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>

1 files changed, 12 insertions, 9 deletions

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 772a43fea825..73cc26e321de 100644
--- 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;
         }