Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware
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- Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware
- From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- Date: Wed, 14 Feb 2018 09:57:32 +0100
- Cc: Ingo Molnar <mingo@xxxxxxxxxx>, Dave Hansen <dave@xxxxxxxx>, hpa@xxxxxxxxx, tglx@xxxxxxxxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, dwmw@xxxxxxxxxxxx, linux-tip-commits@xxxxxxxxxxxxxxx, Borislav Petkov <bp@xxxxxxxxx>, Arjan van de Ven <arjan@xxxxxxxxxxxxx>
- In-reply-to: <20180214085614.GT25181@hirez.programming.kicks-ass.net>
- References: <1518362359-1005-1-git-send-email-dwmw@amazon.co.uk> <tip-670c3e8da87fa4046a55077b1409cf250865a203@git.kernel.org> <20180212102211.cdrrqqd4hdw7xu5y@gmail.com> <a85c28b8-1255-3264-bb4a-89e85e3c6009@sr71.net> <20180212165835.GO25181@hirez.programming.kicks-ass.net> <20180213075540.3lkikkpgjoe6ocjk@gmail.com> <5c3ba123-abbe-f153-7b75-a89d31d25c72@linux.intel.com> <20180214085614.GT25181@hirez.programming.kicks-ass.net>
- User-agent: Mutt/1.9.2 (2017-12-15)
On Wed, Feb 14, 2018 at 09:56:14AM +0100, Peter Zijlstra wrote:
> 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 */
s/from/more/
typing hard.
> this_cpu_dec_return() // 1->0
> wrmsr(SPEC_CTRL, 0);
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