arm64: Allow mismatched 32-bit EL0 support

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@arm.com>
Link: https://lore.kernel.org/r/20210608180313.11502-3-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 650de92..9bb9d11 100644 (file)
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -637,9 +637,15 @@ 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 33e5330..5238901 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -108,6 +108,24 @@ 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
  * will be used to determine if a new booting CPU should
@@ -775,7 +793,7 @@ 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;
@@ -871,7 +889,7 @@ 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);
@@ -990,6 +1008,22 @@ static void relax_cpu_ftr_reg(u32 sys_id, int field)
        WARN_ON(!ftrp->width);
 }
 
+static void lazy_init_32bit_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)
 {
@@ -1161,6 +1195,7 @@ 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)) {
+               lazy_init_32bit_cpu_features(info, boot);
                taint |= update_32bit_cpu_features(cpu, &info->aarch32,
                                                   &boot->aarch32);
        }
@@ -1273,6 +1308,28 @@ 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;
@@ -1891,10 +1948,9 @@ 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,
@@ -2403,7 +2459,7 @@ 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:
@@ -2448,7 +2504,7 @@ 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);
@@ -2623,7 +2679,7 @@ 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++)
@@ -2634,6 +2690,14 @@ 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);
@@ -2698,11 +2762,7 @@ 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();
@@ -2837,6 +2897,34 @@ 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 21fbdda..49305c2 100644 (file)
--- 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