bpf: Prepare bpf_prog_put() to be called from irq context.

Currently bpf_prog_put() is called from the task context only.
With addition of bpf timers the timer related helpers will start calling
bpf_prog_put() from irq-saved region and in rare cases might drop
the refcnt to zero.
To address this case, first, convert bpf_prog_free_id() to be irq-save
(this is similar to bpf_map_free_id), and, second, defer non irq
appropriate calls into work queue.
For example:
bpf_audit_prog() is calling kmalloc and wake_up_interruptible,
bpf_prog_kallsyms_del_all()->bpf_ksym_del()->spin_unlock_bh().
They are not safe with irqs disabled.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210715005417.78572-2-alexei.starovoitov@gmail.com

kernel/bpf/syscall.c

@@ -1699,6 +1699,8 @@ static int bpf_prog_alloc_id(struct bpf_prog *prog)
 
 void bpf_prog_free_id(struct bpf_prog *prog, bool do_idr_lock)
 {
+	unsigned long flags;
+
 	/* cBPF to eBPF migrations are currently not in the idr store.
 	 * Offloaded programs are removed from the store when their device
 	 * disappears - even if someone grabs an fd to them they are unusable,
@@ -1708,7 +1710,7 @@ void bpf_prog_free_id(struct bpf_prog *prog, bool do_idr_lock)
 		return;
 
 	if (do_idr_lock)
-		spin_lock_bh(&prog_idr_lock);
+		spin_lock_irqsave(&prog_idr_lock, flags);
 	else
 		__acquire(&prog_idr_lock);
 
@@ -1716,7 +1718,7 @@ void bpf_prog_free_id(struct bpf_prog *prog, bool do_idr_lock)
 	prog->aux->id = 0;
 
 	if (do_idr_lock)
-		spin_unlock_bh(&prog_idr_lock);
+		spin_unlock_irqrestore(&prog_idr_lock, flags);
 	else
 		__release(&prog_idr_lock);
 }
@@ -1752,14 +1754,32 @@ static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
 	}
 }
 
+static void bpf_prog_put_deferred(struct work_struct *work)
+{
+	struct bpf_prog_aux *aux;
+	struct bpf_prog *prog;
+
+	aux = container_of(work, struct bpf_prog_aux, work);
+	prog = aux->prog;
+	perf_event_bpf_event(prog, PERF_BPF_EVENT_PROG_UNLOAD, 0);
+	bpf_audit_prog(prog, BPF_AUDIT_UNLOAD);
+	__bpf_prog_put_noref(prog, true);
+}
+
 static void __bpf_prog_put(struct bpf_prog *prog, bool do_idr_lock)
 {
-	if (atomic64_dec_and_test(&prog->aux->refcnt)) {
-		perf_event_bpf_event(prog, PERF_BPF_EVENT_PROG_UNLOAD, 0);
-		bpf_audit_prog(prog, BPF_AUDIT_UNLOAD);
+	struct bpf_prog_aux *aux = prog->aux;
+
+	if (atomic64_dec_and_test(&aux->refcnt)) {
 		/* bpf_prog_free_id() must be called first */
 		bpf_prog_free_id(prog, do_idr_lock);
-		__bpf_prog_put_noref(prog, true);
+
+		if (in_irq() || irqs_disabled()) {
+			INIT_WORK(&aux->work, bpf_prog_put_deferred);
+			schedule_work(&aux->work);
+		} else {
+			bpf_prog_put_deferred(&aux->work);
+		}
 	}
 }