Re: [PATCH 1/1] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped

[Vlastimil Babka <vbabka@suse.cz>]

On 7/28/25 19:37, Suren Baghdasaryan wrote:
> On Mon, Jul 28, 2025 at 10:19 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>> > +      */
>> > +     if (unlikely(vma->vm_mm != mm)) {
>> > +             /*
>> > +              * __mmdrop() is a heavy operation and we don't need RCU
>> > +              * protection here. Release RCU lock during these operations.
>> > +              */
>> > +             rcu_read_unlock();
>> > +             mmgrab(vma->vm_mm);
>> > +             vma_refcount_put(vma);
>>
>> The vma can go away here.
> 
> No, the vma can't go away here because we are holding vm_refcnt. So,
> the vma and its mm are stable up until vma_refcount_put() drops
> vm_refcnt.

But that's exactly what we're doing here?

>>
>> > +             mmdrop(vma->vm_mm);

And here we reference the vma again?

>> So we need to copy the vma->vm_mm first?
>>
>> > +             rcu_read_lock();
>> > +             return NULL;
>> > +     }
>> > +
>> >       /*
>> >        * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
>> >        * False unlocked result is impossible because we modify and check