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

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Date:   Mon, 23 Mar 2020 15:02:09 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Brown <broonie@...nel.org>,
        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 02:39:55PM +0000, Catalin Marinas wrote:
> On Mon, Mar 23, 2020 at 01:57:22PM +0000, Mark Rutland wrote:
> > On Mon, Mar 23, 2020 at 01:24:12PM +0000, Mark Brown wrote:
> > > 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;
> > > > +}

> > I think it would be best to document the current behaviour, as it's a
> > simple ABI that we can guarantee, and the dynamic linker will have to be
> > aware of BTI in order to do the right thing anyhow.
> 
> 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)?

Also worth noting that an old dynamic linker won't have ARM64_ELF_BTI
set, so the kernel will not enable BTI for this.

Mark.