linux-kernel - Re: [PATCH -next v5 2/3] x86/mce: set MCE_IN_KERNEL_COPYIN for DEFAULT_MCE

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Date: Wed, 7 Feb 2024 13:29:42 +0100
From: Borislav Petkov <bp@...en8.de>
To: Tong Tiangen <tongtiangen@...wei.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	wangkefeng.wang@...wei.com,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <naoya.horiguchi@....com>,
	linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
	linux-mm@...ck.org, Guohanjun <guohanjun@...wei.com>
Subject: Re: [PATCH -next v5 2/3] x86/mce: set MCE_IN_KERNEL_COPYIN for
 DEFAULT_MCE_SAFE exception

On Sun, Feb 04, 2024 at 04:26:26PM +0800, Tong Tiangen wrote:
> diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
> index bca780fa5e57..b2cce1b6c96d 100644
> --- a/arch/x86/kernel/cpu/mce/severity.c
> +++ b/arch/x86/kernel/cpu/mce/severity.c
> @@ -292,11 +292,11 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>  	case EX_TYPE_UACCESS:
>  		if (!copy_user)
>  			return IN_KERNEL;
> +		fallthrough;
> +	case EX_TYPE_DEFAULT_MCE_SAFE:
>  		m->kflags |= MCE_IN_KERNEL_COPYIN;
>  		fallthrough;

I knew something was still bugging me here and this is still wrong.

Let's imagine this flow:

copy_mc_to_user() - note *src is kernel memory
|-> copy_mc_enhanced_fast_string or copy_mc_fragile - it's the same thing
  |-> -#MC, exception type EX_TYPE_DEFAULT_MCE_SAFE
    |-> error_context():
       case EX_TYPE_DEFAULT_MCE_SAFE:
                m->kflags |= MCE_IN_KERNEL_COPYIN;

MCE_IN_KERNEL_COPYIN does kill_me_never():

	pr_err("Kernel accessed poison in user space at %llx\n", p->mce_addr);

but that's reading from kernel memory!

IOW, I *think* that switch statement should be this:

	switch (fixup_type) {
	case EX_TYPE_UACCESS:
	case EX_TYPE_DEFAULT_MCE_SAFE:
		if (!copy_user)
			return IN_KERNEL;

		m->kflags |= MCE_IN_KERNEL_COPYIN;
		fallthrough;

	case EX_TYPE_FAULT_MCE_SAFE:
		m->kflags |= MCE_IN_KERNEL_RECOV;
		return IN_KERNEL_RECOV;

	default:
		return IN_KERNEL;
	}

Provided I'm not missing a case and provided is_copy_from_user() really
detects all cases properly.

And then patch 3 is wrong because we only can handle "copy in" - not
just any copy.

Thx.

-- 
Regards/Gruss,
    Boris.

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