linux-kernel - Re: [PATCH v5 1/5] arm64: Add utilities to save restore pointer authentication keys

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Message-ID: <88ccd568-f027-6cdf-b4f1-6709d76102c1@arm.com>
Date:   Thu, 31 Jan 2019 16:20:03 +0000
From:   James Morse <james.morse@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] arm64: Add utilities to save restore pointer
 authentication keys

Hi Amit,

On 28/01/2019 06:58, Amit Daniel Kachhap wrote:
> The keys can be switched either inside an assembly or such
> functions which do not have pointer authentication checks, so a GCC
> attribute is added to enable it.
> 
> A function ptrauth_keys_store is added which is similar to existing
> function ptrauth_keys_switch but saves the key values in memory.
> This may be useful for save/restore scenarios when CPU changes
> privilege levels, suspend/resume etc.


> diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
> index 15d4951..98441ce 100644
> --- a/arch/arm64/include/asm/pointer_auth.h
> +++ b/arch/arm64/include/asm/pointer_auth.h
> @@ -11,6 +11,13 @@
>  
>  #ifdef CONFIG_ARM64_PTR_AUTH
>  /*
> + * Compile the function without pointer authentication instructions. This
> + * allows pointer authentication to be enabled/disabled within the function
> + * (but leaves the function unprotected by pointer authentication).
> + */
> +#define __no_ptrauth	__attribute__((target("sign-return-address=none")))

The documentation[0] for this says 'none' is the default. Will this only
take-effect once the kernel supports pointer-auth for the host? (Is this just
documentation until then?)

('noptrauth' would fit with 'notrace' slightly better)


Thanks,

James

[0]
https://gcc.gnu.org/onlinedocs/gcc/AArch64-Function-Attributes.html#AArch64-Function-Attributes