Messages in this thread | | | Date | Wed, 14 Feb 2018 09:56:14 +0100 | From | Peter Zijlstra <> | Subject | Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware |
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On Tue, Feb 13, 2018 at 05:49:47PM -0800, Tim Chen wrote:
> static inline void firmware_restrict_branch_speculation_start(void) > { > + if (this_cpu_inc_return(spec_ctrl_ibrs_fw_depth) == 1) > + alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS, > X86_FEATURE_USE_IBRS_FW); > } > > static inline void firmware_restrict_branch_speculation_end(void) > { > + if (this_cpu_dec_return(spec_ctrl_ibrs_fw_depth) == 0) > + alternative_msr_write(MSR_IA32_SPEC_CTRL, 0, > + X86_FEATURE_USE_IBRS_FW); > }
At the very least this must disable and re-enable preemption, such that we guarantee we inc/dec the same counter. ISTR some firmware calls (EFI) actually are preemptible so that wouldn't work.
Further, consider:
this_cpu_inc_return() // 0->1 <NMI> this_cpu_inc_return() // 1->2 call_broken_arse_firmware() this_cpu_dec_return() // 2->1 </NMI> wrmsr(SPEC_CTRL, IBRS);
/* from dodgy firmware crap */
this_cpu_dec_return() // 1->0 wrmsr(SPEC_CTRL, 0);
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