[PATCH] perf/bpf: Fix lockless access to parent_event->prog in perf_event_alloc()

During fork(), perf_event_alloc() reads parent_event->prog locklessly
which can race with a concurrent bpf_perf_link_release() clearing and
freeing the prog via perf_event_detach_bpf_prog(). This can result in
a NULL pointer dereference or use-after-free in bpf_prog_inc().

Fix by holding bpf_event_mutex when inheriting the BPF program during
fork(). Make bpf_event_mutex non-static and declare it extern in
perf_event.h so it is accessible from kernel/events/core.c.

Fixes: 85192dbf4de0 ("bpf: Convert bpf_prog refcnt to atomic64_t")
Signed-off-by: Deepanshu Kartikey <[email protected]>
---
 include/linux/perf_event.h | 2 ++
 kernel/events/core.c       | 8 +++++---
 kernel/trace/bpf_trace.c   | 2 +-
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 48d851fbd8ea..5c6dabb6dccb 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -2136,4 +2136,6 @@ static inline void perf_lopwr_cb(bool mode)
 }
 #endif
 
+extern struct mutex bpf_event_mutex;
+
 #endif /* _LINUX_PERF_EVENT_H */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index ba5bd6a78fe7..a89e33c47073 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13433,12 +13433,15 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 		overflow_handler = parent_event->overflow_handler;
 		context = parent_event->overflow_handler_context;
 #if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_EVENT_TRACING)
-		if (parent_event->prog) {
-			struct bpf_prog *prog = parent_event->prog;
-
+		struct bpf_prog *prog;
+
+		mutex_lock(&bpf_event_mutex);
+		prog = parent_event->prog;
+		if (prog) {
 			bpf_prog_inc(prog);
 			event->prog = prog;
 		}
+		mutex_unlock(&bpf_event_mutex);
 #endif
 	}
 
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 75495a5c3507..730fc22a9daa 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1944,7 +1944,7 @@ const struct bpf_verifier_ops perf_event_verifier_ops = {
 const struct bpf_prog_ops perf_event_prog_ops = {
 };
 
-static DEFINE_MUTEX(bpf_event_mutex);
+DEFINE_MUTEX(bpf_event_mutex);
 
 #define BPF_TRACE_MAX_PROGS 64
 
-- 
2.43.0