linux-kernel - Re: [PATCH v10 00/13] arm64: Branch Target Identification support

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Date:   Mon, 23 Mar 2020 13:24:12 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Szabolcs Nagy <szabolcs.nagy@....com>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Paul Elliott <paul.elliott@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Amit Kachhap <amit.kachhap@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Marc Zyngier <maz@...nel.org>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        "H . J . Lu " <hjl.tools@...il.com>,
        Andrew Jones <drjones@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Kristina Martšenko <kristina.martsenko@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Florian Weimer <fweimer@...hat.com>,
        Sudakshina Das <sudi.das@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        nd@....com
Subject: Re: [PATCH v10 00/13] arm64: Branch Target Identification support

On Mon, Mar 23, 2020 at 12:21:44PM +0000, Catalin Marinas wrote:
> On Fri, Mar 20, 2020 at 05:39:46PM +0000, Szabolcs Nagy wrote:

> +int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> +                        bool has_interp, bool is_interp)
> +{
> +       if (is_interp != has_interp)
> +               return prot;
> +
> +       if (!(state->flags & ARM64_ELF_BTI))
> +               return prot;
> +
> +       if (prot & PROT_EXEC)
> +               prot |= PROT_BTI;
> +
> +       return prot;
> +}

> At a quick look, for dynamic binaries we have has_interp == true and
> is_interp == false. I don't know why but, either way, the above code
> needs a comment with some justification.

I don't really know for certain either, I inherited this code as is with
the understanding that this was all agreed with the toolchain and libc
people - the actual discussion that lead to the decisions being made
happened before I was involved.  My understanding is that the idea was
that the dynamic linker would be responsible for mapping everything in
dynamic applications other than itself but other than consistency I
don't know why.  I guess it defers more decision making to userspace but
I'm having a hard time thinking of sensible cases where one might wish
to make a decision other than enabling PROT_BTI.

I'd be perfectly happy to drop the check if that makes more sense to
people, otherwise I can send a patch adding a comment explaining the
situation.

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