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Subject[PATCH 2/3] perf: Fix sample_max_stack maximum check
Date
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>
---
kernel/events/callchain.c | 21 ++++++++++++---------
1 file 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;
}

--
2.13.6
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