Re: [PATCH v2] x86/mce: set MCE_IN_KERNEL_COPYIN for all MC-Safe Copy

From: Kefeng Wang <wangkefeng.wang@huawei.com>
To: Borislav Petkov <bp@alien8.de>, Youquan Song <youquan.song@intel.com>
Cc: <tony.luck@intel.com>, <naoya.horiguchi@nec.com>,
	<tglx@linutronix.de>, <mingo@redhat.com>,
	<dave.hansen@linux.intel.com>, <x86@kernel.org>,
	<akpm@linux-foundation.org>, <linux-edac@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
	<jane.chu@oracle.com>
Subject: Re: [PATCH v2] x86/mce: set MCE_IN_KERNEL_COPYIN for all MC-Safe Copy
Date: Fri, 26 May 2023 20:18:09 +0800	[thread overview]
Message-ID: <e816734d-e6f5-b990-c86d-ac7d5f1c94c0@huawei.com> (raw)
In-Reply-To: <20230526070952.GAZHBbQNAWZJP6tOXv@nazgul.local>



On 2023/5/26 15:09, Borislav Petkov wrote:
> On Fri, May 26, 2023 at 02:32:42PM +0800, Kefeng Wang wrote:
>> The best way to fix them is set MCE_IN_KERNEL_COPYIN for MC-Safe Copy,
>> then let the core do_machine_check() to isolate corrupted page instead
>> of doing it one-by-one.
> 
> No, this whole thing is confused.
> 
>   * Indicates an MCE that happened in kernel space while copying data
>   * from user.
> 
> #define MCE_IN_KERNEL_COPYIN
> 
> This is a very specific exception type: EX_TYPE_COPY which got added by
> 
>    278b917f8cb9 ("x86/mce: Add _ASM_EXTABLE_CPY for copy user access")
> 
> but Linus then removed all such user copy exception points in
> 
>    034ff37d3407 ("x86: rewrite '__copy_user_nocache' function")
> 
> So now that EX_TYPE_COPY never happens.

Is this broken the recover when kernel was copying from user space?

+ Youquan  could you help to check it?

> 
> And what you're doing is lumping the handling for
> EX_TYPE_DEFAULT_MCE_SAFE and EX_TYPE_FAULT_MCE_SAFE together and saying
> that the MCE happened while copying data from user.
> 
> And XSTATE_OP() is one example where this is not really the case.
> 

Oh, for XSTATE_OP(), it uses EX_TYPE_DEFAULT_MCE_SAFE, but I'm focus on 
EX_TYPE_DEFAULT_MCE_SAFE, which use copy_mc (arch/x86/lib/copy_mc_64.S),
like I maintained in changelog, CoW/Coredump/nvdimm/dax, they use 
copy_mc_xxx function,  sorry for mixed them up.


> So no, this is not correct.

so only add MCE_IN_KERNEL_COPYIN for EX_TYPE_DEFAULT_MCE_SAFE?

diff --git a/arch/x86/kernel/cpu/mce/severity.c 
b/arch/x86/kernel/cpu/mce/severity.c
index c4477162c07d..6d2587994623 100644
--- a/arch/x86/kernel/cpu/mce/severity.c
+++ b/arch/x86/kernel/cpu/mce/severity.c
@@ -293,11 +293,11 @@ static noinstr int error_context(struct mce *m, 
struct pt_regs *regs)
         case EX_TYPE_COPY:
                 if (!copy_user)
                         return IN_KERNEL;
+               fallthrough;
+       case EX_TYPE_DEFAULT_MCE_SAFE:
                 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;

Correct me if I am wrong, thanks for you reviewing.


> 

