Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag

David Hildenbrand
2022-04-22T18:00:19Z

On 22.04.22 15:12, Catalin Marinas wrote:
> On Fri, Apr 22, 2022 at 01:04:31PM +0200, David Hildenbrand wrote:
>> On 22.04.22 12:28, Catalin Marinas wrote:
>>> On Thu, Apr 21, 2022 at 06:37:49PM +0100, David Hildenbrand wrote:
>>>> Note that in the (FOLL_WRITE|FOLL_FORCE) we only require VM_MAYWRITE on
>>>> the vma and trigger a write fault. As the VMA is not VM_WRITE, we won't
>>>> actually map the PTE writable, but set it dirty. GUP will retry, find a
>>>> R/O pte that is dirty and where it knows that it broke COW and will
>>>> allow the read access, although the PTE is R/O.
>>>>
>>>> That mechanism is required to e.g., set breakpoints in R/O MAP_PRIVATE
>>>> kernel sections, but it's used elsewhere for page pinning as well.
>>>>
>>>> My gut feeling is that GUP(FOLL_WRITE|FOLL_FORCE) could be used right
>>>> now to bypass that mechanism, I might be wrong.
>>>
>>> GUP can be used to bypass this. But if an attacker can trigger such GUP
>>> paths via a syscall (e.g. ptrace(PTRACE_POKEDATA)), I think we need the
>>> checks on those paths (and reject the syscall) rather than on
>>> mmap/mprotect(). This would be covered by something like CAP_SYS_PTRACE.
>>
>> I was told that RDMA uses FOLL_FORCE|FOLL_WRITE and is available to
>> unprivileged users.
>
> Ah, do they really need this? At a quick search, ib_umem_get() for
> example:
>
> unsigned int gup_flags = FOLL_WRITE;
> ...
> if (!umem->writable)
> gup_flags |= FOLL_FORCE;
>
> I guess with a new MDWE flag we can make the GUP code ignore FOLL_FORCE
> if VM_EXEC.
>

It's, for example, required when you have a MAP_PRIVATE but PROT_READ
mapping and want to take a reliable R/O (!) pin. Without
FOLL_FORCE|FOLL_WRITE you'd be pinning a (shared zeropage, pagecache)
page that will get replaced by an anonymous page in the COW handler,
after mprotect(PROT_READ|PROT_WRITE) followed by a write access. That
was an issue for RDMA in the past, that's why we have that handling in
place IIRC.

Yes, it's ugly.

--
Thanks,

David / dhildenb