[PATCH v2] arm64: Enable data independent timing (DIT) in the kernel

Ard Biesheuvel
2022-11-07T17:25:25Z

The ARM architecture revision v8.4 introduces a data independent timing
control (DIT) which can be set at any exception level, and instructs the
CPU to avoid optimizations that may result in a correlation between the
execution time of certain instructions and the value of the data they
operate on.

The DIT bit is part of PSTATE, and is therefore context switched as
usual, given that it becomes part of the saved program state (SPSR) when
taking an exception. We have also defined a hwcap for DIT, and so user
space can discover already whether or nor DIT is available. This means
that, as far as user space is concerned, DIT is wired up and fully
functional.

In the kernel, however, we never bothered with DIT: we disable at it
boot (i.e., INIT_PSTATE_EL1 has DIT cleared) and ignore the fact that we
might run with DIT enabled if user space happened to set it.

Currently, we have no idea whether or not running privileged code with
DIT disabled on a CPU that implements support for it may result in a
side channel that exposes privileged data to unprivileged user space
processes, so let's be cautious and just enable DIT while running in the
kernel if supported by all CPUs.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Adam Langley <agl@google.com>
Link: https://lore.kernel.org/all/YwgCrqutxmX0W72r@gmail.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
v2:
- enable DIT on resume-from-suspend path
- tidy up some issues spotted by Eric
- avoid some code duplication in SET_PSTATE_xxx macro definitions
- tweak the commit log so that it doesn't read as if we are fixing an
actual vulnerability

arch/arm64/include/asm/cpufeature.h | 5 +++++
arch/arm64/include/asm/sysreg.h | 12 ++++++++----
arch/arm64/kernel/cpufeature.c | 17 +++++++++++++++++
arch/arm64/kernel/entry.S | 3 +++
arch/arm64/kernel/suspend.c | 2 ++
arch/arm64/tools/cpucaps | 1 +
6 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index f73f11b5504254be..f44579bca9f8107e 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -875,6 +875,11 @@ static inline bool cpu_has_pan(void)
ID_AA64MMFR1_EL1_PAN_SHIFT);
}

+static inline bool cpu_has_dit(void)
+{
+ return cpus_have_const_cap(ARM64_HAS_DIT);
+}
+
#ifdef CONFIG_ARM64_AMU_EXTN
/* Check whether the cpu supports the Activity Monitors Unit (AMU) */
extern bool cpu_has_amu_feat(int cpu);
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 7d301700d1a93692..1f3f52ce407fe942 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -90,20 +90,24 @@
*/
#define pstate_field(op1, op2) ((op1) << Op1_shift | (op2) << Op2_shift)
#define PSTATE_Imm_shift CRm_shift
+#define SET_PSTATE(x, r) __emit_inst(0xd500401f | PSTATE_ ## r | ((!!x) << PSTATE_Imm_shift))

#define PSTATE_PAN pstate_field(0, 4)
#define PSTATE_UAO pstate_field(0, 3)
#define PSTATE_SSBS pstate_field(3, 1)
+#define PSTATE_DIT pstate_field(3, 2)
#define PSTATE_TCO pstate_field(3, 4)

-#define SET_PSTATE_PAN(x) __emit_inst(0xd500401f | PSTATE_PAN | ((!!x) << PSTATE_Imm_shift))
-#define SET_PSTATE_UAO(x) __emit_inst(0xd500401f | PSTATE_UAO | ((!!x) << PSTATE_Imm_shift))
-#define SET_PSTATE_SSBS(x) __emit_inst(0xd500401f | PSTATE_SSBS | ((!!x) << PSTATE_Imm_shift))
-#define SET_PSTATE_TCO(x) __emit_inst(0xd500401f | PSTATE_TCO | ((!!x) << PSTATE_Imm_shift))
+#define SET_PSTATE_PAN(x) SET_PSTATE((x), PAN)
+#define SET_PSTATE_UAO(x) SET_PSTATE((x), UAO)
+#define SET_PSTATE_SSBS(x) SET_PSTATE((x), SSBS)
+#define SET_PSTATE_DIT(x) SET_PSTATE((x), DIT)
+#define SET_PSTATE_TCO(x) SET_PSTATE((x), TCO)

#define set_pstate_pan(x) asm volatile(SET_PSTATE_PAN(x))
#define set_pstate_uao(x) asm volatile(SET_PSTATE_UAO(x))
#define set_pstate_ssbs(x) asm volatile(SET_PSTATE_SSBS(x))
+#define set_pstate_dit(x) asm volatile(SET_PSTATE_DIT(x))

#define __SYS_BARRIER_INSN(CRm, op2, Rt) \
__emit_inst(0xd5000000 | sys_insn(0, 3, 3, (CRm), (op2)) | ((Rt) & 0x1f))
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6062454a90674317..74ceec411ea8b597 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2077,6 +2077,11 @@ static void cpu_trap_el0_impdef(const struct arm64_cpu_capabilities *__unused)
sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_TIDCP);
}

+static void cpu_enable_dit(const struct arm64_cpu_capabilities *__unused)
+{
+ set_pstate_dit(1);
+}
+
/* Internal helper functions to match cpu capability type */
static bool
cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
@@ -2640,6 +2645,18 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.matches = has_cpuid_feature,
.cpu_enable = cpu_trap_el0_impdef,
},
+ {
+ .desc = "Data independent timing control (DIT)",
+ .capability = ARM64_HAS_DIT,
+ .type = ARM64_CPUCAP_SYSTEM_FEATURE,
+ .sys_reg = SYS_ID_AA64PFR0_EL1,
+ .sign = FTR_UNSIGNED,
+ .field_pos = ID_AA64PFR0_EL1_DIT_SHIFT,
+ .field_width = 4,
+ .min_field_value = ID_AA64PFR0_EL1_DIT_IMP,
+ .matches = has_cpuid_feature,
+ .cpu_enable = cpu_enable_dit,
+ },
{},
};

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index e28137d64b7688e2..11cb99c4d298784d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -197,6 +197,9 @@ alternative_cb_end
.endm

.macro kernel_entry, el, regsize = 64
+ .if \el == 0
+ alternative_insn nop, SET_PSTATE_DIT(1), ARM64_HAS_DIT
+ .endif
.if \regsize == 32
mov w0, w0 // zero upper 32 bits of x0
.endif
diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index 8b02d310838f9240..3032a82ea51a19f7 100644
--- a/arch/arm64/kernel/suspend.c
+++ b/arch/arm64/kernel/suspend.c
@@ -60,6 +60,8 @@ void notrace __cpu_suspend_exit(void)
* PSTATE was not saved over suspend/resume, re-enable any detected
* features that might not have been set correctly.
*/
+ if (cpu_has_dit())
+ set_pstate_dit(1);
__uaccess_enable_hw_pan();

/*
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index f1c0347ec31a85c7..a86ee376920a08dd 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -20,6 +20,7 @@ HAS_CNP
HAS_CRC32
HAS_DCPODP
HAS_DCPOP
+HAS_DIT
HAS_E0PD
HAS_ECV
HAS_EPAN
--
2.35.1