[PATCH bpf v2] bpf: keep trampoline progs alive until image release

[Junseo Lim <zirajs7@gmail.com>]

arch_prepare_bpf_trampoline() embeds program pointers in the generated
image and passes them to __bpf_prog_enter_recur(). After
bpf_trampoline_update() replaces the image, bpf_tramp_image_put() can keep
the old image executable past a normal RCU grace period.

A detached non-sleepable prog can therefore be freed before a preempted
task reaches rcu_read_lock_dont_migrate() in __bpf_prog_enter_recur(),
leading to a use-after-free.

Keep image-local prog refs and drop them from bpf_tramp_image_free().

Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
Reported-by: Sechang Lim <rhkrqnwk98@gmail.com>
Signed-off-by: Junseo Lim <zirajs7@gmail.com>
---
v1 -> v2:
- Factor prog ref get/put logic into helpers, per Leon Hwang's suggestion.

v1: https://lore.kernel.org/bpf/20260815071927.147049-1-zirajs7@gmail.com/T/

 include/linux/bpf.h     |  5 +++++
 kernel/bpf/trampoline.c | 30 ++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 7719f6528445..bacb6bc2e27b 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1376,6 +1376,11 @@ struct bpf_tramp_image {
 		struct rcu_head rcu;
 		struct work_struct work;
 	};
+#ifdef CONFIG_PREEMPTION
+	/* Programs called from this image must outlive deferred image freeing. */
+	struct bpf_prog *progs[BPF_MAX_TRAMP_LINKS];
+	int nr_progs;
+#endif
 };
 
 struct bpf_trampoline {
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 1a721fc4bef5..33383d1dc1e0 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -529,8 +529,36 @@ bpf_trampoline_get_progs(const struct bpf_trampoline *tr, int *total, bool *ip_a
 	return tnodes;
 }
 
+static void bpf_tramp_image_get_progs(struct bpf_tramp_image *im,
+				      struct bpf_tramp_nodes *tnodes)
+{
+#ifdef CONFIG_PREEMPTION
+	int i, kind;
+
+	for (kind = 0; kind < BPF_TRAMP_MAX; kind++)
+		for (i = 0; i < tnodes[kind].nr_nodes; i++) {
+			struct bpf_prog *prog = tnodes[kind].nodes[i]->link->prog;
+
+			bpf_prog_inc(prog);
+			im->progs[im->nr_progs++] = prog;
+		}
+#endif
+}
+
+static void bpf_tramp_image_put_progs(struct bpf_tramp_image *im)
+{
+#ifdef CONFIG_PREEMPTION
+	int i;
+
+	for (i = 0; i < im->nr_progs; i++)
+		bpf_prog_put(im->progs[i]);
+#endif
+}
+
 static void bpf_tramp_image_free(struct bpf_tramp_image *im)
 {
+	bpf_tramp_image_put_progs(im);
+
 	bpf_image_ksym_del(&im->ksym);
 	arch_free_bpf_trampoline(im->image, im->size);
 	bpf_jit_uncharge_modmem(im->size);
@@ -740,6 +768,8 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 		goto out;
 	}
 
+	bpf_tramp_image_get_progs(im, tnodes);
+
 	err = arch_prepare_bpf_trampoline(im, im->image, im->image + size,
 					  &tr->func.model, tr->flags, tnodes,
 					  tr->func.addr);
-- 
2.55.0