[PATCH v2] perf/bpf: Fix lockless access to parent_event->prog in perf_event_alloc()

[Deepanshu Kartikey <kartikey406@gmail.com>]

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

Fix by using READ_ONCE() to atomically read parent_event->prog into a
local variable and bpf_prog_inc_not_zero() to safely increment the
reference count only if the program is still alive. This handles both
tracing and non-tracing event types without requiring any additional
locking.

Fixes: 85192dbf4de0 ("bpf: Convert bpf_prog refcnt to atomic64_t")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
v2:
- Drop bpf_event_mutex approach which was wrong for non-tracing events
  as perf_event_free_bpf_handler() operates locklessly
- Use READ_ONCE() + bpf_prog_inc_not_zero() instead which handles both
  tracing and non-tracing event types safely without any lock
- Remove extern bpf_event_mutex from perf_event.h (no longer needed)
- Remove static removal from bpf_trace.c (no longer needed)
---
 kernel/events/core.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ba5bd6a78fe7..39755bfacc55 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13433,11 +13433,13 @@ 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;
-
-			bpf_prog_inc(prog);
-			event->prog = prog;
+		struct bpf_prog *prog;
+
+		prog = READ_ONCE(parent_event->prog);
+		if (prog) {
+			prog = bpf_prog_inc_not_zero(prog);
+			if (!IS_ERR(prog))
+				event->prog = prog;
 		}
 #endif
 	}
-- 
2.43.0