Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed

[Josh Poimboeuf]

On Thu, Apr 11, 2024 at 05:15:22PM -0700, Pawan Gupta wrote:
> > + * Do either a direct or an indirect call, depending on whether indirect calls
> > + * are considered safe.
> > + */
> > +#define __do_syscall(table, func_direct, nr, regs)			\
> > +({									\
> > +	unsigned long __rax, __rdi, __rsi;				\
> > +									\
> > +	asm_inline volatile(						\
> > +		ALTERNATIVE("call " __stringify(func_direct) "\n\t",	\
> > +			    ANNOTATE_RETPOLINE_SAFE			\
> > +			    "call *%[func_ptr]\n\t",			\
> 
> This will likely not insert the lfence before the indirect call in
> spectre_v2=eibrs,lfence mode. As X86_FEATURE_INDIRECT_SAFE is not
> cleared when eIBRS is enabled, this will not be converted to direct
> call.

Hm, I think the problem here is that SPECTRE_V2_EIBRS_LFENCE confusingly
sets X86_FEATURE_RETPOLINE.  So the following bit unintentionally takes
effect:

	/* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
		spec_ctrl_disable_kernel_rrsba();
		if (rrsba_disabled)
			return;
	}

If RRSBA gets disabled (which is likely), bhi_select_mitigation()
returns early and X86_FEATURE_INDIRECT_SAFE doesn't get cleared.

"LFENCE; CALL" is most definitely not a retpoline, so it's weird for
SPECTRE_V2_EIBRS_LFENCE to be setting X86_FEATURE_RETPOLINE.  We should
fix that.

Honestly, I think SPECTRE_V2_EIBRS_LFENCE is obsolete anyway.  It was
originally intended to be a BHI mitigation, but the real-world
benchmarks I've seen are showing it to be quite a bit slower than the
actual BHI mitigations.

Plus it's only a partial fix because the speculative window after the
branch can still be big enough to do multiple loads.

For similar reasons I'm thinking we should also remove the non-eIBRS
version (SPECTRE_V2_LFENCE).

I'll make some patches to do that, with warnings printed if somebody
tries to use them.  They can just fall back to the (more secure and
generally faster) defaults.

> [...]
> > @@ -1720,6 +1744,7 @@ static void __init spectre_v2_select_mitigation(void)
> >  
> >  	case SPECTRE_V2_CMD_RETPOLINE_LFENCE:
> >  		pr_err(SPECTRE_V2_LFENCE_MSG);
> > +		setup_clear_cpu_cap(X86_FEATURE_INDIRECT_SAFE);
> 
> I don't know if it intentional, this seems to be the duplicate of
> X86_FEATURE_INDIRECT_SAFE clear later in SPECTRE_V2_LFENCE mode. Also it
> seems a bit odd to do this here in SPECTRE_V2_CMD handling.

Yeah, I accidentally left that in from an earlier implementation.  It's
harmless but I'll clean that up too with a new patch unless Ingo wants
to remove that line.

-- 
Josh