linux-kernel - Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT

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Date:   Wed, 4 Nov 2020 11:02:32 +0200
From:   Topi Miettinen <toiwoton@...il.com>
To:     Mark Brown <broonie@...nel.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>
Cc:     libc-alpha@...rceware.org, Jeremy Linton <jeremy.linton@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/4] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ
 #26831]

On 3.11.2020 19.34, Mark Brown wrote:
> On Tue, Nov 03, 2020 at 10:25:37AM +0000, Szabolcs Nagy wrote:
> 
>> Re-mmap executable segments instead of mprotecting them in
>> case mprotect is seccomp filtered.
> 
>> For the kernel mapped main executable we don't have the fd
>> for re-mmap so linux needs to be updated to add BTI. (In the
>> presence of seccomp filters for mprotect(PROT_EXEC) the libc
>> cannot change BTI protection at runtime based on user space
>> policy so it is better if the kernel maps BTI compatible
>> binaries with PROT_BTI by default.)
> 
> Given that there were still some ongoing discussions on a more robust
> kernel interface here and there seem to be a few concerns with this
> series should we perhaps just take a step back and disable this seccomp
> filter in systemd on arm64, at least for the time being?

Filtering mprotect() and mmap() with seccomp also protects BTI, since 
without it the attacker could remove PROT_BTI from existing pages, or 
map new pages without BTI. This would be possible even with SARA or 
SELinux execmem protections enabled, since they don't care about PROT_BTI.

-Topi