linux-kernel - Re: [PATCH 0/4] MDWE without inheritance

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Message-ID: <ccc1a37b-f0f4-82da-22e8-d7b2a88afb0c@gmail.com>
Date:   Mon, 8 May 2023 20:21:16 +0300
From:   Topi Miettinen <toiwoton@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Florent Revest <revest@...omium.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        anshuman.khandual@....com, joey.gouly@....com, mhocko@...e.com,
        keescook@...omium.org, david@...hat.com, izbyshev@...ras.ru,
        nd@....com, broonie@...nel.org, szabolcs.nagy@....com,
        lennart@...ttering.net
Subject: Re: [PATCH 0/4] MDWE without inheritance

On 8.5.2023 17.10, Catalin Marinas wrote:
> I think we should keep the original behaviour of systemd here, otherwise
> they won't transition to the new interface and keep using the SECCOMP
> BPF approach (which, in addition, prevents glibc from setting PROT_BTI
> on an already executable mapping).

Systemd has transitioned to prctl(PR_SET_MDWE) method since release of 
v253, so the original behaviour definitely should be kept.

> To me MDWE is not about preventing JITs but rather ensuring buggy
> programs don't end up with WX mappings. We ended up this way because of
> the SECCOMP BPF limitations (just guessing, I haven't been involved in
> its design). With a no-inherit MDWE, one can introduce an additional
> policy for systemd. It would be a sysadmin decision which one to enable
> and maybe current (inherit) MDWE will disappear in time.

There could be a new setting for this, like 
MemoryDenyWriteExecute=no-inherit. I'd only use it for those special 
cases where MemoryDenyWriteExecute=yes can't be used.

> x86 has protection keys and arm64 will soon have permission overlays
> that allow user-space to toggle between RX and RW (Joey is looking at
> the arm64 support). I'm not sure how we'll end up implemented this on
> arm64 (and haven't looked at x86) but I have a suspicion MDWE will get
> in the way as the base page table permission will probably need
> PROT_WRITE|PROT_EXEC.

Wouldn't those features defeat any gains from MDWE? The features 
probably should be forbidden with MemoryDenyWriteExecute=yes.

-Topi