Re: [PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/24/26 09:39, Mike Rapoport wrote:
> On Mon, Mar 23, 2026 at 09:20:18PM +0100, David Hildenbrand (Arm) wrote:
>> follow_pfnmap_start() suffers from two problems:
>>
>> (1) We are not re-fetching the pmd/pud after taking the PTL
>>
>> Therefore, we are not properly stabilizing what the lock lock actually
> 
>                                                     ^ lock lock

Thanks, Andrew can you fix that up? Thanks!

> 
>> protects. If there is concurrent zapping, we would indicate to the
>> caller that we found an entry, however, that entry might already have
>> been invalidated, or contain a different PFN after taking the lock.
>>
>> Properly use pmdp_get() / pudp_get() after taking the lock.
>>
>> (2) pmd_leaf() / pud_leaf() are not well defined on non-present entries
>>
>> pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries.
>>
>> There is no real guarantee that pmd_leaf()/pud_leaf() returns something
>> reasonable on non-present entries. Most architectures indeed either
>> perform a present check or make it work by smart use of flags.
>>
>> However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(),
>> and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby
>> pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not
>> do that.
>>
>> Let's check pmd_present()/pud_present() before assuming "the is a
>> present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page
>> table handling code that traverses user page tables does.
>>
>> Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP,
>> (1) is likely more relevant than (2). It is questionable how often (1)
>> would actually trigger, but let's CC stable to be sure.
>>
>> This was found by code inspection.
>>
>> Fixes: 6da8e9634bb7 ("mm: new follow_pfnmap API")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> 
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
>> ---
>> Gave it a quick test in a VM with MM selftests etc, but I am not sure if
>> I actually trigger the follow_pfnmap machinery.
> 
> Most probably not :)
> KVM selftests might, didn't really dig into that. But I doubt any selftest
> would trigger potential races there.

I was wondering whether generic_access_phys() would somehow trigger on
an ordinary system when doing some more involved activities in user space :)

-- 
Cheers,

David