Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

Dave Hansen
2018-02-12T16:20:02Z

On 02/12/2018 02:22 AM, Ingo Molnar wrote:
>> +static inline void firmware_restrict_branch_speculation_end(void)
>> +{
>> + alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
>> + X86_FEATURE_USE_IBRS_FW);
> BTW., there's a detail that only occurred to me today, this enabling/disabling
> sequence is not NMI safe, and it might be called from NMI context:

FWIW, Tim Chen and I talked about this a bunch. We ended up just
saving/restoring the MSR verbatim in the NMI handler the same way we do
CR3, stashing it in a high general-purpose-register (r%12?). That costs
a RDMSR (at least) and an WRMSR (which you can optimize out). We have a
patch for that somewhere if anybody wants it.

We also came to the same conclusion that it's a rather challenging thing
to exploit these cases, especially when you consider that we can easily
do RSB stuffing on NMI exit just before going back to the firmware.