[PATCH v3 1/3] bpf: propagate cb_access from freplace programs

[Weiming Shi <bestswngs@gmail.com>]

bpf_prog_run_save_cb() and bpf_prog_run_clear_cb() use the target
program's cb_access flag to decide whether skb->cb must be saved or
cleared.  An extension program can introduce ctx->cb[] access behind a
target which does not access it itself, leaving protocol-owned control
block contents visible to the replacement and preventing the wrapper
from restoring them.

Make cb_access independently addressable and propagate it to the target
before activating a replacement.  Wait for wrappers which observed the
old value, and snapshot the flag once per invocation so save and restore
decisions remain paired.

Cc: stable@vger.kernel.org
Fixes: be8704ff07d2 ("bpf: Introduce dynamic program extensions")
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Assisted-by: LLM
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 include/linux/bpf.h    | 2 +-
 include/linux/filter.h | 7 ++++---
 kernel/bpf/syscall.c   | 6 ++++++
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e57af902560c3..606e7cf397af2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1864,11 +1864,11 @@ struct bpf_prog_aux {
 
 struct bpf_prog {
 	u16			pages;		/* Number of allocated pages */
+	bool			cb_access;	/* Is control block accessed? */
 	u32			jited:1,	/* Is our filter JIT'ed? */
 				jit_requested:1,/* archs need to JIT the prog */
 				jit_required:1,	/* program strictly requires JIT compiler */
 				gpl_compatible:1, /* Is filter GPL compatible? */
-				cb_access:1,	/* Is control block accessed? */
 				dst_needed:1,	/* Do we need dst entry? */
 				blinding_requested:1, /* needs constant blinding */
 				blinded:1,	/* Was blinded */
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 39decde7fc730..788c2d625db4a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1047,16 +1047,17 @@ static inline u32 __bpf_prog_run_save_cb(const struct bpf_prog *prog,
 	const struct sk_buff *skb = ctx;
 	u8 *cb_data = bpf_skb_cb(skb);
 	u8 cb_saved[BPF_SKB_CB_LEN];
+	bool cb_access = READ_ONCE(prog->cb_access);
 	u32 res;
 
-	if (unlikely(prog->cb_access)) {
+	if (unlikely(cb_access)) {
 		memcpy(cb_saved, cb_data, sizeof(cb_saved));
 		memset(cb_data, 0, sizeof(cb_saved));
 	}
 
 	res = bpf_prog_run(prog, skb);
 
-	if (unlikely(prog->cb_access))
+	if (unlikely(cb_access))
 		memcpy(cb_data, cb_saved, sizeof(cb_saved));
 
 	return res;
@@ -1079,7 +1080,7 @@ static inline u32 bpf_prog_run_clear_cb(const struct bpf_prog *prog,
 	u8 *cb_data = bpf_skb_cb(skb);
 	u32 res;
 
-	if (unlikely(prog->cb_access))
+	if (unlikely(READ_ONCE(prog->cb_access)))
 		memset(cb_data, 0, BPF_SKB_CB_LEN);
 
 	res = bpf_prog_run_pin_on_cpu(prog, skb);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c7bc9ba9b331f..43a29e47c8d01 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3812,6 +3812,12 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 	if (err)
 		goto out_unlock;
 
+	if (prog->type == BPF_PROG_TYPE_EXT && READ_ONCE(prog->cb_access)) {
+		WRITE_ONCE(tgt_prog->cb_access, true);
+		/* Drain runs that observed cb_access=false before enabling freplace. */
+		synchronize_rcu();
+	}
+
 	err = bpf_trampoline_link_prog(&link->link.node, tr, tgt_prog);
 	if (err) {
 		bpf_link_cleanup(&link_primer);
-- 
2.55.0