next seccomp_run_filters() can return SECCOMP_RET_USER_NOTIF, right?
>> seccomp_do_user_notification() doesn't check recheck_after_trace and it simply
>> does n.data = sd.
>>
>> Doesn't this mean that "unotif.data = *(knotif->data)" can hit NULL ?
>>
>> seccomp_run_filters() does populate_seccomp_data() in this case, but this
>> won't affect "seccomp_data *sd" passed to seccomp_do_user_notification().

Woo, yeah, good catch. :)

> Oof, yes, you're right. Seems like there are no other users of sd in
> __seccomp_filter(). Seems to me like we can just do the
> populate_seccomp_data() one level higher in __seccomp_filter()?

Agreed.

>
> Tycho
>
>
> From 9e0f75ea51a2c328567910df3122a236ebeccab0 Mon Sep 17 00:00:00 2001
> From: Tycho Andersen <tycho@tycho.ws>
> Date: Tue, 30 Oct 2018 09:51:14 -0600
> Subject: [PATCH] seccomp: hoist struct seccomp_data recalculation higher
>
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> ---
>  kernel/seccomp.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 4c5fb6ced4cd..1525cb753ad2 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -257,7 +257,6 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
>  static u32 seccomp_run_filters(const struct seccomp_data *sd,
>                                struct seccomp_filter **match)
>  {
> -       struct seccomp_data sd_local;
>         u32 ret = SECCOMP_RET_ALLOW;
>         /* Make sure cross-thread synced filter points somewhere sane. */
>         struct seccomp_filter *f =
> @@ -267,11 +266,6 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>         if (unlikely(WARN_ON(f == NULL)))
>                 return SECCOMP_RET_KILL_PROCESS;
>
> -       if (!sd) {
> -               populate_seccomp_data(&sd_local);
> -               sd = &sd_local;
> -       }
> -
>         /*
>          * All filters in the list are evaluated and the lowest BPF return
>          * value always takes priority (ignoring the DATA).
> @@ -821,6 +815,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>         u32 filter_ret, action;
>         struct seccomp_filter *match = NULL;
>         int data;
> +       struct seccomp_data sd_local;
>
>         /*
>          * Make sure that any changes to mode from another thread have
> @@ -828,6 +823,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
>          */
>         rmb();
>
> +       if (!sd) {
> +               populate_seccomp_data(&sd_local);
> +               sd = &sd_local;
> +       }
> +
>         filter_ret = seccomp_run_filters(sd, &match);
>         data = filter_ret & SECCOMP_RET_DATA;
>         action = filter_ret & SECCOMP_RET_ACTION_FULL;
> --
> 2.17.1
>

Looks good to me, yes.

-- 
Kees Cook