linux-kernel - Re: [PATCH v2 3/5] x86/mce: add EX_TYPE_EFAULT_REG as in-kernel recovery context to fix copy-from-user operations regression

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Message-ID: <20250218164800.GNZ7S5wL1A4dTaySOP@fat_crate.local>
Date: Tue, 18 Feb 2025 17:48:00 +0100
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Shuai Xue <xueshuai@...ux.alibaba.com>, tony.luck@...el.com,
	nao.horiguchi@...il.com, tglx@...utronix.de, mingo@...hat.com,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linmiaohe@...wei.com, akpm@...ux-foundation.org,
	jpoimboe@...nel.org, linux-edac@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	baolin.wang@...ux.alibaba.com, tianruidong@...ux.alibaba.com
Subject: Re: [PATCH v2 3/5] x86/mce: add EX_TYPE_EFAULT_REG as in-kernel
 recovery context to fix copy-from-user operations regression

On Tue, Feb 18, 2025 at 03:15:35PM +0100, Peter Zijlstra wrote:
> diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
> index dac4d64dfb2a..cfdae25eacd7 100644
> --- a/arch/x86/kernel/cpu/mce/severity.c
> +++ b/arch/x86/kernel/cpu/mce/severity.c
> @@ -301,18 +301,19 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>  	instrumentation_end();
>  
>  	switch (fixup_type) {
> -	case EX_TYPE_UACCESS:
> -		if (!copy_user)
> -			return IN_KERNEL;
> -		m->kflags |= MCE_IN_KERNEL_COPYIN;
> -		fallthrough;
> -
>  	case EX_TYPE_FAULT_MCE_SAFE:
>  	case EX_TYPE_DEFAULT_MCE_SAFE:
>  		m->kflags |= MCE_IN_KERNEL_RECOV;
>  		return IN_KERNEL_RECOV;
>  
>  	default:
> +		if (copy_user) {

As said on chat, if we can make is_copy_from_user() *always* correctly detect
user access, then sure but I'm afraid EX_TYPE_UACCESS being generated at the
handful places where we do user memory access is there for a reason as it
makes it pretty explicit.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette