[PATCH 5.10 41/58] arm64: Use the clearbhb instruction in mitigations

From: Greg Kroah-Hartman
Date: Thu Mar 10 2022 - 09:36:03 EST

From: James Morse <james.morse@xxxxxxx>

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@xxxxxxxxxxxxxxx>
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
[ modified for stable: Use a KVM vector template instead of alternatives,
removed bitmap of mitigations ]
Signed-off-by: James Morse <james.morse@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/arm64/include/asm/assembler.h | 7 +++++++
arch/arm64/include/asm/cpufeature.h | 13 +++++++++++++
arch/arm64/include/asm/insn.h | 1 +
arch/arm64/include/asm/kvm_asm.h | 2 ++
arch/arm64/include/asm/sysreg.h | 1 +
arch/arm64/include/asm/vectors.h | 7 +++++++
arch/arm64/kernel/cpufeature.c | 1 +
arch/arm64/kernel/entry.S | 8 ++++++++
arch/arm64/kernel/proton-pack.c | 12 ++++++++++++
arch/arm64/kvm/hyp/smccc_wa.S | 9 +++++++++
10 files changed, 61 insertions(+)

--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -98,6 +98,13 @@
.endm

/*
+ * Clear Branch History instruction
+ */
+ .macro clearbhb
+ hint #22
+ .endm
+
+/*
* Speculation barrier
*/
.macro sb
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -621,6 +621,19 @@ 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/insn.h
+++ b/arch/arm64/include/asm/insn.h
@@ -65,6 +65,7 @@ enum aarch64_insn_hint_cr_op {
AARCH64_INSN_HINT_PSB = 0x11 << 5,
AARCH64_INSN_HINT_TSB = 0x12 << 5,
AARCH64_INSN_HINT_CSDB = 0x14 << 5,
+ AARCH64_INSN_HINT_CLEARBHB = 0x16 << 5,

AARCH64_INSN_HINT_BTI = 0x20 << 5,
AARCH64_INSN_HINT_BTIC = 0x22 << 5,
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -37,6 +37,7 @@
#define __SMCCC_WORKAROUND_1_SMC_SZ 36
#define __SMCCC_WORKAROUND_3_SMC_SZ 36
#define __SPECTRE_BHB_LOOP_SZ 44
+#define __SPECTRE_BHB_CLEARBHB_SZ 12

#define KVM_HOST_SMCCC_ID(id) \
ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL, \
@@ -205,6 +206,7 @@ extern char __smccc_workaround_3_smc[__S
extern char __spectre_bhb_loop_k8[__SPECTRE_BHB_LOOP_SZ];
extern char __spectre_bhb_loop_k24[__SPECTRE_BHB_LOOP_SZ];
extern char __spectre_bhb_loop_k32[__SPECTRE_BHB_LOOP_SZ];
+extern char __spectre_bhb_clearbhb[__SPECTRE_BHB_LOOP_SZ];

/*
* Obtain the PC-relative address of a kernel symbol
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -689,6 +689,7 @@
#define ID_AA64ISAR1_GPI_IMP_DEF 0x1

/* id_aa64isar2 */
+#define ID_AA64ISAR2_CLEARBHB_SHIFT 28
#define ID_AA64ISAR2_RPRES_SHIFT 4
#define ID_AA64ISAR2_WFXT_SHIFT 0

--- a/arch/arm64/include/asm/vectors.h
+++ b/arch/arm64/include/asm/vectors.h
@@ -32,6 +32,12 @@ 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 */

/*
@@ -43,6 +49,7 @@ 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/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -211,6 +211,7 @@ 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_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR2_RPRES_SHIFT, 4, 0),
ARM64_FTR_END,
};
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -827,6 +827,7 @@ 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
@@ -843,6 +844,11 @@ 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
@@ -919,6 +925,7 @@ SYM_CODE_START_NOALIGN(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
SYM_CODE_END(tramp_vectors)
@@ -981,6 +988,7 @@ SYM_CODE_START(__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 */
SYM_CODE_END(__bp_harden_el1_vectors)
.popsection
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -824,6 +824,7 @@ int arch_prctl_spec_ctrl_get(struct task
* - 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.
@@ -949,6 +950,9 @@ 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;

@@ -987,6 +991,8 @@ static int kvm_bhb_get_vecs_size(const c
start == __spectre_bhb_loop_k24 ||
start == __spectre_bhb_loop_k32)
return __SPECTRE_BHB_LOOP_SZ;
+ else if (start == __spectre_bhb_clearbhb)
+ return __SPECTRE_BHB_CLEARBHB_SZ;

return 0;
}
@@ -1027,6 +1033,7 @@ static void kvm_setup_bhb_slot(const cha
#define __spectre_bhb_loop_k8 NULL
#define __spectre_bhb_loop_k24 NULL
#define __spectre_bhb_loop_k32 NULL
+#define __spectre_bhb_clearbhb NULL

static void kvm_setup_bhb_slot(const char *hyp_vecs_start) { }
#endif /* CONFIG_KVM */
@@ -1046,6 +1053,11 @@ 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);
+ 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/kvm/hyp/smccc_wa.S
+++ b/arch/arm64/kvm/hyp/smccc_wa.S
@@ -96,3 +96,12 @@ SYM_DATA_START(__spectre_bhb_loop_k32)
1: .org __spectre_bhb_loop_k32 + __SPECTRE_BHB_LOOP_SZ
.org 1b
SYM_DATA_END(__spectre_bhb_loop_k32)
+
+ .global __spectre_bhb_clearbhb
+SYM_DATA_START(__spectre_bhb_clearbhb)
+ esb
+ clearbhb
+ isb
+1: .org __spectre_bhb_clearbhb + __SPECTRE_BHB_CLEARBHB_SZ
+ .org 1b
+SYM_DATA_END(__spectre_bhb_clearbhb)