[PATCH v2] powerpc/perf: Fix kernel address leak to userspace via BHRB buffer

[Madhavan Srinivasan <maddy@linux.vnet.ibm.com>]

The current Branch History Rolling Buffer (BHRB) code does
not check for any privilege levels before updating the data
from BHRB. This leaks 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>
---
Changelog -v1:
 - Added comment.

 arch/powerpc/perf/core-book3s.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index f89bbd54ecec..37d24c22557d 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/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:
-- 
2.7.4