Re: [BUG] hard-to-hit mm_struct UAF due to insufficiently careful vma_refcount_put() wrt SLAB_TYPESAFE_BY_RCU

[Vlastimil Babka]

On 7/24/25 04:30, Suren Baghdasaryan wrote:
> So, I think vma_refcount_put() can mmgrab(vma->mm) before calling
> __refcount_dec_and_test(), to stabilize that mm and then mmdrop()
> after it calls rcuwait_wake_up(). What do you think about this
> approach, folks?

Yeah except it would be wasteful to do for all vma_refcount_put(). Should be
enough to have this version (as Jann suggested) for inval_end_read: part of
lock_vma_under_rcu. I think we need it also for the vma_refcount_put() done
in vma_start_read() when we fail the seqcount check? I think in that case
the same thing can be happening too, just with different race windows?

Also as Jann suggested, maybe it's not great (or even safe) to perform
__mmdrop() under rcu? And maybe some vma_start_read() users are even more
restricted? Maybe then we'd need to make __mmdrop_delayed() not RT-only, and
use that.

>>
>> Vlastimil is right that if A1 was able to successfully elevate vma's
>> refcount then:
>> 1. vma must be attached to some valid mm. This is true because if the
>> vma is detached, vma_start_read() would not be able to elevate its
>> refcount. Once vma_start_read() elevates the refcount, vma will not
>> detach from under us because vma_mark_detached() will block until no
>> readers are using the vma.
>> 2. vma->mm can't be destroyed from under us because of that
>> exit_mmap()->vma_mark_detached() which again will ensure no readers
>> are holding a reference to the vmas of that mm.
>>
>> So, a special version of vma_refcount_put() that takes mm as a
>> parameter and does mmgrab/mmdrop before using that mm might work. I'll
>> do some more digging and maybe test this solution with your reproducer
>> to see if that works as I would expect.
>> Thanks,
>> Suren.