Re: [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE)

Kees Cook
2022-04-21T17:41:49Z

On Thu, Apr 21, 2022 at 06:24:21PM +0100, Catalin Marinas wrote:
> On Thu, Apr 21, 2022 at 09:42:23AM -0700, Kees Cook wrote:
> > On Thu, Apr 21, 2022 at 04:35:15PM +0100, Catalin Marinas wrote:
> > > Do we want the "was PROT_WRITE" or we just reject mprotect(PROT_EXEC) if
> > > the vma is not already PROT_EXEC? The latter is closer to the current
> > > systemd approach. The former allows an mprotect(PROT_EXEC) if the
> > > mapping was PROT_READ only for example.
> > >
> > > I'd drop the "was PROT_WRITE" for now if the aim is a drop-in
> > > replacement for BPF MDWE.
> >
> > I think "was PROT_WRITE" is an important part of the defense that
> > couldn't be done with a simple seccomp filter (which is why the filter
> > ended up being a problem in the first place).
>
> I would say "was PROT_WRITE" is slightly more relaxed than "is not
> already PROT_EXEC". The seccomp filter can't do "is not already
> PROT_EXEC" either since it only checks the mprotect() arguments, not the
> current vma flags.
>
> So we have (with sub-cases):
>
> 1. Current BPF filter:
>
> a) mmap(PROT_READ|PROT_WRITE|PROT_EXEC); // fails
>
> b) mmap(PROT_READ|PROT_EXEC);
> mprotect(PROT_READ|PROT_EXEC|PROT_BTI); // fails
>
> c) mmap(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // fails
>
> d) mmap(PROT_READ|PROT_WRITE);
> mprotect(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // fails
>
> 2. "is not already PROT_EXEC":
>
> a) mmap(PROT_READ|PROT_WRITE|PROT_EXEC); // fails
>
> b) mmap(PROT_READ|PROT_EXEC);
> mprotect(PROT_READ|PROT_EXEC|PROT_BTI); // passes
>
> c) mmap(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // fails
>
> d) mmap(PROT_READ|PROT_WRITE);
> mprotect(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // fails
>
> 3. "is or was not PROT_WRITE":
>
> a) mmap(PROT_READ|PROT_WRITE|PROT_EXEC); // fails
>
> b) mmap(PROT_READ|PROT_EXEC);
> mprotect(PROT_READ|PROT_EXEC|PROT_BTI); // passes
>
> c) mmap(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // passes
>
> d) mmap(PROT_READ|PROT_WRITE);
> mprotect(PROT_READ);
> mprotect(PROT_READ|PROT_EXEC); // fails

[edited above to show each case]

restated what was already summarized:
Problem is 1.b. 2 and 3 solve it. 3 is more relaxed (c passes).

> If we don't care about 3.c, we might as well go for (2). I don't mind,
> already went for (3) in this series. I think either of them would not be
> a regression on MDWE, unless there is some test that attempts 3.c and
> expects it to fail.

I should stop arguing for a less restrictive mode. ;) It just feels weird
that the combinations are API-mediated, rather than logically defined:
I can do PROT_READ|PROT_EXEC with mmap but not mprotect under 2. As
opposed to saying "the vma cannot be executable if it is or ever was
writable". I find the latter much easier to reason about as far as the
expectations of system state.

So, I'd still prefer 3, as that was the _goal_ of the systemd MDWE
seccomp filter, but yes, 2 does provide the same protection while
allowing BTI.

--
Kees Cook