Re: [RFC PATCH] mm/mmap: Fix uprobe anon page be overwritten when expanding vma during mremap

[Pu Lehui <pulehui@huaweicloud.com>]

On 2025/5/27 19:42, Lorenzo Stoakes wrote:
> On Mon, May 26, 2025 at 08:46:07PM +0200, David Hildenbrand wrote:
>> On 26.05.25 17:48, Oleg Nesterov wrote:
>>> Hi Lehui,
>>>
>>> As I said, I don't understand mm/, so can't comment, but...
>>>
>>> On 05/26, Pu Lehui wrote:
>>>>
>>>> To make things simpler, perhaps we could try post-processing, that is:
>>>>
>>>> diff --git a/mm/mremap.c b/mm/mremap.c
>>>> index 83e359754961..46a757fd26dc 100644
>>>> --- a/mm/mremap.c
>>>> +++ b/mm/mremap.c
>>>> @@ -240,6 +240,11 @@ static int move_ptes(struct pagetable_move_control
>>>> *pmc,
>>>>                   if (pte_none(ptep_get(old_pte)))
>>>>                           continue;
>>>>
>>>> +               /* skip move pte when expanded range has uprobe */
>>>> +               if (unlikely(pte_present(*new_pte) &&
>>>> +                            vma_has_uprobes(pmc->new, new_addr, new_addr +
>>>> PAGE_SIZE)))
> 
> This feels like a horrible hack, note that we also move page tables at higher
> page table levels _anyway_ so this would be broken by that (unless uprobes split
> huge mappings).

Got it. Won't do this try...

> 
> If it's uprobe code that's putting the new PTE in place, then this is
> just... yeah. I'm with David's suggestion of just disallowing this scenario, I
> really dislike the idea that we're ok with an invalid condition being ok, only
> to cover off this one specific case.
> 
> 
>>>> +                       continue;
>>>> +
>>>
>>> I was thinking about
>>>
>>> 	WARN_ON(!pte_none(*new_pte))
>>>
>>> at the start of the main loop.
>>>
>>> Obviously not to fix the problem, but rather to make it more explicit.
>>
>> Yeah, WARN_ON_ONCE().
>>
>> We really should fix the code to not install uprobes into the area we are
>> moving.
>>
>> Likely, the correct fix will be to pass the range as well to uprobe_mmap(),
>> and passing that range to build_probe_list().
>>
>> Only when growing using mremap(), we want to call it on the extended range
>> only.
> 
> We might be able to implement a simpler version of the proposed patch though
> which might avoid us needing to do something like this.
> 
> Having a look...
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>