linux-hardening - Re: [PATCH 3/4] aarch64: Use mmap to add PROT

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Message-ID: <87v9embufl.fsf@oldenburg2.str.redhat.com>
Date:   Tue, 03 Nov 2020 11:34:22 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     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.deacon@....com>,
        Mark Brown <broonie@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Topi Miettinen <toiwoton@...il.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 3/4] aarch64: Use mmap to add PROT_BTI instead of
 mprotect [BZ #26831]

* Szabolcs Nagy:

> Re-mmap executable segments if possible instead of using mprotect
> to add PROT_BTI. This allows using BTI protection with security
> policies that prevent mprotect with PROT_EXEC.
>
> If the fd of the ELF module is not available because it was kernel
> mapped then mprotect is used and failures are ignored.  It is
> expected that linux kernel will add PROT_BTI when mapping a module
> (current linux as of version 5.9 does not do this).
>
> Computing the mapping parameters follows the logic of
> _dl_map_object_from_fd more closely now.

What's the performance of this on execve-heavy workloads, such as kernel
or glibc builds?  Hopefully it's cheap because these mappings have not
been faulted in yet.

Thanks,
Florian
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