Re: [PATCH v3 net-next] bpf: fix bpf_perf_event_read() helper

From: Wangnan (F)
Date: Thu Oct 22 2015 - 22:22:57 EST




On 2015/10/23 8:10, Alexei Starovoitov wrote:
Fix safety checks for bpf_perf_event_read():
- only non-inherited events can be added to perf_event_array map
(do this check statically at map insertion time)
- dynamically check that event is local and !pmu->count
Otherwise buggy bpf program can cause kernel splat.

Also fix error path after perf_event_attrs()
and remove redundant 'extern'.

Fixes: 35578d798400 ("bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter")
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
v2->v3:
. refactor checks based on Wangnan's and Peter's feedback
while refactoring realized that these two issues need fixes as well:
. fix perf_event_attrs() error path
. remove redundant extern

v1->v2: fix compile in case of !CONFIG_PERF_EVENTS

Even in the worst case the crash is not possible.
Only warn_on_once, so imo net-next is ok.

include/linux/bpf.h | 1 -
kernel/bpf/arraymap.c | 25 ++++++++++++++++---------
kernel/trace/bpf_trace.c | 7 ++++++-
3 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e3a51b74e275..75718fa28260 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -194,7 +194,6 @@ extern const struct bpf_func_proto bpf_map_lookup_elem_proto;
extern const struct bpf_func_proto bpf_map_update_elem_proto;
extern const struct bpf_func_proto bpf_map_delete_elem_proto;
-extern const struct bpf_func_proto bpf_perf_event_read_proto;
extern const struct bpf_func_proto bpf_get_prandom_u32_proto;
extern const struct bpf_func_proto bpf_get_smp_processor_id_proto;
extern const struct bpf_func_proto bpf_tail_call_proto;
diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index e3cfe46b074f..3f4c99e06c6b 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -292,16 +292,23 @@ static void *perf_event_fd_array_get_ptr(struct bpf_map *map, int fd)
attr = perf_event_attrs(event);
if (IS_ERR(attr))
- return (void *)attr;
+ goto err;
- if (attr->type != PERF_TYPE_RAW &&
- !(attr->type == PERF_TYPE_SOFTWARE &&
- attr->config == PERF_COUNT_SW_BPF_OUTPUT) &&
- attr->type != PERF_TYPE_HARDWARE) {
- perf_event_release_kernel(event);
- return ERR_PTR(-EINVAL);
- }
- return event;
+ if (attr->inherit)
+ goto err;
+

Since Peter suggest it is pointless for a system-wide perf_event
has inherit bit set [1], I think it should be safe to enable
system-wide perf_event pass this check?

I'll check code to make sure.

[1] http://lkml.kernel.org/r/20151022124142.GQ17308@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

+ if (attr->type == PERF_TYPE_RAW)
+ return event;
+
+ if (attr->type == PERF_TYPE_HARDWARE)
+ return event;
+
+ if (attr->type == PERF_TYPE_SOFTWARE &&
+ attr->config == PERF_COUNT_SW_BPF_OUTPUT)
+ return event;
+err:
+ perf_event_release_kernel(event);
+ return ERR_PTR(-EINVAL);
}
static void perf_event_fd_array_put_ptr(void *ptr)
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 47febbe7998e..003df3887287 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -199,6 +199,11 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
if (!event)
return -ENOENT;
+ /* make sure event is local and doesn't have pmu::count */
+ if (event->oncpu != smp_processor_id() ||
+ event->pmu->count)
+ return -EINVAL;
+
/*
* we don't know if the function is run successfully by the
* return value. It can be judged in other places, such as
@@ -207,7 +212,7 @@ static u64 bpf_perf_event_read(u64 r1, u64 index, u64 r3, u64 r4, u64 r5)
return perf_event_read_local(event);
}
-const struct bpf_func_proto bpf_perf_event_read_proto = {
+static const struct bpf_func_proto bpf_perf_event_read_proto = {
.func = bpf_perf_event_read,
.gpl_only = false,
.ret_type = RET_INTEGER,


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