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

[Linus Torvalds]

On Tue, Aug 9, 2022 at 1:07 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> >         /* But FOLL_FORCE has no effect on shared mappings */
> >         if (vma->vm_flags & MAP_SHARED)
> >                 return false;
>
> I'd actually rather check for MAP_MAYSHARE here, which is even stronger.
> Thoughts?

Hmm. Adding the test for both is basically free (all those vm_flags
checks end up being a bit mask and compare), so no objections.

For some reason I though VM_SHARED and VM_MAYSHARE end up always being
the same bits, and it was a mistake to make them two bits in the first
place (unlike the read/write/exec bits that can are about mprotect),

But as there are two bits, I'm sure somebody ends up touching one and
not the other.

So yeah, I don't see any downside to just checking both bits.

[ That said, is_cow_mapping() only checks VM_SHARED, so if they are
ever different, that's a potential source of confusion ]

               Linus