powerpc/perf: Prevent kernel address leak to userspace via BHRB buffer

The current Branch History Rolling Buffer (BHRB) code does not check
for any privilege levels before updating the data from BHRB. This
could leak kernel addresses to userspace even when profiling only with
userspace privileges. Add proper checks to prevent it.

Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/perf/core-book3s.c

@@ -457,6 +457,16 @@ static void power_pmu_bhrb_read(struct cpu_hw_events *cpuhw)
  /* invalid entry */
  continue;
 
+ /*
+ * BHRB rolling buffer could very much contain the kernel
+ * addresses at this point. Check the privileges before
+ * exporting it to userspace (avoid exposure of regions
+ * where we could have speculative execution)
+ */
+ if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN) &&
+ is_kernel_addr(addr))
+ continue;
+
  /* Branches are read most recent first (ie. mfbhrb 0 is
  * the most recent branch).
  * There are two types of valid entries: