[PATCH 4.19 44/57] arm64: Use the clearbhb instruction in mitigations

Greg Kroah-Hartman
2022-03-21T14:00:36Z

From: James Morse <james.morse@arm.com>

commit 228a26b912287934789023b4132ba76065d9491c upstream.

Future CPUs may implement a clearbhb instruction that is sufficient
to mitigate SpectreBHB. CPUs that implement this instruction, but
not CSV2.3 must be affected by Spectre-BHB.

Add support to use this instruction as the BHB mitigation on CPUs
that support it. The instruction is in the hint space, so it will
be treated by a NOP as older CPUs.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[ modified for stable: Use a KVM vector template instead of alternatives ]
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/include/asm/assembler.h | 7 +++++++
arch/arm64/include/asm/cpufeature.h | 13 +++++++++++++
arch/arm64/include/asm/sysreg.h | 3 +++
arch/arm64/include/asm/vectors.h | 7 +++++++
arch/arm64/kernel/cpu_errata.c | 14 ++++++++++++++
arch/arm64/kernel/cpufeature.c | 1 +
arch/arm64/kernel/entry.S | 8 ++++++++
arch/arm64/kvm/hyp/hyp-entry.S | 6 ++++++
8 files changed, 59 insertions(+)

--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -127,6 +127,13 @@
.endm

/*
+ * Clear Branch History instruction
+ */
+ .macro clearbhb
+ hint #22
+ .endm
+
+/*
* Sanitise a 64-bit bounded index wrt speculation, returning zero if out
* of bounds.
*/
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -497,6 +497,19 @@ static inline bool supports_csv2p3(int s
return csv2_val == 3;
}

+static inline bool supports_clearbhb(int scope)
+{
+ u64 isar2;
+
+ if (scope == SCOPE_LOCAL_CPU)
+ isar2 = read_sysreg_s(SYS_ID_AA64ISAR2_EL1);
+ else
+ isar2 = read_sanitised_ftr_reg(SYS_ID_AA64ISAR2_EL1);
+
+ return cpuid_feature_extract_unsigned_field(isar2,
+ ID_AA64ISAR2_CLEARBHB_SHIFT);
+}
+
static inline bool system_supports_32bit_el0(void)
{
return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -527,6 +527,9 @@
#define ID_AA64ISAR1_JSCVT_SHIFT 12
#define ID_AA64ISAR1_DPB_SHIFT 0

+/* id_aa64isar2 */
+#define ID_AA64ISAR2_CLEARBHB_SHIFT 28
+
/* id_aa64pfr0 */
#define ID_AA64PFR0_CSV3_SHIFT 60
#define ID_AA64PFR0_CSV2_SHIFT 56
--- a/arch/arm64/include/asm/vectors.h
+++ b/arch/arm64/include/asm/vectors.h
@@ -33,6 +33,12 @@ enum arm64_bp_harden_el1_vectors {
* canonical vectors.
*/
EL1_VECTOR_BHB_FW,
+
+ /*
+ * Use the ClearBHB instruction, before branching to the canonical
+ * vectors.
+ */
+ EL1_VECTOR_BHB_CLEAR_INSN,
#endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */

/*
@@ -44,6 +50,7 @@ enum arm64_bp_harden_el1_vectors {
#ifndef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
#define EL1_VECTOR_BHB_LOOP -1
#define EL1_VECTOR_BHB_FW -1
+#define EL1_VECTOR_BHB_CLEAR_INSN -1
#endif /* !CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */

/* The vectors to use on return from EL0. e.g. to remap the kernel */
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -106,6 +106,8 @@ extern char __spectre_bhb_loop_k24_start
extern char __spectre_bhb_loop_k24_end[];
extern char __spectre_bhb_loop_k32_start[];
extern char __spectre_bhb_loop_k32_end[];
+extern char __spectre_bhb_clearbhb_start[];
+extern char __spectre_bhb_clearbhb_end[];

static void __copy_hyp_vect_bpi(int slot, const char *hyp_vecs_start,
const char *hyp_vecs_end)
@@ -969,6 +971,7 @@ static void update_mitigation_state(enum
* - Mitigated by a branchy loop a CPU specific number of times, and listed
* in our "loop mitigated list".
* - Mitigated in software by the firmware Spectre v2 call.
+ * - Has the ClearBHB instruction to perform the mitigation.
* - Has the 'Exception Clears Branch History Buffer' (ECBHB) feature, so no
* software mitigation in the vectors is needed.
* - Has CSV2.3, so is unaffected.
@@ -1108,6 +1111,9 @@ bool is_spectre_bhb_affected(const struc
if (supports_csv2p3(scope))
return false;

