[PATCH bpf-next v4 2/4] bpf: Prevent extending tail callee prog with freplace prog

[Leon Hwang <leon.hwang@linux.dev>]

Alongside previous patch, the infinite loop issue caused by combination of
tailcal and freplace can be prevented completely.

The previous patch can not prevent the use case that updates a prog to
prog_array map and then extends subprog of the prog with freplace prog.

This patch fixes the case by preventing extending a prog, which has been
updated to prog_array map, with freplace prog.

If a prog has been updated to prog_array map, it or its subprog can not
be extended by freplace prog.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 include/linux/bpf.h   |  3 ++-
 kernel/bpf/arraymap.c |  9 ++++++++-
 kernel/bpf/syscall.c  | 11 +++++++++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index aac6d2f42830c..dc19ad99e2857 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1484,7 +1484,8 @@ struct bpf_prog_aux {
 	bool exception_cb;
 	bool exception_boundary;
 	bool is_extended; /* true if extended by freplace program */
-	struct mutex ext_mutex; /* mutex for is_extended */
+	u32 prog_array_member_cnt; /* counts how many times as member of prog_array */
+	struct mutex ext_mutex; /* mutex for is_extended and prog_array_member_cnt */
 	struct bpf_arena *arena;
 	/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
 	const struct btf_type *attach_func_proto;
diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 4a4de4f014be9..91b5bdf4dc72d 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -957,6 +957,8 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map,
 
 	mutex_lock(&prog->aux->ext_mutex);
 	is_extended = prog->aux->is_extended;
+	if (!is_extended)
+		prog->aux->prog_array_member_cnt++;
 	mutex_unlock(&prog->aux->ext_mutex);
 	if (is_extended)
 		/* Extended prog can not be tail callee. It's to prevent a
@@ -974,8 +976,13 @@ static void *prog_fd_array_get_ptr(struct bpf_map *map,
 
 static void prog_fd_array_put_ptr(struct bpf_map *map, void *ptr, bool need_defer)
 {
+	struct bpf_prog *prog = ptr;
+
+	mutex_lock(&prog->aux->ext_mutex);
+	prog->aux->prog_array_member_cnt--;
+	mutex_unlock(&prog->aux->ext_mutex);
 	/* bpf_prog is freed after one RCU or tasks trace grace period */
-	bpf_prog_put(ptr);
+	bpf_prog_put(prog);
 }
 
 static u32 prog_fd_array_sys_lookup_elem(void *ptr)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index db17c52fa35db..4beec9729f742 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3287,6 +3287,17 @@ static int bpf_extend_prog(struct bpf_tracing_link *link,
 	int err = 0;
 
 	mutex_lock(&aux->ext_mutex);
+	if (aux->prog_array_member_cnt) {
+		/* Program extensions can not extend target prog when the target
+		 * prog has been updated to any prog_array map as tail callee.
+		 * It's to prevent a potential infinite loop like:
+		 * tgt prog entry -> tgt prog subprog -> freplace prog entry
+		 * --tailcall-> tgt prog entry.
+		 */
+		err = -EINVAL;
+		goto out_unlock;
+	}
+
 	err = bpf_trampoline_link_prog(&link->link, tr);
 	if (err)
 		goto out_unlock;
-- 
2.44.0