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

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Message-ID: <20200323153228.GE4948@sirena.org.uk>
Date:   Mon, 23 Mar 2020 15:32:28 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        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 02:55:46PM +0000, Mark Rutland wrote:
> On Mon, Mar 23, 2020 at 02:39:55PM +0000, Catalin Marinas wrote:

> > So this means that the interpreter will have to mprotect(PROT_BTI) the
> > text section of the primary executable.

> Yes, but after fixing up any relocations in that section it's going to
> have to call mprotect() on it anyhow (e.g. in order to make it
> read-only), and in doing so would throw away BTI unless it was BTI
> aware.

Ah, of course - I forgot that's not a read/modify/write cycle.  I'll
send the comment version.

> > That's a valid point. If we have an old dynamic linker and the kernel
> > enabled BTI automatically for the main executable, could things go wrong
> > (e.g. does the PLT need to be BTI-aware)?

> I believe that a PLT in an unguarded page needs no special treatment. A
> PLT within a guarded page needs to be built specially for BTI.

Unguarded stuff is unaffected.

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