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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 18, 2025 at 01:54:08PM +0100, Peter Zijlstra wrote:

> ---
> diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
> index dac4d64dfb2a..cb021058165f 100644
> --- a/arch/x86/kernel/cpu/mce/severity.c
> +++ b/arch/x86/kernel/cpu/mce/severity.c
> @@ -300,13 +300,12 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>  	copy_user  = is_copy_from_user(regs);
>  	instrumentation_end();
>  
> -	switch (fixup_type) {
> -	case EX_TYPE_UACCESS:
> -		if (!copy_user)
> -			return IN_KERNEL;
> -		m->kflags |= MCE_IN_KERNEL_COPYIN;
> -		fallthrough;
> +	if (copy_user) {
> +		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_COPYIN;

Typing is hard, obviously that second should'be been _RECOV.

> +		return IN_KERNEL_RECOV

But why are we having that bit *and* a return value saying the same
thing?

> +	}
>  
> +	switch (fixup_type) {
>  	case EX_TYPE_FAULT_MCE_SAFE:
>  	case EX_TYPE_DEFAULT_MCE_SAFE:
>  		m->kflags |= MCE_IN_KERNEL_RECOV;