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

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Date:   Mon, 23 Mar 2020 12:21:44 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Szabolcs Nagy <szabolcs.nagy@....com>
Cc:     Mark Brown <broonie@...nel.org>, 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 Fri, Mar 20, 2020 at 05:39:46PM +0000, Szabolcs Nagy wrote:
> The 03/16/2020 16:50, Mark Brown wrote:
> > This patch series implements support for ARMv8.5-A Branch Target
> > Identification (BTI), which is a control flow integrity protection
> > feature introduced as part of the ARMv8.5-A extensions.
> 
> i was playing with this and it seems the kernel does not add
> PROT_BTI to non-static executables (i.e. there is an interpreter).
> 
> i thought any elf that the kernel maps would get PROT_BTI from the
> kernel. (i want to remove the mprotect in glibc when not necessary)

I haven't followed the early discussions but I think this makes sense.

> i tested by linking a hello world exe with -Wl,-z,force-bti (and
> verified that the property note is there) and expected it to crash
> (with SIGILL) when the dynamic linker jumps to _start in the exe,
> but it executed without errors (if i do the mprotect in glibc then
> i get SIGILL as expected).
> 
> is this deliberate? does the kernel map static exe and dynamic
> linked exe differently?

I think the logic is in patch 5:

+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.

-- 
Catalin