Re: [PATCH ARM64 v4.4 V3 07/44] arm64: uaccess: Prevent speculative use of the current addr_limit

From: Mark Rutland <mark.rutland@arm•com>
To: Viresh Kumar <viresh.kumar@linaro•org>
Cc: Julien Thierry <Julien.Thierry@arm•com>,
	Marc Zyngier <marc.zyngier@arm•com>,
	Catalin Marinas <catalin.marinas@arm•com>,
	Will Deacon <will.deacon@arm•com>,
	stable@vger•kernel.org, mark.brown@arm•com,
	Russell King <rmk+kernel@arm•linux.org.uk>,
	linux-arm-kernel@lists•infradead.org
Subject: Re: [PATCH ARM64 v4.4 V3 07/44] arm64: uaccess: Prevent speculative use of the current addr_limit
Date: Fri, 30 Aug 2019 10:40:54 +0100	[thread overview]
Message-ID: <20190830094053.GG46475@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <dbe69b13f77052abf5d342b2775b1ebdbcce241a.1567077734.git.viresh.kumar@linaro.org>

On Thu, Aug 29, 2019 at 05:03:52PM +0530, Viresh Kumar wrote:
> From: Will Deacon <will.deacon@arm•com>
> 
> commit c2f0ad4fc089cff81cef6a13d04b399980ecbfcc upstream.
> 
> A mispredicted conditional call to set_fs could result in the wrong
> addr_limit being forwarded under speculation to a subsequent access_ok
> check, potentially forming part of a spectre-v1 attack using uaccess
> routines.
> 
> This patch prevents this forwarding from taking place, but putting heavy
> barriers in set_fs after writing the addr_limit.
> 
> Reviewed-by: Mark Rutland <mark.rutland@arm•com>
> Signed-off-by: Will Deacon <will.deacon@arm•com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm•com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro•org>

Reviewed-by: Mark Rutland <mark.rutland@arm•com> [v4.4 backport]

Mark.

> ---
>  arch/arm64/include/asm/uaccess.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 75363d723262..fc11c50af558 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -62,6 +62,13 @@ extern int fixup_exception(struct pt_regs *regs);
>  static inline void set_fs(mm_segment_t fs)
>  {
>  	current_thread_info()->addr_limit = fs;
> +
> +	/*
> +	 * Prevent a mispredicted conditional call to set_fs from forwarding
> +	 * the wrong address limit to access_ok under speculation.
> +	 */
> +	dsb(nsh);
> +	isb();
>  }
>  
>  #define segment_eq(a, b)	((a) == (b))
> -- 
> 2.21.0.rc0.269.g1a574e7a288b
> 

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