+ if (supports_clearbhb(scope))
+ return true;
+
if (spectre_bhb_loop_affected(scope))
return true;

@@ -1148,6 +1154,8 @@ static const char *kvm_bhb_get_vecs_end(
return __spectre_bhb_loop_k24_end;
else if (start == __spectre_bhb_loop_k32_start)
return __spectre_bhb_loop_k32_end;
+ else if (start == __spectre_bhb_clearbhb_start)
+ return __spectre_bhb_clearbhb_end;

return NULL;
}
@@ -1187,6 +1195,7 @@ static void kvm_setup_bhb_slot(const cha
#define __spectre_bhb_loop_k8_start NULL
#define __spectre_bhb_loop_k24_start NULL
#define __spectre_bhb_loop_k32_start NULL
+#define __spectre_bhb_clearbhb_start NULL

static void kvm_setup_bhb_slot(const char *hyp_vecs_start) { };
#endif
@@ -1206,6 +1215,11 @@ void spectre_bhb_enable_mitigation(const
pr_info_once("spectre-bhb mitigation disabled by command line option\n");
} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
state = SPECTRE_MITIGATED;
+ } else if (supports_clearbhb(SCOPE_LOCAL_CPU)) {
+ kvm_setup_bhb_slot(__spectre_bhb_clearbhb_start);
+ this_cpu_set_vectors(EL1_VECTOR_BHB_CLEAR_INSN);
+
+ state = SPECTRE_MITIGATED;
} else if (spectre_bhb_loop_affected(SCOPE_LOCAL_CPU)) {
switch (spectre_bhb_loop_affected(SCOPE_SYSTEM)) {
case 8:
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -151,6 +151,7 @@ static const struct arm64_ftr_bits ftr_i
};

static const struct arm64_ftr_bits ftr_id_aa64isar2[] = {
+ ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_HIGHER_SAFE, ID_AA64ISAR2_CLEARBHB_SHIFT, 4, 0),
ARM64_FTR_END,
};

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -981,6 +981,7 @@ alternative_else_nop_endif
#define BHB_MITIGATION_NONE 0
#define BHB_MITIGATION_LOOP 1
#define BHB_MITIGATION_FW 2
+#define BHB_MITIGATION_INSN 3

.macro tramp_ventry, vector_start, regsize, kpti, bhb
.align 7
@@ -997,6 +998,11 @@ alternative_else_nop_endif
__mitigate_spectre_bhb_loop x30
.endif // \bhb == BHB_MITIGATION_LOOP

+ .if \bhb == BHB_MITIGATION_INSN
+ clearbhb
+ isb
+ .endif // \bhb == BHB_MITIGATION_INSN
+
.if \kpti == 1
/*
* Defend against branch aliasing attacks by pushing a dummy
@@ -1073,6 +1079,7 @@ ENTRY(tramp_vectors)
#ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
generate_tramp_vector kpti=1, bhb=BHB_MITIGATION_LOOP
generate_tramp_vector kpti=1, bhb=BHB_MITIGATION_FW
+ generate_tramp_vector kpti=1, bhb=BHB_MITIGATION_INSN
#endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
generate_tramp_vector kpti=1, bhb=BHB_MITIGATION_NONE
END(tramp_vectors)
@@ -1135,6 +1142,7 @@ ENTRY(__bp_harden_el1_vectors)
#ifdef CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY
generate_el1_vector bhb=BHB_MITIGATION_LOOP
generate_el1_vector bhb=BHB_MITIGATION_FW
+ generate_el1_vector bhb=BHB_MITIGATION_INSN
#endif /* CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY */
END(__bp_harden_el1_vectors)
.popsection
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -392,4 +392,10 @@ ENTRY(__spectre_bhb_loop_k32_start)
ldp x0, x1, [sp, #(8 * 0)]
add sp, sp, #(8 * 2)
ENTRY(__spectre_bhb_loop_k32_end)
+
+ENTRY(__spectre_bhb_clearbhb_start)
+ esb
+ clearbhb
+ isb
+ENTRY(__spectre_bhb_clearbhb_end)
#endif