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

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Date: Thu, 8 Feb 2024 14:21:05 +0800
From: Tong Tiangen <tongtiangen@...wei.com>
To: Borislav Petkov <bp@...en8.de>
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



在 2024/2/7 20:29, Borislav Petkov 写道:
> 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.

That makes sense. I'll check that point out later.

Many thanks.
Tong.
> 
> Thx.
>