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