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

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;