perf: Fix sample_max_stack maximum check

The syzbot hit KASAN bug in perf_callchain_store having the entry stored
behind the allocated bounds [1].

We miss the sample_max_stack check for the initial event that allocates
callchain buffers. This missing check allows to create an event with
sample_max_stack value bigger than the global sysctl maximum:

  # sysctl -a | grep perf_event_max_stack
  kernel.perf_event_max_stack = 127

  # perf record -vv -C 1 -e cycles/max-stack=256/ kill
  ...
  perf_event_attr:
    size                             112
    ...
    sample_max_stack                 256
  ------------------------------------------------------------
  sys_perf_event_open: pid -1  cpu 1  group_fd -1  flags 0x8 = 4

Note the '-C 1', which forces perf record to create just single event.
Otherwise it opens event for every cpu, then the sample_max_stack check
fails on the second event and all's fine.

The fix is to run the sample_max_stack check also for the first event
with callchains.

[1] https://marc.info/?l=linux-kernel&m=152352732920874&w=2

Reported-by: syzbot+7c449856228b63ac951e@syzkaller.appspotmail.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: syzkaller-bugs@googlegroups.com
Cc: x86@kernel.org
Fixes: 97c79a38cd45 ("perf core: Per event callchain limit")
Link: http://lkml.kernel.org/r/20180415092352.12403-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 772a43f..73cc26e 100644 (file)
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -119,19 +119,22 @@ int get_callchain_buffers(int event_max_stack)
                goto exit;
        }
 
+       /*
+        * If requesting per event more than the global cap,
+        * return a different error to help userspace figure
+        * this out.
+        *
+        * And also do it here so that we have &callchain_mutex held.
+        */
+       if (event_max_stack > sysctl_perf_event_max_stack) {
+               err = -EOVERFLOW;
+               goto exit;
+       }
+
        if (count > 1) {
                /* If the allocation failed, give up */
                if (!callchain_cpus_entries)
                        err = -ENOMEM;
-               /*
-                * If requesting per event more than the global cap,
-                * return a different error to help userspace figure
-                * this out.
-                *
-                * And also do it here so that we have &callchain_mutex held.
-                */
-               if (event_max_stack > sysctl_perf_event_max_stack)
-                       err = -EOVERFLOW;
                goto exit;
        }