[PATCH bpf-next] seg6: Ensure that seg6_bpf_srh_states can only be accessed from input_action_end_bpf()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Initially I assumed that the per-CPU variable is `seg6_bpf_srh_states'
is first initialized in input_action_end_bpf() and then accessed during
the bpf_prog_run_save_cb() invocation by the eBPF via the BPF callbacks.
syzbot demonstrated that is possible to invoke the BPF callbacks (and
access `seg6_bpf_srh_states') without entering input_action_end_bpf()
first.

The valid path via input_action_end_bpf() is invoked within NAPI
context which means it has bpf_net_context set. This can be used to
identify the "valid" calling path.

Set in input_action_end_bpf() the BPF_RI_F_SEG6_STATE bit to signal the
valid calling path and clear it at the end. Check for the context and
the bit in bpf_lwt_seg6.*() and abort if missing.

Reported-by: syzbot+608a2acde8c5a101d07d@syzkaller.appspotmail.com
Fixes: d1542d4ae4dfd ("seg6: Use nested-BH locking for seg6_bpf_srh_states.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/filter.h | 24 ++++++++++++++++++++++++
 net/core/filter.c      |  6 ++++++
 net/ipv6/seg6_local.c  |  3 +++
 3 files changed, 33 insertions(+)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 0bbd2585e6def..cadddb25ff4db 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -739,6 +739,7 @@ struct bpf_nh_params {
 #define BPF_RI_F_CPU_MAP_INIT	BIT(2)
 #define BPF_RI_F_DEV_MAP_INIT	BIT(3)
 #define BPF_RI_F_XSK_MAP_INIT	BIT(4)
+#define BPF_RI_F_SEG6_STATE	BIT(5)
 
 struct bpf_redirect_info {
 	u64 tgt_index;
@@ -856,6 +857,29 @@ static inline void bpf_net_ctx_get_all_used_flush_lists(struct list_head **lh_ma
 		*lh_xsk = lh;
 }
 
+static inline bool bpf_net_ctx_seg6_state_avail(void)
+{
+	struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
+
+	if (!bpf_net_ctx)
+		return false;
+	return bpf_net_ctx->ri.kern_flags & BPF_RI_F_SEG6_STATE;
+}
+
+static inline void bpf_net_ctx_seg6_state_set(void)
+{
+	struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
+
+	bpf_net_ctx->ri.kern_flags |= BPF_RI_F_SEG6_STATE;
+}
+
+static inline void bpf_net_ctx_seg6_state_clr(void)
+{
+	struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
+
+	bpf_net_ctx->ri.kern_flags &= ~BPF_RI_F_SEG6_STATE;
+}
+
 /* Compute the linear packet data range [data, data_end) which
  * will be accessed by various program types (cls_bpf, act_bpf,
  * lwt, ...). Subsystems allowing direct data access must (!)
diff --git a/net/core/filter.c b/net/core/filter.c
index 403d23faf22e1..ea5bc4a4a6a23 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6459,6 +6459,8 @@ BPF_CALL_4(bpf_lwt_seg6_store_bytes, struct sk_buff *, skb, u32, offset,
 	void *srh_tlvs, *srh_end, *ptr;
 	int srhoff = 0;
 
+	if (!bpf_net_ctx_seg6_state_avail())
+		return -EINVAL;
 	lockdep_assert_held(&srh_state->bh_lock);
 	if (srh == NULL)
 		return -EINVAL;
@@ -6516,6 +6518,8 @@ BPF_CALL_4(bpf_lwt_seg6_action, struct sk_buff *, skb,
 	int hdroff = 0;
 	int err;
 
+	if (!bpf_net_ctx_seg6_state_avail())
+		return -EINVAL;
 	lockdep_assert_held(&srh_state->bh_lock);
 	switch (action) {
 	case SEG6_LOCAL_ACTION_END_X:
@@ -6593,6 +6597,8 @@ BPF_CALL_3(bpf_lwt_seg6_adjust_srh, struct sk_buff *, skb, u32, offset,
 	int srhoff = 0;
 	int ret;
 
+	if (!bpf_net_ctx_seg6_state_avail())
+		return -EINVAL;
 	lockdep_assert_held(&srh_state->bh_lock);
 	if (unlikely(srh == NULL))
 		return -EINVAL;
diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index c74705ead9849..3e3a48b7266b5 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -1429,6 +1429,7 @@ static int input_action_end_bpf(struct sk_buff *skb,
 	 * bpf_prog_run_save_cb().
 	 */
 	local_lock_nested_bh(&seg6_bpf_srh_states.bh_lock);
+	bpf_net_ctx_seg6_state_set();
 	srh_state = this_cpu_ptr(&seg6_bpf_srh_states);
 	srh_state->srh = srh;
 	srh_state->hdrlen = srh->hdrlen << 3;
@@ -1452,6 +1453,7 @@ static int input_action_end_bpf(struct sk_buff *skb,
 
 	if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
 		goto drop;
+	bpf_net_ctx_seg6_state_clr();
 	local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock);
 
 	if (ret != BPF_REDIRECT)
@@ -1460,6 +1462,7 @@ static int input_action_end_bpf(struct sk_buff *skb,
 	return dst_input(skb);
 
 drop:
+	bpf_net_ctx_seg6_state_clr();
 	local_unlock_nested_bh(&seg6_bpf_srh_states.bh_lock);
 	kfree_skb(skb);
 	return -EINVAL;
-- 
2.45.2