[PATCH v7 03/22] arm64: Allow mismatched 32-bit EL0 support

From: Will Deacon
Date: Tue May 25 2021 - 11:15:35 EST

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When confronted with a mixture of CPUs, some of which support 32-bit
applications and others which don't, we quite sensibly treat the system
as 64-bit only for userspace and prevent execve() of 32-bit binaries.

Unfortunately, some crazy folks have decided to build systems like this
with the intention of running 32-bit applications, so relax our
sanitisation logic to continue to advertise 32-bit support to userspace
on these systems and track the real 32-bit capable cores in a cpumask
instead. For now, the default behaviour remains but will be tied to
a command-line option in a later patch.

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
arch/arm64/include/asm/cpufeature.h | 8 +-
arch/arm64/kernel/cpufeature.c | 114 ++++++++++++++++++++++++----
arch/arm64/tools/cpucaps | 3 +-
3 files changed, 110 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 338840c00e8e..603bf4160cd6 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -630,9 +630,15 @@ static inline bool cpu_supports_mixed_endian_el0(void)
return id_aa64mmfr0_mixed_endian_el0(read_cpuid(ID_AA64MMFR0_EL1));
}

+const struct cpumask *system_32bit_el0_cpumask(void);
+DECLARE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
+
static inline bool system_supports_32bit_el0(void)
{
- return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);
+ u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
+
+ return static_branch_unlikely(&arm64_mismatched_32bit_el0) ||
+ id_aa64pfr0_32bit_el0(pfr0);
}

static inline bool system_supports_4kb_granule(void)
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index a4db25cd7122..4194a47de62d 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -107,6 +107,24 @@ DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
bool arm64_use_ng_mappings = false;
EXPORT_SYMBOL(arm64_use_ng_mappings);

+/*
+ * Permit PER_LINUX32 and execve() of 32-bit binaries even if not all CPUs
+ * support it?
+ */
+static bool __read_mostly allow_mismatched_32bit_el0;
+
+/*
+ * Static branch enabled only if allow_mismatched_32bit_el0 is set and we have
+ * seen at least one CPU capable of 32-bit EL0.
+ */
+DEFINE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
+
+/*
+ * Mask of CPUs supporting 32-bit EL0.
+ * Only valid if arm64_mismatched_32bit_el0 is enabled.
+ */
+static cpumask_var_t cpu_32bit_el0_mask __cpumask_var_read_mostly;
+
/*
* Flag to indicate if we have computed the system wide
* capabilities based on the boot time active CPUs. This
@@ -767,7 +785,7 @@ static void __init sort_ftr_regs(void)
* Any bits that are not covered by an arm64_ftr_bits entry are considered
* RES0 for the system-wide value, and must strictly match.
*/
-static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
+static void init_cpu_ftr_reg(u32 sys_reg, u64 new)
{
u64 val = 0;
u64 strict_mask = ~0x0ULL;
@@ -863,7 +881,7 @@ static void __init init_cpu_hwcaps_indirect_list(void)

static void __init setup_boot_cpu_capabilities(void);

-static void __init init_32bit_cpu_features(struct cpuinfo_32bit *info)
+static void init_32bit_cpu_features(struct cpuinfo_32bit *info)
{
init_cpu_ftr_reg(SYS_ID_DFR0_EL1, info->reg_id_dfr0);
init_cpu_ftr_reg(SYS_ID_DFR1_EL1, info->reg_id_dfr1);
@@ -979,6 +997,22 @@ static void relax_cpu_ftr_reg(u32 sys_id, int field)
WARN_ON(!ftrp->width);
}

+static void update_mismatched_32bit_el0_cpu_features(struct cpuinfo_arm64 *info,
+ struct cpuinfo_arm64 *boot)
+{
+ static bool boot_cpu_32bit_regs_overridden = false;
+
+ if (!allow_mismatched_32bit_el0 || boot_cpu_32bit_regs_overridden)
+ return;
+
+ if (id_aa64pfr0_32bit_el0(boot->reg_id_aa64pfr0))
+ return;
+
+ boot->aarch32 = info->aarch32;
+ init_32bit_cpu_features(&boot->aarch32);
+ boot_cpu_32bit_regs_overridden = true;
+}
+
static int update_32bit_cpu_features(int cpu, struct cpuinfo_32bit *info,
struct cpuinfo_32bit *boot)
{
@@ -1139,6 +1173,7 @@ void update_cpu_features(int cpu,
* (e.g. SYS_ID_AA64PFR0_EL1), so we call it last.
*/
if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
+ update_mismatched_32bit_el0_cpu_features(info, boot);
taint |= update_32bit_cpu_features(cpu, &info->aarch32,
&boot->aarch32);
}
@@ -1251,6 +1286,28 @@ has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
return feature_matches(val, entry);
}

