On 09/20/2018 04:38 PM, Lendacky, Thomas wrote:
> On 09/19/2018 04:35 PM, Tim Chen wrote:
>> This patchset provides an option to apply IBPB and STIBP mitigation
>> to only non-dumpable processes.
>>
>> Jiri's patch to harden spectre_v2 makes IBPB and STIBP available for
>> general spectre v2 app to app mitigation. IBPB will be issued for
>> switching to an app that's not ptraceable by the previous
>> app and STIBP will be always turned on.
>>
>> However, leaving STIBP on all the time is expensive for certain
>> applications that have frequent indirect branches. One such application
>> is perlbench in the SpecInt Rate 2006 test suite which shows a
>> 21% reduction in throughput. Other application like bzip2 in
>> the same test suite with minimal indirct branches have
>> only a 0.7% reduction in throughput. IBPB will also impose
>> overhead during context switches.
>>
>> App to app exploit is in general difficult
>> due to address space layout randomization in apps and
>> the need to know an app's address space layout ahead of time.
>> Users may not wish to incur app to app performance
>> overhead from IBPB and STIBP for general non security sensitive apps
>> and use these mitigations only for non-dumpable apps.
>>
>> The first patch provides a lite option for spectre_v2 app to app
>> mitigation where IBPB is only issued for security sensitive
>> non-dumpable app. The second patch extends this option
>> where STIBP is only issued for non-dumpable app.
>>
>> The changes apply to intel cpus affected by spectre_v2. Tom,
>> can you update the STIBP changes for AMD cpus on
>> __speculative_store_bypass_update and x86_virt_spec_ctrl
>> to update the SPEC_CTRL msr for AMD cpu?
>
> Hi Tim,
>
> Let me think about this a bit, since it can get a bit tricky if
> I want to avoid multiple MSR writes when only one may have been
> needed (assuming SSBD is not using the SPEC_CTRL MSR).
I think something along the lines of the following would work and
prevent any extra MSR writes for AMD when SSBD is not using the
SPEC_CTRL MSR. Let me know what you think (tglx, especially, since
he was heavily involved in this part):
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 878301d..d093d85 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -397,26 +397,59 @@ static __always_inline void
amd_set_ssb_virt_state(unsigned long tifn)
static __always_inline void intel_set_spec_ctrl_state(unsigned long tifn)
{
- u64 msr = x86_spec_ctrl_base | ssbd_tif_to_spec_ctrl(tifn)
- | stibp_tif_to_spec_ctrl(tifn);
+ u64 msr = x86_spec_ctrl_base;
+
+ /*
+ * AMD we may have used a different method to update SSBD, so
+ * we need to be sure we are using the SPEC_CTRL MSR for SSBD.
+ */
+ if (static_cpu_has(X86_FEATURE_SSBD))
+ x86_spec_ctrl_base |= ssbd_tif_to_spec_ctrl(tifn);
+
+ x86_spec_ctrl_base |= stibp_tif_to_spec_ctrl(tifn);
wrmsrl(MSR_IA32_SPEC_CTRL, msr);
}
-static __always_inline void __speculative_store_bypass_update(unsigned long
tifn)
+static __always_inline void __speculative_store_bypass_update(unsigned long
tifp,
+ unsigned long
tifn)
{
- if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
- amd_set_ssb_virt_state(tifn);
- else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
- amd_set_core_ssb_state(tifn);
- else
- intel_set_spec_ctrl_state(tifn);
+ bool stibp = !!((tifp ^ tifn) & _TIF_STIBP);
+ bool ssbd = !!((tifp ^ tifn) & _TIF_SSBD);
+
+ if (!ssbd && !stibp)
+ return;
+
+ if (ssbd) {
+ /*
+ * For AMD, try these methods first. The ssbd variable will
+ * reflect if the SPEC_CTRL MSR method is needed.
+ */
+ ssbd = false;
+
+ if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
+ amd_set_ssb_virt_state(tifn);
+ else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
+ amd_set_core_ssb_state(tifn);
+ else
+ ssbd = true;
+ }
+
+ /* Avoid a possible extra MSR write, recheck the flags */
+ if (!ssbd && !stibp)
+ return;
+
+ intel_set_spec_ctrl_state(tifn);
}
void speculative_store_bypass_update(unsigned long tif)
{
+ /*
+ * On this path we're forcing the update, so use ~tif as the
+ * previous flags.
+ */
preempt_disable();
- __speculative_store_bypass_update(tif);
+ __speculative_store_bypass_update(~tif, tif);
preempt_enable();
}
@@ -452,8 +485,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct
task_struct *next_p,
if ((tifp ^ tifn) & _TIF_NOCPUID)
set_cpuid_faulting(!!(tifn & _TIF_NOCPUID));
- if ((tifp ^ tifn) & (_TIF_SSBD | _TIF_STIBP))
- __speculative_store_bypass_update(tifn);
+ __speculative_store_bypass_update(tifp, tifn);
}
/*
Thanks,
Tom
>
> Thanks,
> Tom
>
>>
>> Thanks.
>>
>> Tim
>>
>> Tim Chen (2):
>> x86/speculation: Option to select app to app mitigation for spectre_v2
>> x86/speculation: Provide application property based STIBP protection
>>
>> Documentation/admin-guide/kernel-parameters.txt | 11 +++
>> arch/x86/include/asm/msr-index.h | 3 +-
>> arch/x86/include/asm/nospec-branch.h | 9 ++
>> arch/x86/include/asm/spec-ctrl.h | 12 +++
>> arch/x86/include/asm/thread_info.h | 4 +-
>> arch/x86/kernel/cpu/bugs.c | 105
>> ++++++++++++++++++++++--
>> arch/x86/kernel/process.c | 9 +-
>> arch/x86/mm/tlb.c | 41 ++++++++-
>> 8 files changed, 179 insertions(+), 15 deletions(-)
>>
