Patch "powerpc/perf: Dereference BHRB entries safely" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/perf: Dereference BHRB entries safely

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-perf-dereference-bhrb-entries-safely.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ravi Bangoria <[email protected]>
Date: Tue, 12 Dec 2017 17:59:15 +0530
Subject: powerpc/perf: Dereference BHRB entries safely

From: Ravi Bangoria <[email protected]>

commit f41d84dddc66b164ac16acf3f584c276146f1c48 upstream.

It's theoretically possible that branch instructions recorded in BHRB (Branch History Rolling Buffer) entries have already been unmapped before they are processed by the kernel. Hence, trying to dereference such memory location will result in a crash. eg:

    Unable to handle kernel paging request for data at address 0xd000000019c41764
    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 it by deferefencing the addresses safely.

---
 arch/powerpc/perf/core-book3s.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

@@ -401,8 +401,12 @@ 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_read(&instr, (void *)addr, sizeof(instr)))
+			return 0;
+
+		return branch_target(&instr);
+	}
 
 	/* Userspace: need copy instruction here then translate it */
 	pagefault_disable();

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/powerpc-perf-dereference-bhrb-entries-safely.patch