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

David Hildenbrand
2022-08-09T20:24:02Z

On 09.08.22 22:20, Linus Torvalds wrote:
> On Tue, Aug 9, 2022 at 1:14 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> But as there are two bits, I'm sure somebody ends up touching one and
>> not the other.
>
> Ugh. The nommu code certainly does odd things here. That just looks wrong.
>
> And the hugetlb code does something horrible too, but never *sets* it,
> so it looks like some odd subset of VM_SHARED.

mm/mmap.c:do_mmap() contains the magic bit

switch (flags & MAP_TYPE) {
case MAP_SHARED:
...
case MAP_SHARED_VALIDATE:
...
vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
fallthrough;

VM_SHARED semantics are confusing.

--
Thanks,

David / dhildenb