Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]

[Catalin Marinas]

On Wed, Nov 04, 2020 at 11:55:57AM +0200, Topi Miettinen wrote:
> On 4.11.2020 11.29, Florian Weimer wrote:
> > * Will Deacon:
> > 
> > > Is there real value in this seccomp filter if it only looks at mprotect(),
> > > or was it just implemented because it's easy to do and sounds like a good
> > > idea?
> > 
> > It seems bogus to me.  Everyone will just create alias mappings instead,
> > just like they did for the similar SELinux feature.  See “Example code
> > to avoid execmem violations” in:
> > 
> >    <https://www.akkadia.org/drepper/selinux-mem.html>
[...]
> > As you can see, this reference implementation creates a PROT_WRITE
> > mapping aliased to a PROT_EXEC mapping, so it actually reduces security
> > compared to something that generates the code in an anonymous mapping
> > and calls mprotect to make it executable.
[...]
> If a service legitimately needs executable and writable mappings (due to
> JIT, trampolines etc), it's easy to disable the filter whenever really
> needed with "MemoryDenyWriteExecute=no" (which is the default) in case of
> systemd or a TE rule like "allow type_t self:process { execmem };" for
> SELinux. But this shouldn't be the default case, since there are many
> services which don't need W&X.

I think Drepper's point is that separate X and W mappings, with enough
randomisation, would be more secure than allowing W&X at the same
address (but, of course, less secure than not having W at all, though
that's not always possible).

> I'd also question what is the value of BTI if it can be easily circumvented
> by removing PROT_BTI with mprotect()?

Well, BTI is a protection against JOP attacks. The assumption here is
that an attacker cannot invoke mprotect() to disable PROT_BTI. If it
can, it's probably not worth bothering with a subsequent JOP attack, it
can already call functions directly.

I see MDWX not as a way of detecting attacks but rather plugging
inadvertent security holes in certain programs. On arm64, such hardening
currently gets in the way of another hardening feature, BTI.

-- 
Catalin