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

Catalin Marinas
2022-04-22T13:12:47Z

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.

--
Catalin