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 09:28:33PM +0800, Shuai Xue wrote:

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

NOINSTR_VALIDATION=y helps

> > >   	/* 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.

Well, then write it like so. Afaict, you don't care what the actual
exception type is, just that there is one, for the copy_user case.

diff --git a/arch/x86/kernel/cpu/mce/severity.c b/arch/x86/kernel/cpu/mce/severity.c
index dac4d64dfb2a..cfdae25eacd7 100644
--- a/arch/x86/kernel/cpu/mce/severity.c
+++ b/arch/x86/kernel/cpu/mce/severity.c
@@ -301,18 +301,19 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	instrumentation_end();
 
 	switch (fixup_type) {
-	case EX_TYPE_UACCESS:
-		if (!copy_user)
-			return IN_KERNEL;
-		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;
 
 	default:
+		if (copy_user) {
+			m->kflags |= MCE_IN_KERNEL_COPYIN | MCE_IN_KERNEL_RECOV;
+			return IN_KERNEL_RECOV;
+		}
+		fallthrough;
+
+	case EX_TYPE_NONE:
 		return IN_KERNEL;
 	}
 }