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

[Kefeng Wang]

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.


>