bpf: Make sure internal and UAPI bpf_redirect flags don't overlap

The bpf_redirect_info is shared between the SKB and XDP redirect paths,
and the two paths use the same numeric flag values in the ri->flags
field (specifically, BPF_F_BROADCAST == BPF_F_NEXTHOP). This means that
if skb bpf_redirect_neigh() is used with a non-NULL params argument and,
subsequently, an XDP redirect is performed using the same
bpf_redirect_info struct, the XDP path will get confused and end up
crashing, which syzbot managed to trigger.

With the stack-allocated bpf_redirect_info, the structure is no longer
shared between the SKB and XDP paths, so the crash doesn't happen
anymore. However, different code paths using identically-numbered flag
values in the same struct field still seems like a bit of a mess, so
this patch cleans that up by moving the flag definitions together and
redefining the three flags in BPF_F_REDIRECT_INTERNAL to not overlap
with the flags used for XDP. It also adds a BUILD_BUG_ON() check to make
sure the overlap is not re-introduced by mistake.

Fixes: e624d4ed ("xdp: Extend xdp_redirect_map with broadcast support")
Reported-by: syzbot+cca39e6e84a367a7e6f6@syzkaller.appspotmail.com
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://syzkaller.appspot.com/bug?extid=cca39e6e84a367a7e6f6
Link: https://lore.kernel.org/bpf/20240920125625.59465-1-toke@redhat.com

include/uapi/linux/bpf.h

@@ -6047,11 +6047,6 @@ enum {
 	BPF_F_MARK_ENFORCE		= (1ULL << 6),
 };
 
-/* BPF_FUNC_clone_redirect and BPF_FUNC_redirect flags. */
-enum {
-	BPF_F_INGRESS			= (1ULL << 0),
-};
-
 /* BPF_FUNC_skb_set_tunnel_key and BPF_FUNC_skb_get_tunnel_key flags. */
 enum {
 	BPF_F_TUNINFO_IPV6		= (1ULL << 0),
@@ -6198,10 +6193,12 @@ enum {
 	BPF_F_BPRM_SECUREEXEC	= (1ULL << 0),
 };
 
-/* Flags for bpf_redirect_map helper */
+/* Flags for bpf_redirect and bpf_redirect_map helpers */
 enum {
-	BPF_F_BROADCAST		= (1ULL << 3),
-	BPF_F_EXCLUDE_INGRESS	= (1ULL << 4),
+	BPF_F_INGRESS		= (1ULL << 0), /* used for skb path */
+	BPF_F_BROADCAST		= (1ULL << 3), /* used for XDP path */
+	BPF_F_EXCLUDE_INGRESS	= (1ULL << 4), /* used for XDP path */
+#define BPF_F_REDIRECT_FLAGS (BPF_F_INGRESS | BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS)
 };
 
 #define __bpf_md_ptr(type, name)	\

net/core/filter.c

@@ -2438,9 +2438,9 @@ static int __bpf_redirect_neigh(struct sk_buff *skb, struct net_device *dev,
 
 /* Internal, non-exposed redirect flags. */
 enum {
-	BPF_F_NEIGH	= (1ULL << 1),
-	BPF_F_PEER	= (1ULL << 2),
-	BPF_F_NEXTHOP	= (1ULL << 3),
+	BPF_F_NEIGH	= (1ULL << 16),
+	BPF_F_PEER	= (1ULL << 17),
+	BPF_F_NEXTHOP	= (1ULL << 18),
 #define BPF_F_REDIRECT_INTERNAL	(BPF_F_NEIGH | BPF_F_PEER | BPF_F_NEXTHOP)
 };
 
@@ -2450,6 +2450,8 @@ BPF_CALL_3(bpf_clone_redirect, struct sk_buff *, skb, u32, ifindex, u64, flags)
 	struct sk_buff *clone;
 	int ret;
 
+	BUILD_BUG_ON(BPF_F_REDIRECT_INTERNAL & BPF_F_REDIRECT_FLAGS);
+
 	if (unlikely(flags & (~(BPF_F_INGRESS) | BPF_F_REDIRECT_INTERNAL)))
 		return -EINVAL;