Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB
From: KarimAllah Ahmed
Date: Sun Jan 21 2018 - 13:30:13 EST
On 01/21/2018 07:06 PM, Borislav Petkov wrote:
On Sun, Jan 21, 2018 at 09:49:06AM +0000, David Woodhouse wrote:
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Expose indirect_branch_prediction_barrier() for use in subsequent patches.
[karahmed: remove the special-casing of skylake for using IBPB (wtf?),
switch to using ALTERNATIVES instead of static_cpu_has]
[dwmw2: set up ax/cx/dx in the asm too so it gets NOP'd out]
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/nospec-branch.h | 16 ++++++++++++++++
arch/x86/kernel/cpu/bugs.c | 7 +++++++
3 files changed, 24 insertions(+)
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 8c9e5c0..cf28399 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -207,6 +207,7 @@
#define X86_FEATURE_RETPOLINE_AMD ( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
#define X86_FEATURE_INTEL_PPIN ( 7*32+14) /* Intel Processor Inventory Number */
+#define X86_FEATURE_IBPB ( 7*32+16) /* Using Indirect Branch Prediction Barrier */
Right, and as AMD has a separate bit for this in CPUID_80000008_EBX[12],
we probably don't really need the synthetic bit here but simply use the
one at (13*32+12) - word 13.
#define X86_FEATURE_AMD_PRED_CMD ( 7*32+17) /* Prediction Command MSR (AMD) */
#define X86_FEATURE_MBA ( 7*32+18) /* Memory Bandwidth Allocation */
#define X86_FEATURE_RSB_CTXSW ( 7*32+19) /* Fill RSB on context switches */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 4ad4108..c333c95 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -218,5 +218,21 @@ static inline void vmexit_fill_RSB(void)
#endif
}
+static inline void indirect_branch_prediction_barrier(void)
I like ibp_barrier() better.
+{
+ unsigned long ax, cx, dx;
+
+ asm volatile(ALTERNATIVE("",
+ "movl %[msr], %%ecx\n\t"
+ "movl %[val], %%eax\n\t"
+ "movl $0, %%edx\n\t"
+ "wrmsr",
+ X86_FEATURE_IBPB)
+ : "=a" (ax), "=c" (cx), "=d" (dx)
+ : [msr] "i" (MSR_IA32_PRED_CMD),
+ [val] "i" (PRED_CMD_IBPB)
+ : "memory");
+}
Btw, we can simplify this a bit by dropping the inputs and marking the 3
GPRs as clobbered:
alternative_input("",
"mov $0x49, %%ecx\n\t"
"mov $1, %%eax\n\t"
"xor %%edx, %%edx\n\t"
"wrmsr\n\t",
X86_FEATURE_IBPB,
ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
The "memory" clobber is probably not really needed but it wouldn't
hurt...
Also, above says:
switch to using ALTERNATIVES instead of static_cpu_has]
Why?
if (static_cpu_has(X86_FEATURE_IBPB))
wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
It can't get any more readable than this. Why even f*ck with
alternatives?
Because static_cpu_has is an indirect branch which will cause
speculation and
we have to avoid that.
David told me that Peter was working on a fix for static_cpu_has to
avoid the
speculation but I do not know what is the status of this.
+
#endif /* __ASSEMBLY__ */
#endif /* __NOSPEC_BRANCH_H__ */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 390b3dc..96548ff 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -249,6 +249,13 @@ static void __init spectre_v2_select_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
pr_info("Filling RSB on context switch\n");
}
+
+ /* Initialize Indirect Branch Prediction Barrier if supported */
+ if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+ boot_cpu_has(X86_FEATURE_AMD_PRED_CMD)) {
+ setup_force_cpu_cap(X86_FEATURE_IBPB);
+ pr_info("Enabling Indirect Branch Prediction Barrier\n");
We don't really need the pr_info as "ibpb" will appear in /proc/cpuinfo.
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