linux-kernel - Re: [PATCH -next v4 4/7] arm64: add copy_{to, from}

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Date:   Wed, 4 May 2022 11:26:40 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Tong Tiangen <tongtiangen@...wei.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Robin Murphy <robin.murphy@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Kefeng Wang <wangkefeng.wang@...wei.com>,
        Xie XiuQi <xiexiuqi@...wei.com>,
        Guohanjun <guohanjun@...wei.com>
Subject: Re: [PATCH -next v4 4/7] arm64: add copy_{to, from}_user to machine
 check safe

On Wed, Apr 20, 2022 at 03:04:15AM +0000, Tong Tiangen wrote:
> Add copy_{to, from}_user() to machine check safe.
> 
> If copy fail due to hardware memory error, only the relevant processes are
> affected, so killing the user process and isolate the user page with
> hardware memory errors is a more reasonable choice than kernel panic.

Just to make sure I understand - we can only recover if the fault is in
a user page. That is, for a copy_from_user(), we can only handle the
faults in the source address, not the destination.

> diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> index 34e317907524..480cc5ac0a8d 100644
> --- a/arch/arm64/lib/copy_from_user.S
> +++ b/arch/arm64/lib/copy_from_user.S
> @@ -25,7 +25,7 @@
>  	.endm
>  
>  	.macro strb1 reg, ptr, val
> -	strb \reg, [\ptr], \val
> +	USER_MC(9998f, strb \reg, [\ptr], \val)
>  	.endm

So if I got the above correctly, why do we need an exception table entry
for the store to the kernel address?

-- 
Catalin