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

[David Hildenbrand <david@redhat.com>]

On 27.05.25 15:38, Pu Lehui wrote:
> Hi David,
> 
> On 2025/5/27 2:46, 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)))
>>>> +                       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.
> Alright, so let's try this direction.
> 
>>
>> Likely, the correct fix will be to pass the range as well to
>> uprobe_mmap(), and passing that range to build_probe_list().
> 
> It will be great. But IIUC, the range we expand to is already included
> when entering uprobe_mmap and also build_probe_list.

Right, you'd have to communicate that information through all layers 
(expanded range).

As an alternative, maybe we can really call handle_vma_uprobe() after 
moving the pages.

uprobe_write_opcode() should detect that the uprobe is already installed 
(verify_opcode() will return 0) and just return.

-- 
Cheers,

David / dhildenb