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

[Pawan Gupta]

On Thu, Apr 11, 2024 at 08:57:40PM -0700, Josh Poimboeuf wrote:
> 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

I think it is intentional, more on it below.

> 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.

I could be completely wrong here, but my guess is, it is needed because
the thunk call inserted by the compiler with X86_FEATURE_RETPOLINE
provides room for adding the extra lfence.

In order to prefix lfence(3 bytes) indirect call is first converted to
call __x86_indirect_thunk_reg, which has a 5 byte opcode. At runtime,
thunk call is patched to "lfence;call *reg", which is also 3+2=5 bytes.

Thunk call is anyways needed because, there are indirect
calls opcodes that are 3 byte long e.g. call *%r8. So, wherever possible
lfence+call* is inlined, otherwise lfence is executed in a call to thunk,
which then does jmp *%reg.

> Honestly, I think SPECTRE_V2_EIBRS_LFENCE is obsolete anyway.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.

Thats fair.