[PATCH] powerpc/perf: Dereference bhrb entries safely

[Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>]

It may very well happen that branch instructions recorded by
bhrb entries already get unmapped before they get processed by
the kernel. Hence, trying to dereference such memory location
will endup in a crash. Ex,

    Unable to handle kernel paging request for data at address 0xc008000019c41764
    Faulting instruction address: 0xc000000000084a14
    NIP [c000000000084a14] branch_target+0x4/0x70
    LR [c0000000000eb828] record_and_restart+0x568/0x5c0
    Call Trace:
    [c0000000000eb3b4] record_and_restart+0xf4/0x5c0 (unreliable)
    [c0000000000ec378] perf_event_interrupt+0x298/0x460
    [c000000000027964] performance_monitor_exception+0x54/0x70
    [c000000000009ba4] performance_monitor_common+0x114/0x120

Fix this by deferefencing them safely.

Suggested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
---
 arch/powerpc/perf/core-book3s.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 9e3da168d54c..5a68d2effdf9 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -410,8 +410,11 @@ static __u64 power_pmu_bhrb_to(u64 addr)
 	int ret;
 	__u64 target;
 
-	if (is_kernel_addr(addr))
-		return branch_target((unsigned int *)addr);
+	if (is_kernel_addr(addr)) {
+		if (probe_kernel_address((void *)addr, instr))
+			return 0;
+		return branch_target(&instr);
+	}
 
 	/* Userspace: need copy instruction here then translate it */
 	pagefault_disable();
-- 
2.13.6