Re: [PATCH] mm/rmap: Add anon_vma lifetime debug check

[David Hildenbrand <david@redhat.com>]

On 25.07.25 13:08, Jann Horn wrote:
> On Thu, Jul 24, 2025 at 11:56 PM David Hildenbrand <david@redhat.com> wrote:
>> On 24.07.25 21:13, Jann Horn wrote:
>>> If an anon page is mapped into userspace, its anon_vma must be alive,
>>> otherwise rmap walks can hit UAF.
>>>
>>> There have been syzkaller reports a few months ago[1][2] of UAF in rmap
>>> walks that seems to indicate that there can be pages with elevated mapcount
>>> whose anon_vma has already been freed, but I think we never figured out
>>> what the cause is; and syzkaller only hit these UAFs when memory pressure
>>> randomly caused reclaim to rmap-walk the affected pages, so it of course
>>> didn't manage to create a reproducer.
>>>
>>> Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous pages to
>>> hopefully catch such issues more reliably.
>>>
>>> Implementation note: I'm checking IS_ENABLED(CONFIG_DEBUG_VM) because,
>>> unlike the checks above, this one would otherwise be hard to write such
>>> that it completely compiles away in non-debug builds by itself, without
>>> looking extremely ugly.
>>>
>>> [1] https://lore.kernel.org/r/67abaeaf.050a0220.110943.0041.GAE@google.com
>>> [2] https://lore.kernel.org/r/67a76f33.050a0220.3d72c.0028.GAE@google.com
>>>
>>> Signed-off-by: Jann Horn <jannh@google.com>
>>> ---
>>>    include/linux/rmap.h | 13 +++++++++++++
>>>    1 file changed, 13 insertions(+)
>>>
>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>> index c4f4903b1088..ba694c436f59 100644
>>> --- a/include/linux/rmap.h
>>> +++ b/include/linux/rmap.h
>>> @@ -449,6 +449,19 @@ static inline void __folio_rmap_sanity_checks(const struct folio *folio,
>>>        default:
>>>                VM_WARN_ON_ONCE(true);
>>>        }
>>> +
>>> +     /*
>>> +      * Anon folios must have an associated live anon_vma as long as they're
>>> +      * mapped into userspace.
>>> +      * Part of the purpose of the atomic_read() is to make KASAN check that
>>> +      * the anon_vma is still alive.
>>> +      */
>>> +     if (IS_ENABLED(CONFIG_DEBUG_VM) && PageAnonNotKsm(page)) {
>>
>> 1) You probably don't need the CONFIG_DEBUG_VM check: the
>> VM_WARN_ON_FOLIO should make everything get optimized out ... right?
> 
> The PageAnonNotKsm() check is outside the VM_WARN_ON_FOLIO(), and it
> uses page_folio(), which uses _compound_head(), which does
> READ_ONCE(page->compound_head); and READ_ONCE() unfortunately doesn't
> just mean "I want a read without tearing", it also (intentionally)
> prevents the compiler from removing the read when it sees that it's
> not being used for anything.

That makes sense, the volatile read will not get optimized out.

> 
>> 2) We have a folio here, so ... better
>>
>> if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
>>          ...
>> }
> 
> Hrm, okay. It kind of irks me to write it as two checks when really I
> want to ask "is it this one specific type", but yeah, will change it.

Well, ksm is a subtype of anon.

folio_test_anon_not_ksm()

let's rather ... not :)

-- 
Cheers,

David / dhildenb