Subject: Re: [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831]

From: Catalin Marinas <[email-masked]>
Date: Thu, 3 Dec 2020 17:30:06 +0000

Hi Szabolcs,

On Fri, Nov 27, 2020 at 01:19:16PM +0000, Szabolcs Nagy wrote:
> This is v2 of
> https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html
> 
> To enable BTI support, re-mmap executable segments instead of
> mprotecting them in case mprotect is seccomp filtered.
> 
> I would like linux to change to map the main exe with PROT_BTI when
> that is marked as BTI compatible. From the linux side i heard the
> following concerns about this:
> - it's an ABI change so requires some ABI bump. (this is fine with
>   me, i think glibc does not care about backward compat as nothing
>   can reasonably rely on the current behaviour, but if we have a
>   new bit in auxv or similar then we can save one mprotect call.)

I'm not concerned about the ABI change but there are workarounds like a
new auxv bit.

> - in case we discover compatibility issues with user binaries it's
>   better if userspace can easily disable BTI (e.g. removing the
>   mprotect based on some env var, but if kernel adds PROT_BTI and
>   mprotect is filtered then we have no reliable way to remove that
>   from executables. this problem already exists for static linked
>   exes, although admittedly those are less of a compat concern.)

This is our main concern. For static binaries, the linker could detect,
in theory, potential issues when linking and not set the corresponding
ELF information.

At runtime, a dynamic linker could detect issues and avoid enabling BTI.
In both cases, it's a (static or dynamic) linker decision that belongs
in user-space.

> - ideally PROT_BTI would be added via a new syscall that does not
>   interfere with PROT_EXEC filtering. (this does not conflict with
>   the current patches: even with a new syscall we need a fallback.)

This can be discussed as a long term solution.

> - solve it in systemd (e.g. turn off the filter, use better filter):
>   i would prefer not to have aarch64 (or BTI) specific policy in
>   user code. and there was no satisfying way to do this portably.

I agree. I think the best for now (as a back-portable glibc fix) is to
ignore the mprotect(PROT_EXEC|PROT_BTI) error that the dynamic loader
gets. BTI will be disabled if MDWX is enabled.

In the meantime, we should start (continue) looking at a solution that
works for both systemd and the kernel and be generic enough for other
architectures. The stateless nature of the current SECCOMP approach is
not suitable for this W^X policy. Kees had some suggestions here but the
thread seems to have died:

https://lore.kernel.org/kernel-hardening/202010221256.A4F95FD11@keescook/

-- 
Catalin

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