[PATCH bpf v3] bpf: Fix potential UAF when reading bpf link info

[Pu Lehui]

From: Pu Lehui <pulehui@xxxxxxxxxx>

In bpf_link_show_fdinfo and bpf_link_get_info_by_fd, link->prog is
accessed without holding any locks. If the prog is concurrently replaced
via bpf_link_update, the old prog can be freed, leading to a potential
UAF issue.

Fix this by accessing link->prog under RCU protection to safely fetch
the pointer and guarantee its lifetime while reading its fields.

Fixes: 0c991ebc8c69 ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
Reviewed-by: Amery Hung <ameryhung@xxxxxxxxx>
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
v3：
- Initialize prog_id while link->prog may be NULL. (Andrii)
- Remove misleading commit msg. (Andrii)
- Add Reviewed-by tags by Amery and Emil.
- Separate from patchset [0]. (Andrii)

Link: https://lore.kernel.org/bpf/f87b53c0-8f00-45a6-82db-8242fa9b143f@xxxxxxxxxxxxxxx [0]

v2: https://lore.kernel.org/bpf/20260721041048.1394085-1-pulehui@xxxxxxxxxxxxxxx
- Remove rcu_read_lock_trace as rcu_read_unlock is sufficient. (Mykyta)

v1: https://lore.kernel.org/bpf/20260720033055.1215477-1-pulehui@xxxxxxxxxxxxxxx

 kernel/bpf/syscall.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6db306d23b47..70dca14378bb 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3471,9 +3471,10 @@ static const char *bpf_link_type_strs[] = {
 static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 {
 	const struct bpf_link *link = filp->private_data;
-	const struct bpf_prog *prog = link->prog;
+	const struct bpf_prog *prog;
 	enum bpf_link_type type = link->type;
 	char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
+	u32 prog_id = 0;
 
 	if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
 		if (link->type == BPF_LINK_TYPE_KPROBE_MULTI)
@@ -3490,13 +3491,20 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 	}
 	seq_printf(m, "link_id:\t%u\n", link->id);
 
+	rcu_read_lock();
+	prog = READ_ONCE(link->prog);
 	if (prog) {
 		bin2hex(prog_tag, prog->tag, sizeof(prog->tag));
+		prog_id = prog->aux->id;
+	}
+	rcu_read_unlock();
+
+	if (prog) {
 		seq_printf(m,
 			   "prog_tag:\t%s\n"
 			   "prog_id:\t%u\n",
 			   prog_tag,
-			   prog->aux->id);
+			   prog_id);
 	}
 	if (link->ops->show_fdinfo)
 		link->ops->show_fdinfo(link, m);
@@ -5535,6 +5543,7 @@ static int bpf_link_get_info_by_fd(struct file *file,
 {
 	struct bpf_link_info __user *uinfo = u64_to_user_ptr(attr->info.info);
 	struct bpf_link_info info;
+	const struct bpf_prog *prog;
 	u32 info_len = attr->info.info_len;
 	int err;
 
@@ -5549,8 +5558,12 @@ static int bpf_link_get_info_by_fd(struct file *file,
 
 	info.type = link->type;
 	info.id = link->id;
-	if (link->prog)
-		info.prog_id = link->prog->aux->id;
+
+	rcu_read_lock();
+	prog = READ_ONCE(link->prog);
+	if (prog)
+		info.prog_id = prog->aux->id;
+	rcu_read_unlock();
 
 	if (link->ops->fill_link_info) {
 		err = link->ops->fill_link_info(link, &info);
-- 
2.34.1