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

[Shuai Xue <xueshuai@linux.alibaba.com>]

Hi, Peter

在 2025/2/18 20:54, Peter Zijlstra 写道:
> On Mon, Feb 17, 2025 at 02:33:33PM +0800, Shuai Xue wrote:
> 
>> diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
>> index dac4d64dfb2a..14c2d71c3ce1 100644
>> --- a/arch/x86/kernel/cpu/mce/severity.c
>> +++ b/arch/x86/kernel/cpu/mce/severity.c
>> @@ -16,6 +16,7 @@
>>   #include <asm/traps.h>
>>   #include <asm/insn.h>
>>   #include <asm/insn-eval.h>
>> +#include <linux/extable.h>
>>   
>>   #include "internal.h"
>>   
>> @@ -285,7 +286,8 @@ static bool is_copy_from_user(struct pt_regs *regs)
>>    */
>>   static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>>   {
>> -	int fixup_type;
>> +	const struct exception_table_entry *e;
>> +	int fixup_type, imm;
>>   	bool copy_user;
>>   
>>   	if ((m->cs & 3) == 3)
>> @@ -294,9 +296,14 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>>   	if (!mc_recoverable(m->mcgstatus))
>>   		return IN_KERNEL;
>>   
>> +	e = search_exception_tables(m->ip);
>> +	if (!e)
>> +		return IN_KERNEL;
> 
> You didn't actually build this, did you? Or did you ignore the extra
> noinstr warnings?

I did build and test this patch set on it. But I did not find any warnings.
Could you provide more details?

> 
>>   	/* Allow instrumentation around external facilities usage. */
>>   	instrumentation_begin();
>> -	fixup_type = ex_get_fixup_type(m->ip);
>> +	fixup_type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
>> +	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
>>   	copy_user  = is_copy_from_user(regs);
>>   	instrumentation_end();
>>   
>> @@ -304,9 +311,13 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
>>   	case EX_TYPE_UACCESS:
>>   		if (!copy_user)
>>   			return IN_KERNEL;
>> -		m->kflags |= MCE_IN_KERNEL_COPYIN;
>> -		fallthrough;
>> -
>> +		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
>> +		return IN_KERNEL_RECOV;
>> +	case EX_TYPE_IMM_REG:
>> +		if (!copy_user || imm != -EFAULT)
>> +			return IN_KERNEL;
>> +		m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
>> +		return IN_KERNEL_RECOV;
> 
> Maybe I'm justnot understanding things, but what's wrong with something
> like the below; why do we care about the ex-type if we know its a MOV
> reading from userspace?
> 
> The less we muck about with the extable here, the better.

We need to make sure that we have register a fixup handler for the copy_user
case.  If no fixup handler found, the PC accessing posion will trigger #MCE
again and again resulting a hardlock up.

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

Thanks.
Shuai