[PATCH v2.1] x86/speculation: Use IBRS if available before calling into firmware

David Woodhouse
2018-02-11T15:19:28Z

Retpoline means the kernel is safe because it has no indirect branches.
But firmware isn't, so use IBRS for firmware calls if it's available.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
Helps to include the right header file.

arch/x86/include/asm/apm.h | 6 ++++++
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/efi.h | 17 +++++++++++++++--
arch/x86/include/asm/nospec-branch.h | 37 +++++++++++++++++++++++++++---------
arch/x86/kernel/cpu/bugs.c | 12 +++++++++++-
drivers/watchdog/hpwdt.c | 3 +++
6 files changed, 64 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/apm.h b/arch/x86/include/asm/apm.h
index 4d4015d..c356098 100644
--- a/arch/x86/include/asm/apm.h
+++ b/arch/x86/include/asm/apm.h
@@ -7,6 +7,8 @@
#ifndef _ASM_X86_MACH_DEFAULT_APM_H
#define _ASM_X86_MACH_DEFAULT_APM_H

+#include <asm/nospec-branch.h>
+
#ifdef APM_ZERO_SEGS
# define APM_DO_ZERO_SEGS \
"pushl %%ds\n\t" \
@@ -32,6 +34,7 @@ static inline void apm_bios_call_asm(u32 func, u32 ebx_in, u32 ecx_in,
* N.B. We do NOT need a cld after the BIOS call
* because we always save and restore the flags.
*/
+ firmware_restrict_branch_speculation_start();
__asm__ __volatile__(APM_DO_ZERO_SEGS
"pushl %%edi\n\t"
"pushl %%ebp\n\t"
@@ -44,6 +47,7 @@ static inline void apm_bios_call_asm(u32 func, u32 ebx_in, u32 ecx_in,
"=S" (*esi)
: "a" (func), "b" (ebx_in), "c" (ecx_in)
: "memory", "cc");
+ firmware_restrict_branch_speculation_end();
}

static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
@@ -56,6 +60,7 @@ static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
* N.B. We do NOT need a cld after the BIOS call
* because we always save and restore the flags.
*/
+ firmware_restrict_branch_speculation_start();
__asm__ __volatile__(APM_DO_ZERO_SEGS
"pushl %%edi\n\t"
"pushl %%ebp\n\t"
@@ -68,6 +73,7 @@ static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
"=S" (si)
: "a" (func), "b" (ebx_in), "c" (ecx_in)
: "memory", "cc");
+ firmware_restrict_branch_speculation_end();
return error;
}

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 73b5fff..66c1434 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -211,6 +211,7 @@
#define X86_FEATURE_RSB_CTXSW ( 7*32+19) /* "" Fill RSB on context switches */

#define X86_FEATURE_USE_IBPB ( 7*32+21) /* "" Indirect Branch Prediction Barrier enabled */
+#define X86_FEATURE_USE_IBRS_FW ( 7*32+22) /* "" Use IBRS during runtime firmware calls */

/* Virtualization flags: Linux defined, word 8 */
#define X86_FEATURE_TPR_SHADOW ( 8*32+ 0) /* Intel TPR Shadow */
diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index 85f6ccb..a399c1e 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -6,6 +6,7 @@
#include <asm/pgtable.h>
#include <asm/processor-flags.h>
#include <asm/tlb.h>
+#include <asm/nospec-branch.h>

/*
* We map the EFI regions needed for runtime services non-contiguously,
@@ -36,8 +37,18 @@

extern asmlinkage unsigned long efi_call_phys(void *, ...);

-#define arch_efi_call_virt_setup() kernel_fpu_begin()
-#define arch_efi_call_virt_teardown() kernel_fpu_end()
+#define arch_efi_call_virt_setup() \
+({ \
+ kernel_fpu_begin(); \
+ firmware_restrict_branch_speculation_start(); \
+})
+
+#define arch_efi_call_virt_teardown() \
+({ \
+ firmware_restrict_branch_speculation_end(); \
+ kernel_fpu_end(); \
+})
+

