Re: [PATCH RFC 05/19] mm: add early FAULT_FLAG_WRITE consistency checks

[David Hildenbrand <david@redhat.com>]

On 07.11.22 20:03, Nadav Amit wrote:
> On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@redhat.com> wrote:
> 
>> !! External Email
>>
>> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
>> care in all other handlers and might get "surprises" if we forget to do
>> so.
>>
>> Write faults without VM_MAYWRITE don't make any sense, and our
>> maybe_mkwrite() logic could have hidden such abuse for now.
>>
>> Write faults without VM_WRITE on something that is not a COW mapping is
>> similarly broken, and e.g., do_wp_page() could end up placing an
>> anonymous page into a shared mapping, which would be bad.
>>
>> This is a preparation for reliable R/O long-term pinning of pages in
>> private mappings, whereby we want to make sure that we will never break
>> COW in a read-only private mapping.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/memory.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index fe131273217a..826353da7b23 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>>                  */
>>                 if (!is_cow_mapping(vma->vm_flags))
>>                         *flags &= ~FAULT_FLAG_UNSHARE;
>> +       } else if (*flags & FAULT_FLAG_WRITE) {
>> +               /* Write faults on read-only mappings are impossible ... */
>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
>> +                       return VM_FAULT_SIGSEGV;
>> +               /* ... and FOLL_FORCE only applies to COW mappings. */
>> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
>> +                                !is_cow_mapping(vma->vm_flags)))
>> +                       return VM_FAULT_SIGSEGV;
> 
> Not sure about the WARN_*(). Seems as if it might trigger in benign even if
> rare scenarios, e.g., mprotect() racing with page-fault.
> 

We most certainly would want to catch any such broken/racy cases. There 
are no benign cases I could possibly think of.

Page faults need the mmap lock in read. mprotect() / VMA changes need 
the mmap lock in write. Whoever calls handle_mm_fault() is supposed to 
properly check VMA permissions.


-- 
Thanks,

David / dhildenb


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