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: <20201023090451.GB25736@gaia>
Date:   Fri, 23 Oct 2020 10:04:51 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Szabolcs Nagy <szabolcs.nagy@....com>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        systemd-devel@...ts.freedesktop.org,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, libc-alpha@...rceware.org,
        Dave Martin <dave.martin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [systemd-devel] BTI interaction between seccomp filters in
 systemd and glibc mprotect calls, causing service failures

On Fri, Oct 23, 2020 at 07:13:17AM +0100, Szabolcs Nagy wrote:
> The 10/22/2020 10:31, Catalin Marinas wrote:
> > IIUC, the problem is with the main executable which is mapped by the
> > kernel without PROT_BTI. The dynamic loader wants to set PROT_BTI but
> > does not have the original file descriptor to be able to remap. Its only
> > choice is mprotect() and this fails because of the MDWX policy.
> > 
> > Not sure whether the kernel has the right information but could it map
> > the main executable with PROT_BTI if the corresponding PT_GNU_PROPERTY
> > is found? The current ABI states it only sets PROT_BTI for the
> > interpreter who'd be responsible for setting the PROT_BTI on the main
> > executable. I can't tell whether it would break anything but it's worth
> > a try:
> 
> i think it would work, but now i can't easily
> tell from the libc if i have to do the mprotect
> on the main exe or not.
> 
> i guess i can just always mprotect and ignore
> the failure?

I replied to Keys before reading your email. So yeah, still issue
mprotect() but ignore the failure.

-- 
Catalin