/*
* Wrap all the virtual calls in a way that forces the parameters on the stack.
@@ -73,6 +84,7 @@ struct efi_scratch {
efi_sync_low_kernel_mappings(); \
preempt_disable(); \
__kernel_fpu_begin(); \
+ firmware_restrict_branch_speculation_start(); \
\
if (efi_scratch.use_pgd) { \
efi_scratch.prev_cr3 = __read_cr3(); \
@@ -91,6 +103,7 @@ struct efi_scratch {
__flush_tlb_all(); \
} \
\
+ firmware_restrict_branch_speculation_end(); \
__kernel_fpu_end(); \
preempt_enable(); \
})
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 300cc15..788c4da 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -162,17 +162,36 @@ static inline void vmexit_fill_RSB(void)
#endif
}

+#define alternative_msr_write(_msr, _val, _feature) \
+ asm volatile(ALTERNATIVE("", \
+ "movl %[msr], %%ecx\n\t" \
+ "movl %[val], %%eax\n\t" \
+ "movl $0, %%edx\n\t" \
+ "wrmsr", \
+ _feature) \
+ : : [msr] "i" (_msr), [val] "i" (_val) \
+ : "eax", "ecx", "edx", "memory")
+
static inline void indirect_branch_prediction_barrier(void)
{
- asm volatile(ALTERNATIVE("",
- "movl %[msr], %%ecx\n\t"
- "movl %[val], %%eax\n\t"
- "movl $0, %%edx\n\t"
- "wrmsr",
- X86_FEATURE_USE_IBPB)
- : : [msr] "i" (MSR_IA32_PRED_CMD),
- [val] "i" (PRED_CMD_IBPB)
- : "eax", "ecx", "edx", "memory");
+ alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,
+ X86_FEATURE_USE_IBPB);
+}
+
+/*
+ * With retpoline, we must use IBRS to restrict branch prediction
+ * before calling into firmware.
+ */
+static inline void firmware_restrict_branch_speculation_start(void)
+{
+ alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,
+ X86_FEATURE_USE_IBRS_FW);
+}
+
+static inline void firmware_restrict_branch_speculation_end(void)
+{
+ alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
+ X86_FEATURE_USE_IBRS_FW);
}

#endif /* __ASSEMBLY__ */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 61152aa..6f6d763 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -303,6 +303,15 @@ static void __init spectre_v2_select_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
pr_info("Enabling Indirect Branch Prediction Barrier\n");
}
+
+ /*
+ * Retpoline means the kernel is safe because it has no indirect
+ * branches. But firmware isn't, so use IBRS to protect that.
+ */
+ if (boot_cpu_has(X86_FEATURE_IBRS)) {
+ setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
+ pr_info("Enabling Restricted Speculation for firmware calls\n");
+ }
}

#undef pr_fmt
@@ -332,8 +341,9 @@ ssize_t cpu_show_spectre_v2(struct device *dev,
if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
return sprintf(buf, "Not affected\n");

- return sprintf(buf, "%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
+ return sprintf(buf, "%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
boot_cpu_has(X86_FEATURE_USE_IBPB) ? ", IBPB" : "",
+ boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
spectre_v2_module_string());
}
#endif
diff --git a/drivers/watchdog/hpwdt.c b/drivers/watchdog/hpwdt.c
index 67fbe35..bab3721 100644
--- a/drivers/watchdog/hpwdt.c
+++ b/drivers/watchdog/hpwdt.c
@@ -38,6 +38,7 @@
#endif /* CONFIG_HPWDT_NMI_DECODING */
#include <asm/nmi.h>
#include <asm/frame.h>
+#include <asm/nospec-branch.h>

#define HPWDT_VERSION "1.4.0"
#define SECS_TO_TICKS(secs) ((secs) * 1000 / 128)
@@ -486,11 +487,13 @@ static int hpwdt_pretimeout(unsigned int ulReason, struct pt_regs *regs)
if (!hpwdt_nmi_decoding)
return NMI_DONE;

+ firmware_restrict_branch_speculation_start();
spin_lock_irqsave(&rom_lock, rom_pl);
if (!die_nmi_called && !is_icru && !is_uefi)
asminline_call(&cmn_regs, cru_rom_addr);
die_nmi_called = 1;
spin_unlock_irqrestore(&rom_lock, rom_pl);
+ firmware_restrict_branch_speculation_end();

if (allow_kdump)
hpwdt_stop();
--
2.7.4