linux-kernel - Re: [PATCH v6 08/13] arm64: expose user PAC bit positions via ptrace

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Date:   Sun, 9 Dec 2018 09:03:55 -0600
From:   Richard Henderson <richard.henderson@...aro.org>
To:     Kristina Martsenko <kristina.martsenko@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Adam Wallis <awallis@...eaurora.org>,
        Amit Kachhap <amit.kachhap@....com>,
        Andrew Jones <drjones@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave P Martin <dave.martin@....com>,
        Jacob Bramley <jacob.bramley@....com>,
        Kees Cook <keescook@...omium.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 08/13] arm64: expose user PAC bit positions via ptrace

On 12/7/18 12:39 PM, Kristina Martsenko wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> When pointer authentication is in use, data/instruction pointers have a
> number of PAC bits inserted into them. The number and position of these
> bits depends on the configured TCR_ELx.TxSZ and whether tagging is
> enabled. ARMv8.3 allows tagging to differ for instruction and data
> pointers.
> 
> For userspace debuggers to unwind the stack and/or to follow pointer
> chains, they need to be able to remove the PAC bits before attempting to
> use a pointer.
> 
> This patch adds a new structure with masks describing the location of
> the PAC bits in userspace instruction and data pointers (i.e. those
> addressable via TTBR0), which userspace can query via PTRACE_GETREGSET.
> By clearing these bits from pointers (and replacing them with the value
> of bit 55), userspace can acquire the PAC-less versions.
> 
> This new regset is exposed when the kernel is built with (user) pointer
> authentication support, and the address authentication feature is
> enabled. Otherwise, the regset is hidden.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Kristina Martsenko <kristina.martsenko@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Ramana Radhakrishnan <ramana.radhakrishnan@....com>
> Cc: Will Deacon <will.deacon@....com>
> ---
>  arch/arm64/include/asm/pointer_auth.h |  8 ++++++++
>  arch/arm64/include/uapi/asm/ptrace.h  |  7 +++++++
>  arch/arm64/kernel/ptrace.c            | 38 +++++++++++++++++++++++++++++++++++
>  include/uapi/linux/elf.h              |  1 +
>  4 files changed, 54 insertions(+)

Reviewed-by: Richard Henderson <richard.henderson@...aro.org>


r~