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Subject[RFC 05/10] x86/speculation: Add basic IBRS support infrastructure
Date
From: David Woodhouse <dwmw@amazon.co.uk>

Not functional yet; just add the handling for it in the Spectre v2
mitigation selection, and the X86_FEATURE_IBRS flag which will control
the code to be added in later patches.

Also take the #ifdef CONFIG_RETPOLINE from around the RSB-stuffing; IBRS
mode will want that too.

For now we are auto-selecting IBRS on Skylake. We will probably end up
changing that but for now let's default to the safest option.

XX: Do we want a microcode blacklist?

[karahmed: simplify the switch block and get rid of all the magic]

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
---
Documentation/admin-guide/kernel-parameters.txt | 1 +
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/nospec-branch.h | 2 -
arch/x86/kernel/cpu/bugs.c | 108 +++++++++++++++---------
4 files changed, 68 insertions(+), 44 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8122b5f..e597650 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3932,6 +3932,7 @@
retpoline - replace indirect branches
retpoline,generic - google's original retpoline
retpoline,amd - AMD-specific minimal thunk
+ ibrs - Intel: Indirect Branch Restricted Speculation

Not specifying this option is equivalent to
spectre_v2=auto.
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 8ec9588..ae86ad9 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -211,6 +211,7 @@
#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 */
+#define X86_FEATURE_IBRS ( 7*32+21) /* Use IBRS for Spectre v2 safety */

/* Virtualization flags: Linux defined, word 8 */
#define X86_FEATURE_TPR_SHADOW ( 8*32+ 0) /* Intel TPR Shadow */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index c333c95..8759449 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -205,7 +205,6 @@ extern char __indirect_thunk_end[];
*/
static inline void vmexit_fill_RSB(void)
{
-#ifdef CONFIG_RETPOLINE
unsigned long loops;

asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
@@ -215,7 +214,6 @@ static inline void vmexit_fill_RSB(void)
"910:"
: "=r" (loops), ASM_CALL_CONSTRAINT
: : "memory" );
-#endif
}

static inline void indirect_branch_prediction_barrier(void)
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 96548ff..1d5e12f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -79,6 +79,7 @@ enum spectre_v2_mitigation_cmd {
SPECTRE_V2_CMD_RETPOLINE,
SPECTRE_V2_CMD_RETPOLINE_GENERIC,
SPECTRE_V2_CMD_RETPOLINE_AMD,
+ SPECTRE_V2_CMD_IBRS,
};

static const char *spectre_v2_strings[] = {
@@ -87,6 +88,7 @@ static const char *spectre_v2_strings[] = {
[SPECTRE_V2_RETPOLINE_MINIMAL_AMD] = "Vulnerable: Minimal AMD ASM retpoline",
[SPECTRE_V2_RETPOLINE_GENERIC] = "Mitigation: Full generic retpoline",
[SPECTRE_V2_RETPOLINE_AMD] = "Mitigation: Full AMD retpoline",
+ [SPECTRE_V2_IBRS] = "Mitigation: Indirect Branch Restricted Speculation",
};

#undef pr_fmt
@@ -132,9 +134,17 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
spec2_print_if_secure("force enabled on command line.");
return SPECTRE_V2_CMD_FORCE;
} else if (match_option(arg, ret, "retpoline")) {
+ if (!IS_ENABLED(CONFIG_RETPOLINE)) {
+ pr_err("retpoline selected but not compiled in. Switching to AUTO select\n");
+ return SPECTRE_V2_CMD_AUTO;
+ }
spec2_print_if_insecure("retpoline selected on command line.");
return SPECTRE_V2_CMD_RETPOLINE;
} else if (match_option(arg, ret, "retpoline,amd")) {
+ if (!IS_ENABLED(CONFIG_RETPOLINE)) {
+ pr_err("retpoline,amd selected but not compiled in. Switching to AUTO select\n");
+ return SPECTRE_V2_CMD_AUTO;
+ }
if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD) {
pr_err("retpoline,amd selected but CPU is not AMD. Switching to AUTO select\n");
return SPECTRE_V2_CMD_AUTO;
@@ -142,8 +152,19 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
spec2_print_if_insecure("AMD retpoline selected on command line.");
return SPECTRE_V2_CMD_RETPOLINE_AMD;
} else if (match_option(arg, ret, "retpoline,generic")) {
+ if (!IS_ENABLED(CONFIG_RETPOLINE)) {
+ pr_err("retpoline,generic selected but not compiled in. Switching to AUTO select\n");
+ return SPECTRE_V2_CMD_AUTO;
+ }
spec2_print_if_insecure("generic retpoline selected on command line.");
return SPECTRE_V2_CMD_RETPOLINE_GENERIC;
+ } else if (match_option(arg, ret, "ibrs")) {
+ if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
+ pr_err("IBRS selected but no CPU support. Switching to AUTO select\n");
+ return SPECTRE_V2_CMD_AUTO;
+ }
+ spec2_print_if_insecure("IBRS seleted on command line.");
+ return SPECTRE_V2_CMD_IBRS;
} else if (match_option(arg, ret, "auto")) {
return SPECTRE_V2_CMD_AUTO;
}
@@ -156,7 +177,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
return SPECTRE_V2_CMD_NONE;
}

