Re: [PATCH 5/5] mm: check permissions and bounds for vm_ops->access

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

On 11/13/26 21:25, Kuehling, Felix wrote:
> 
> On 2026-11-12 17:09, David Hildenbrand (Arm) wrote:
>> GUP rejects VM_IO and VM_PFNMAP mappings early. __access_remote_vm() then
>> falls back to vm_ops->access without checking VMA permissions and without
>> limiting the request to the VMA size.
>>
>> Consequently, /proc/<pid>/mem can bypass protections even without
>> FOLL_FORCE, or cross into another mapping without checking its
>> permissions.
>>
>> Check VMA permissions before calling ->access and limit each request to
>> the current VMA. Implement similar access permission checks as GUP,
>> however:
>>   (1) Reject writes to CoW mappings because the callbacks access backing
>>       memory directly and cannot handle CoW.
>>   (2) Allow forced writes to non-CoW mappings with VM_MAYWRITE.
>>
>> This intentionally breaks forced writes through legacy amdgpu PROT_NONE,
>> MAP_PRIVATE debugger mappings. The workaround in commit fa5239f2af98
>> ("drm/amdgpu: workaround failed COW checks for Thunk VMAs") cleared
>> VM_MAYWRITE to avoid CoW, but debugger writes still succeeded by
>> exploiting the missing access-permission checks fixed here. Reads remain
>> supported. Userspace must use MAP_SHARED instead; ROCm 5.5.0, released
>> in May 2023, already does so.
> 
> Thank you for considering this use case and checking the ROCm history. The patch is
> 
> Acked-by: Felix Kuehling <felix.kuehling@amd.com>

Thanks a lot for the confirmation, appreciated!

-- 
Cheers,

David