Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

David Hildenbrand
2022-08-09T20:42:20Z

On 09.08.22 22:30, Linus Torvalds wrote:
> On Tue, Aug 9, 2022 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> IIUC VM_MAYSHARE is always set in a MAP_SHARED mapping, but for file
>> mappings we only set VM_SHARED if the file allows for writes
>
> Heh.
>
> This is a horrific hack, and probably should go away.
>
> Yeah, we have that
>
> if (!(file->f_mode & FMODE_WRITE))
> vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
>
>
> but I think that's _entirely_ historical.
>
> Long long ago, in a galaxy far away, we didn't handle shared mmap()
> very well. In fact, we used to not handle it at all.
>
> But nntpd would use write() to update the spool file, adn them read it
> through a shared mmap.
>
> And since our mmap() *was* coherent with people doing write() system
> calls, but didn't handle actual dirty shared mmap, what Linux used to
> do was to just say "Oh, you want a read-only shared file mmap? I can
> do that - I'll just downgrade it to a read-only _private_ mapping, and
> it actually ends up with the same semantics".
>
> And here we are, 30 years later, and it still does that, but it leaves
> the VM_MAYSHARE flag so that /proc/<pid>/maps can show that it's a
> shared mapping.

I was suspecting that this code is full of legacy :)

What would make sense to me is to just have VM_SHARED and make it
correspond to MAP_SHARED, that would at least confuse me less. Once I
have some spare cycles I'll see how easy that might be to achieve.

--
Thanks,

David / dhildenb