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

[Linus Torvalds <torvalds@linux-foundation.org>]

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.

                 Linus