Messages in this thread | | | Subject | Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware | From | Dave Hansen <> | Date | Mon, 12 Feb 2018 08:13:31 -0800 |
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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.
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