perf/x86: Only allow rdpmc if a perf_event is mapped

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, this allows rdpmc for any x86_pmu event, but it's only
useful for self-monitoring tasks.

As a side effect, cap_user_rdpmc will now be false for AMD uncore
events.  This isn't a real regression, since .event_idx is disabled
for these events anyway for the time being.  Whenever that gets
re-added, the cap_user_rdpmc code can be adjusted or refactored
accordingly.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Vince Weaver <vince@deater.net>
Cc: "hillf.zj" <hillf.zj@alibaba-inc.com>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/a2bdb3cf3a1d70c26980d7c6dddfbaa69f3182bf.1414190806.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 40 insertions, 19 deletions

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 73e84a348de1..bec5cff7dc80 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>
@@ -1328,8 +1329,6 @@ x86_pmu_notifier(struct notifier_block *self, unsigned long action, void *hcpu)
                 break;
 
         case CPU_STARTING:
-                if (x86_pmu.attr_rdpmc)
-                        cr4_set_bits(X86_CR4_PCE);
                 if (x86_pmu.cpu_starting)
                         x86_pmu.cpu_starting(cpu);
                 break;
@@ -1805,14 +1804,44 @@ static int x86_pmu_event_init(struct perf_event *event)
                         event->destroy(event);
         }
 
+        if (ACCESS_ONCE(x86_pmu.attr_rdpmc))
+                event->hw.flags |= PERF_X86_EVENT_RDPMC_ALLOWED;
+
         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 (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
+                return;
+
+        if (atomic_inc_return(&current->mm->context.perf_rdpmc_allowed) == 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 (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
+                return;
+
+        if (atomic_dec_and_test(&current->mm->context.perf_rdpmc_allowed))
+                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;
 
-        if (!x86_pmu.attr_rdpmc)
+        if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
                 return 0;
 
         if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
@@ -1830,16 +1859,6 @@ 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_bits(X86_CR4_PCE);
-        else
-                cr4_clear_bits(X86_CR4_PCE);
-}
-
 static ssize_t set_attr_rdpmc(struct device *cdev,
                               struct device_attribute *attr,
                               const char *buf, size_t count)
@@ -1854,11 +1873,7 @@ static ssize_t set_attr_rdpmc(struct device *cdev,
         if (x86_pmu.attr_rdpmc_broken)
                 return -ENOTSUPP;
 
-        if (!!val != !!x86_pmu.attr_rdpmc) {
-                x86_pmu.attr_rdpmc = !!val;
-                on_each_cpu(change_rdpmc, (void *)val, 1);
-        }
-
+        x86_pmu.attr_rdpmc = !!val;
         return count;
 }
 
@@ -1901,6 +1916,9 @@ 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,
@@ -1922,7 +1940,8 @@ void arch_perf_update_userpage(struct perf_event *event,
 
         userpg->cap_user_time = 0;
         userpg->cap_user_time_zero = 0;
-        userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;
+        userpg->cap_user_rdpmc =
+                !!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED);
         userpg->pmc_width = x86_pmu.cntval_bits;
 
         if (!sched_clock_stable())
diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
index 4e6cdb0ddc70..df525d2be1e8 100644
--- a/arch/x86/kernel/cpu/perf_event.h
+++ b/arch/x86/kernel/cpu/perf_event.h
@@ -71,6 +71,8 @@ struct event_constraint {
 #define PERF_X86_EVENT_COMMITTED        0x8 /* event passed commit_txn */
 #define PERF_X86_EVENT_PEBS_LD_HSW      0x10 /* haswell style datala, load */
 #define PERF_X86_EVENT_PEBS_NA_HSW      0x20 /* haswell style datala, unknown */
+#define PERF_X86_EVENT_RDPMC_ALLOWED    0x40 /* grant rdpmc permission */
+
 
 struct amd_nb {
         int nb_id;  /* NorthBridge id */