We currently allow any process to use rdpmc. This significantly weakens the protection offered by PR_TSC_DISABLED, and it could be helpful to users attempting to exploit timing attacks.
Since we can't enable access to individual counters, use a very coarse heuristic to limit access to rdpmc: allow access only when a perf_event is mmapped. This protects seccomp sandboxes. There is plenty of room to further tighen these restrictions. For example, on x86, *all* perf_event mappings set cap_user_rdpmc. This should probably be changed to only apply to perf_events that are accessible using rdpmc. Signed-off-by: Andy Lutomirski <[email protected]> --- arch/x86/include/asm/mmu.h | 2 ++ arch/x86/include/asm/mmu_context.h | 19 ++++++++++++ arch/x86/kernel/cpu/perf_event.c | 60 +++++++++++++++++++++++++++++--------- 3 files changed, 67 insertions(+), 14 deletions(-) diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h index 876e74e8eec7..2e5e4925a63f 100644 --- a/arch/x86/include/asm/mmu.h +++ b/arch/x86/include/asm/mmu.h @@ -19,6 +19,8 @@ typedef struct { struct mutex lock; void __user *vdso; + + atomic_t perf_mmap_count; /* number of mmapped perf_events */ } mm_context_t; #ifdef CONFIG_SMP diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index 04478103df37..10239af45a9d 100644 --- a/arch/x86/include/asm/mmu_context.h +++ b/arch/x86/include/asm/mmu_context.h @@ -19,6 +19,21 @@ static inline void paravirt_activate_mm(struct mm_struct *prev, } #endif /* !CONFIG_PARAVIRT */ +#ifdef CONFIG_PERF_EVENTS +extern struct static_key rdpmc_enabled; + +static inline void load_mm_cr4(struct mm_struct *mm) +{ + if (static_key_true(&rdpmc_enabled) && + atomic_read(&mm->context.perf_mmap_count)) + cr4_set(X86_CR4_PCE); + else + cr4_clear(X86_CR4_PCE); +} +#else +static inline void load_mm_cr4(struct mm_struct *mm) {} +#endif + /* * Used for LDT copy/destruction. */ @@ -53,6 +68,9 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, /* Stop flush ipis for the previous mm */ cpumask_clear_cpu(cpu, mm_cpumask(prev)); + /* Load per-mm CR4 state */ + load_mm_cr4(next); + /* * Load the LDT, if the LDT is different. * @@ -86,6 +104,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, */ load_cr3(next->pgd); trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL); + load_mm_cr4(next); load_LDT_nolock(&next->context); } } diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c index 2401593df4a3..7d42cf60f3b3 100644 --- a/arch/x86/kernel/cpu/perf_event.c +++ b/arch/x86/kernel/cpu/perf_event.c @@ -31,6 +31,7 @@ #include <asm/nmi.h> #include <asm/smp.h> #include <asm/alternative.h> +#include <asm/mmu_context.h> #include <asm/tlbflush.h> #include <asm/timer.h> #include <asm/desc.h> @@ -44,6 +45,8 @@ DEFINE_PER_CPU(struct cpu_hw_events, cpu_hw_events) = { .enabled = 1, }; +struct static_key rdpmc_enabled = STATIC_KEY_INIT_TRUE; + u64 __read_mostly hw_cache_event_ids [PERF_COUNT_HW_CACHE_MAX] [PERF_COUNT_HW_CACHE_OP_MAX] @@ -133,6 +136,7 @@ static int x86_pmu_extra_regs(u64 config, struct perf_event *event) static atomic_t active_events; static DEFINE_MUTEX(pmc_reserve_mutex); +static DEFINE_MUTEX(rdpmc_enable_mutex); #ifdef CONFIG_X86_LOCAL_APIC @@ -1326,8 +1330,6 @@ x86_pmu_notifier(struct notifier_block *self, unsigned long action, void *hcpu) break; case CPU_STARTING: - if (x86_pmu.attr_rdpmc) - cr4_set(X86_CR4_PCE); if (x86_pmu.cpu_starting) x86_pmu.cpu_starting(cpu); break; @@ -1806,6 +1808,27 @@ static int x86_pmu_event_init(struct perf_event *event) return err; } +static void refresh_pce(void *ignored) +{ + if (current->mm) + load_mm_cr4(current->mm); +} + +static void x86_pmu_event_mapped(struct perf_event *event) +{ + if (atomic_inc_return(¤t->mm->context.perf_mmap_count) == 1) + on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); +} + +static void x86_pmu_event_unmapped(struct perf_event *event) +{ + if (!current->mm) + return; + + if (atomic_dec_and_test(¤t->mm->context.perf_mmap_count)) + on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); +} + static int x86_pmu_event_idx(struct perf_event *event) { int idx = event->hw.idx; @@ -1828,16 +1851,6 @@ static ssize_t get_attr_rdpmc(struct device *cdev, return snprintf(buf, 40, "%d\n", x86_pmu.attr_rdpmc); } -static void change_rdpmc(void *info) -{ - bool enable = !!(unsigned long)info; - - if (enable) - cr4_set(X86_CR4_PCE); - else - cr4_clear(X86_CR4_PCE); -} - static ssize_t set_attr_rdpmc(struct device *cdev, struct device_attribute *attr, const char *buf, size_t count) @@ -1852,10 +1865,26 @@ static ssize_t set_attr_rdpmc(struct device *cdev, if (x86_pmu.attr_rdpmc_broken) return -ENOTSUPP; + mutex_lock(&rdpmc_enable_mutex); if (!!val != !!x86_pmu.attr_rdpmc) { - x86_pmu.attr_rdpmc = !!val; - on_each_cpu(change_rdpmc, (void *)val, 1); + if (val) { + static_key_slow_inc(&rdpmc_enabled); + on_each_cpu(refresh_pce, NULL, 1); + smp_wmb(); + x86_pmu.attr_rdpmc = 1; + } else { + /* + * This direction can race against existing + * rdpmc-capable mappings. Try our best regardless. + */ + x86_pmu.attr_rdpmc = 0; + smp_wmb(); + static_key_slow_dec(&rdpmc_enabled); + WARN_ON(static_key_true(&rdpmc_enabled)); + on_each_cpu(refresh_pce, NULL, 1); + } } + mutex_unlock(&rdpmc_enable_mutex); return count; } @@ -1899,6 +1928,9 @@ static struct pmu pmu = { .event_init = x86_pmu_event_init, + .event_mapped = x86_pmu_event_mapped, + .event_unmapped = x86_pmu_event_unmapped, + .add = x86_pmu_add, .del = x86_pmu_del, .start = x86_pmu_start, -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