+const struct cpumask *system_32bit_el0_cpumask(void)
+{
+ if (!system_supports_32bit_el0())
+ return cpu_none_mask;
+
+ if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+ return cpu_32bit_el0_mask;
+
+ return cpu_possible_mask;
+}
+
+static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
+{
+ if (!has_cpuid_feature(entry, scope))
+ return allow_mismatched_32bit_el0;
+
+ if (scope == SCOPE_SYSTEM)
+ pr_info("detected: 32-bit EL0 Support\n");
+
+ return true;
+}
+
static bool has_useable_gicv3_cpuif(const struct arm64_cpu_capabilities *entry, int scope)
{
bool has_sre;
@@ -1869,10 +1926,9 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.cpu_enable = cpu_copy_el2regs,
},
{
- .desc = "32-bit EL0 Support",
- .capability = ARM64_HAS_32BIT_EL0,
+ .capability = ARM64_HAS_32BIT_EL0_DO_NOT_USE,
.type = ARM64_CPUCAP_SYSTEM_FEATURE,
- .matches = has_cpuid_feature,
+ .matches = has_32bit_el0,
.sys_reg = SYS_ID_AA64PFR0_EL1,
.sign = FTR_UNSIGNED,
.field_pos = ID_AA64PFR0_EL0_SHIFT,
@@ -2381,7 +2437,7 @@ static const struct arm64_cpu_capabilities compat_elf_hwcaps[] = {
{},
};

-static void __init cap_set_elf_hwcap(const struct arm64_cpu_capabilities *cap)
+static void cap_set_elf_hwcap(const struct arm64_cpu_capabilities *cap)
{
switch (cap->hwcap_type) {
case CAP_HWCAP:
@@ -2426,7 +2482,7 @@ static bool cpus_have_elf_hwcap(const struct arm64_cpu_capabilities *cap)
return rc;
}

-static void __init setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
+static void setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
{
/* We support emulation of accesses to CPU ID feature registers */
cpu_set_named_feature(CPUID);
@@ -2601,7 +2657,7 @@ static void check_early_cpu_features(void)
}

static void
-verify_local_elf_hwcaps(const struct arm64_cpu_capabilities *caps)
+__verify_local_elf_hwcaps(const struct arm64_cpu_capabilities *caps)
{

for (; caps->matches; caps++)
@@ -2612,6 +2668,14 @@ verify_local_elf_hwcaps(const struct arm64_cpu_capabilities *caps)
}
}

+static void verify_local_elf_hwcaps(void)
+{
+ __verify_local_elf_hwcaps(arm64_elf_hwcaps);
+
+ if (id_aa64pfr0_32bit_el0(read_cpuid(ID_AA64PFR0_EL1)))
+ __verify_local_elf_hwcaps(compat_elf_hwcaps);
+}
+
static void verify_sve_features(void)
{
u64 safe_zcr = read_sanitised_ftr_reg(SYS_ZCR_EL1);
@@ -2676,11 +2740,7 @@ static void verify_local_cpu_capabilities(void)
* on all secondary CPUs.
*/
verify_local_cpu_caps(SCOPE_ALL & ~SCOPE_BOOT_CPU);
-
- verify_local_elf_hwcaps(arm64_elf_hwcaps);
-
- if (system_supports_32bit_el0())
- verify_local_elf_hwcaps(compat_elf_hwcaps);
+ verify_local_elf_hwcaps();

if (system_supports_sve())
verify_sve_features();
@@ -2815,6 +2875,34 @@ void __init setup_cpu_features(void)
ARCH_DMA_MINALIGN);
}

+static int enable_mismatched_32bit_el0(unsigned int cpu)
+{
+ struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
+ bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+
+ if (cpu_32bit) {
+ cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
+ static_branch_enable_cpuslocked(&arm64_mismatched_32bit_el0);
+ setup_elf_hwcaps(compat_elf_hwcaps);
+ }
+
+ return 0;
+}
+
+static int __init init_32bit_el0_mask(void)
+{
+ if (!allow_mismatched_32bit_el0)
+ return 0;
+
+ if (!zalloc_cpumask_var(&cpu_32bit_el0_mask, GFP_KERNEL))
+ return -ENOMEM;
+
+ return cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
+ "arm64/mismatched_32bit_el0:online",
+ enable_mismatched_32bit_el0, NULL);
+}
+subsys_initcall_sync(init_32bit_el0_mask);
+
static void __maybe_unused cpu_enable_cnp(struct arm64_cpu_capabilities const *cap)
{
cpu_replace_ttbr1(lm_alias(swapper_pg_dir));
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 21fbdda7086e..49305c2e6dfd 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -3,7 +3,8 @@
# Internal CPU capabilities constants, keep this list sorted

BTI
-HAS_32BIT_EL0
+# Unreliable: use system_supports_32bit_el0() instead.
+HAS_32BIT_EL0_DO_NOT_USE
HAS_32BIT_EL1
HAS_ADDRESS_AUTH
HAS_ADDRESS_AUTH_ARCH
--
2.31.1.818.g46aad6cb9e-goog

Next message: Will Deacon: "[PATCH v7 04/22] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support"
Previous message: Will Deacon: "[PATCH v7 02/22] arm64: cpuinfo: Split AArch32 registers out into a separate struct"
In reply to: Will Deacon: "[PATCH v7 02/22] arm64: cpuinfo: Split AArch32 registers out into a separate struct"
Next in thread: Will Deacon: "[PATCH v7 04/22] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support"
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