linux-kernel - Re: [systemd-devel] BTI interaction between seccomp filters in systemd and glibc mprotect calls, causing service failures

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Message-ID: <20201022071812.GA324655@gardel-login>
Date:   Thu, 22 Oct 2020 09:18:12 +0200
From:   Lennart Poettering <mzxreary@...inter.de>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, libc-alpha@...rceware.org,
        systemd-devel@...ts.freedesktop.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Brown <broonie@...nel.org>,
        Dave Martin <dave.martin@....com>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in
 systemd and glibc mprotect calls, causing service failures

On Mi, 21.10.20 22:44, Jeremy Linton (jeremy.linton@....com) wrote:

> Hi,
>
> There is a problem with glibc+systemd on BTI enabled systems. Systemd
> has a service flag "MemoryDenyWriteExecute" which uses seccomp to deny
> PROT_EXEC changes. Glibc enables BTI only on segments which are marked as
> being BTI compatible by calling mprotect PROT_EXEC|PROT_BTI. That call is
> caught by the seccomp filter, resulting in service failures.
>
> So, at the moment one has to pick either denying PROT_EXEC changes, or BTI.
> This is obviously not desirable.
>
> Various changes have been suggested, replacing the mprotect with mmap calls
> having PROT_BTI set on the original mapping, re-mmapping the segments,
> implying PROT_EXEC on mprotect PROT_BTI calls when VM_EXEC is already set,
> and various modification to seccomp to allow particular mprotect cases to
> bypass the filters. In each case there seems to be an undesirable attribute
> to the solution.
>
> So, whats the best solution?

Did you see Topi's comments on the systemd issue?

https://github.com/systemd/systemd/issues/17368#issuecomment-710485532

I think I agree with this: it's a bit weird to alter the bits after
the fact. Can't glibc set up everything right from the begining? That
would keep both concepts working.

Lennart

--
Lennart Poettering, Berlin