Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

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

On Mon, Dec 21, 2020 at 2:30 PM Peter Xu <peterx@redhat.com> wrote:
>
> AFAIU mprotect() is the only one who modifies the pte using the mmap write
> lock.  NUMA balancing is also using read mmap lock when changing pte
> protections, while my understanding is mprotect() used write lock only because
> it manipulates the address space itself (aka. vma layout) rather than modifying
> the ptes, so it needs to.

So it's ok to change the pte holding only the PTE lock, if it's a
*one*way* conversion.

That doesn't break the "re-check the PTE contents" model (which
predates _all_ of the rest: NUMA, userfaultfd, everything - it's
pretty much the original model for our page table operations, and goes
back to the dark ages even before SMP and the existence of a page
table lock).

So for example, a COW will always create a different pte (not just
because the page number itself changes - you could imagine a page
getting re-used and changing back - but because it's always a RO->RW
transition).

So two COW operations cannot "undo" each other and fool us into
thinking nothing changed.

Anything that changes RW->RO - like fork(), for example - needs to
take the mmap_lock.

NUMA balancing should be ok wrt COW, because it doesn't do that RW->RO
thing, it uses the present bit.

I think that you are right that NUMA balancing itself might cause
other issues, because it can cause that "pte changed and then came
back" (for numa protectoipn and then a numa fault) all with just the
mmap lock for reading.

However, even that shouldn't matter for COW, because the write protect
bit is the one that proptects the *contents* of the page, so even if
NUMA balancing caused that "load original PTE, then re-check later" to
succeed (despite the PTE actually changing in the middle), the
_contents_ of the page cannot have changed, so COW is ok. NUMA
balancing won't be making a read-only page temporarily writable.

           Linus