Re: [PATCH] ipc,sem: fix use after free on IPC_RMID after a task using same semaphore set exits

[Manfred Spraul]

Hi Herton,

On 08/10/2015 05:31 PM, Herton R. Krzesinski wrote:
> Well without the synchronize_rcu() and with the semid list loop fix I was still
> able to get issues, and I thought the problem is related to racing with IPC_RMID
> on freeary again. This is one scenario I would imagine:
>
>                 A                                                  B
>
> freeary()
>    list_del(&un->list_id)
>    spin_lock(&un->ulp->lock)
>    un->semid = -1
>    list_del_rcu(&un->list_proc)
>      __list_del_entry(&un->list_proc)
>        __list_del(entry->prev, entry->next)      exit_sem()
>          next->prev = prev;                        ...
>          prev->next = next;                        ...
>          ...                                       un = list_entry_rcu(ulp->list_proc.next...)
>      (&un->list_proc)->prev = LIST_POISON2         if (&un->list_proc == &ulp->list_proc) <true, last un removed by thread A>
>    ...                                             kfree(ulp)
>    spin_unlock(&un->ulp->lock) <---- bug
>
> Now that is a very tight window, but I had problems even when I tried this patch
> first:
>
> (...)
> -               if (&un->list_proc == &ulp->list_proc)
> -                       semid = -1;
> -                else
> -                       semid = un->semid;
> +               if (&un->list_proc == &ulp->list_proc) {
> +                       rcu_read_unlock();
What about:
+ spin_unlock_wait(&ulp->lock);
> +                       break;
> +               }
> +               spin_lock(&ulp->lock);
> +               semid = un->semid;
> +               spin_unlock(&ulp->lock);
>
> +               /* exit_sem raced with IPC_RMID, nothing to do */
>                  if (semid == -1) {
>                          rcu_read_unlock();
> -                       break;
> +                       synchronize_rcu();
> +                       continue;
>                  }
> (...)
>
> So even with the bad/uneeded synchronize_rcu() which I had placed there, I could
> still get issues (however the testing on patch above was on an older kernel than
> latest upstream, from RHEL 6, I can test without synchronize_rcu() on latest
> upstream, however the affected code is the same). That's when I thought of
> scenario above. I was able to get this oops:
Adding sleep() usually help, too. But it is ugly, so let's try to 
understand the race and to fix it.

Best regards,
    Manfred
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