powerpc/perf: Dereference BHRB entries safely

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.

Fixes: 691231846ceb ("powerpc/perf: Fix setting of "to" addresses for BHRB")
Suggested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
[mpe: Use probe_kernel_read() which is clearer, tweak change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index d1e65ce545b31f427629853a3803a6b0b4af0d9b..b2ab164a8094e427052e284befca1e4b8b8655a7 100644 (file)
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -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();