[PATCH 6.18.y 2/6] x86/bugs: Enable IBPB flush on BPF JIT allocation

commit a3af84b0fa00ead01fcd0e28b5d773ff25990a0d upstream.

Enable hardening against JIT spraying when Spectre-v2 mitigations are in
use. Specifically, issue an IBPB flush on BPF JIT memory reuse. Skip
enabling the IBPB flush if the BPF dispatcher is already using a retpoline
sequence.

This hardening applies only when BPF-JIT is in use. Guard the enabling
under CONFIG_BPF_JIT so that bugs.c still builds with CONFIG_BPF_JIT=n.

Signed-off-by: Pawan Gupta <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
---
 arch/x86/include/asm/nospec-branch.h |  4 +++
 arch/x86/kernel/cpu/bugs.c           | 50 ++++++++++++++++++++++++++++++++----
 2 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 08ed5a2e46a5..71e9861a16a8 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -386,6 +386,10 @@ extern void srso_alias_return_thunk(void);
 extern void entry_untrain_ret(void);
 extern void write_ibpb(void);
 
+#ifdef CONFIG_BPF_JIT
+extern void bpf_arch_ibpb(void);
+#endif
+
 #ifdef CONFIG_X86_64
 extern void clear_bhb_loop(void);
 #endif
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d7fa03bf51b4..fd0b7880cf7e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -16,6 +16,7 @@
 #include <linux/sched/smt.h>
 #include <linux/pgtable.h>
 #include <linux/bpf.h>
+#include <linux/filter.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
@@ -1796,8 +1797,21 @@ static inline const char *spectre_v2_module_string(void)
 {
 	return spectre_v2_bad_module ? " - vulnerable module loaded" : "";
 }
+
+/*
+ * The "retpoline sequence" is the "call;mov;ret" sequence that
+ * replaces normal indirect branch instructions. Differentiate
+ * *the* retpoline sequence from the LFENCE-prefixed indirect
+ * branches that simply use the retpoline infrastructure.
+ */
+static inline bool retpoline_seq_enabled(void)
+{
+	return boot_cpu_has(X86_FEATURE_RETPOLINE) && !boot_cpu_has(X86_FEATURE_RETPOLINE_LFENCE);
+}
+
 #else
 static inline const char *spectre_v2_module_string(void) { return ""; }
+static inline bool retpoline_seq_enabled(void) { return false; }
 #endif
 
 #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
@@ -2240,8 +2254,7 @@ static void __init bhi_apply_mitigation(void)
 		return;
 
 	/* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
-	if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
-	    !boot_cpu_has(X86_FEATURE_RETPOLINE_LFENCE)) {
+	if (retpoline_seq_enabled()) {
 		spec_ctrl_disable_kernel_rrsba();
 		if (rrsba_disabled)
 			return;
@@ -2383,6 +2396,27 @@ static void __init spectre_v2_update_mitigation(void)
 		pr_info("%s\n", spectre_v2_strings[spectre_v2_enabled]);
 }
 
+#ifdef CONFIG_BPF_JIT
+static void __bpf_arch_ibpb(void *unused)
+{
+	write_ibpb();
+}
+
+void bpf_arch_ibpb(void)
+{
+	on_each_cpu(__bpf_arch_ibpb, NULL, 1);
+}
+
+static bool __init cpu_wants_ibpb_bpf(void)
+{
+	/* A genuine retpoline already neutralizes ring0 indirect predictions */
+	if (retpoline_seq_enabled())
+		return false;
+
+	return boot_cpu_has(X86_FEATURE_IBPB);
+}
+#endif
+
 static void __init spectre_v2_apply_mitigation(void)
 {
 	if (spectre_v2_enabled == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
@@ -2459,6 +2493,14 @@ static void __init spectre_v2_apply_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
 		pr_info("Enabling Restricted Speculation for firmware calls\n");
 	}
+
+#ifdef CONFIG_BPF_JIT
+	if (cpu_wants_ibpb_bpf()) {
+		static_call_update(bpf_arch_pred_flush, bpf_arch_ibpb);
+		static_branch_enable(&bpf_pred_flush_enabled);
+		pr_info("Enabling IBPB for BPF\n");
+	}
+#endif
 }
 
 static void update_stibp_msr(void * __unused)
@@ -3544,9 +3586,7 @@ static const char *spectre_bhi_state(void)
 		return "; BHI: BHI_DIS_S";
 	else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
 		return "; BHI: SW loop, KVM: SW loop";
-	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
-		 !boot_cpu_has(X86_FEATURE_RETPOLINE_LFENCE) &&
-		 rrsba_disabled)
+	else if (retpoline_seq_enabled() && rrsba_disabled)
 		return "; BHI: Retpoline";
 	else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_VMEXIT))
 		return "; BHI: Vulnerable, KVM: SW loop";

-- 
2.43.0