-/* Check for Skylake-like CPUs (for RSB handling) */
+/* Check for Skylake-like CPUs (for RSB and IBRS handling) */
static bool __init is_skylake_era(void)
{
if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
@@ -178,55 +199,58 @@ static void __init spectre_v2_select_mitigation(void)
enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
enum spectre_v2_mitigation mode = SPECTRE_V2_NONE;

- /*
- * If the CPU is not affected and the command line mode is NONE or AUTO
- * then nothing to do.
- */
- if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2) &&
- (cmd == SPECTRE_V2_CMD_NONE || cmd == SPECTRE_V2_CMD_AUTO))
- return;
-
switch (cmd) {
case SPECTRE_V2_CMD_NONE:
+ if (boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
+ pr_err("kernel not compiled with retpoline; no mitigation available!");
return;
-
- case SPECTRE_V2_CMD_FORCE:
- /* FALLTRHU */
- case SPECTRE_V2_CMD_AUTO:
- goto retpoline_auto;
-
- case SPECTRE_V2_CMD_RETPOLINE_AMD:
- if (IS_ENABLED(CONFIG_RETPOLINE))
- goto retpoline_amd;
- break;
- case SPECTRE_V2_CMD_RETPOLINE_GENERIC:
- if (IS_ENABLED(CONFIG_RETPOLINE))
- goto retpoline_generic;
+ case SPECTRE_V2_CMD_IBRS:
+ mode = SPECTRE_V2_IBRS;
+ setup_force_cpu_cap(X86_FEATURE_IBRS);
break;
+ case SPECTRE_V2_CMD_AUTO:
+ if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
+ return;
+ /* Fall through */
+ case SPECTRE_V2_CMD_FORCE:
+ /*
+ * If we have IBRS support, and either Skylake or !RETPOLINE,
+ * then that's what we do.
+ */
+ if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) &&
+ (is_skylake_era() || !retp_compiler())) {
+ mode = SPECTRE_V2_IBRS;
+ setup_force_cpu_cap(X86_FEATURE_IBRS);
+ break;
+ }
+ /* Fall through */
case SPECTRE_V2_CMD_RETPOLINE:
- if (IS_ENABLED(CONFIG_RETPOLINE))
- goto retpoline_auto;
- break;
- }
- pr_err("kernel not compiled with retpoline; no mitigation available!");
- return;
+ case SPECTRE_V2_CMD_RETPOLINE_AMD:
+ if (IS_ENABLED(CONFIG_RETPOLINE) &&
+ boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
+ if (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
+ mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_AMD :
+ SPECTRE_V2_RETPOLINE_MINIMAL_AMD;
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+ break;
+ }

-retpoline_auto:
- if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
- retpoline_amd:
- if (!boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
pr_err("LFENCE not serializing. Switching to generic retpoline\n");
- goto retpoline_generic;
}
- mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_AMD :
- SPECTRE_V2_RETPOLINE_MINIMAL_AMD;
- setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
- setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
- } else {
- retpoline_generic:
- mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_GENERIC :
- SPECTRE_V2_RETPOLINE_MINIMAL;
- setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+ /* Fall through */
+ case SPECTRE_V2_CMD_RETPOLINE_GENERIC:
+ if (IS_ENABLED(CONFIG_RETPOLINE)) {
+ mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_GENERIC :
+ SPECTRE_V2_RETPOLINE_MINIMAL;
+ setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+ break;
+ }
+ /* Fall through */
+ default:
+ if (boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
+ pr_err("kernel not compiled with retpoline; no mitigation available!");
+ return;
}

spectre_v2_enabled = mode;
--
2.7.4
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