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

[Nadav Amit <nadav.amit@gmail.com>]

> On Dec 21, 2020, at 7:19 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Mon, Dec 21, 2020 at 3:22 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> 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.
> 
> Ugh, this is unpleasantly complicated.  I will admit that any API that
> takes an address and more-or-less-blindly marks it RO makes me quite
> nervous even assuming all the relevant locks are held.  At least
> userfaultfd refuses to operate on VM_SHARED VMAs, but we have another
> instance of this (with mmap_sem held for write) in x86:
> mark_screen_rdonly().  Dare I ask how broken this is?  We could likely
> get away with deleting it entirely.

If you only look at the function in isolation, it seems broken. It should
have flushed the TLB before releasing the mmap_lock. After the
mmap_write_unlock() and before the actual flush, a #PF on another thread can
happen, and a similar scenario to the one that is mentioned in this thread
(copying while a stale PTE in the TLBs is not-writeprotected) might happen.

Having said that, I do not know this code and the context in which this
function is called, so I do not know whether there are other mitigating
factors.

Funny, I had a deja-vu and indeed you have already raised (other) TLB issues
with mark_screen_rdonly() 3 years ago. At the time you said "I'd like to
delete it.” [1]

[1] https://lore.kernel.org/patchwork/patch/782486/#976151