[PATCH bpf v2 1/2] bpf: support SKF_NET_OFF and SKF_LL_OFF on skb frags

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

Classic BPF socket filters with SKB_NET_OFF and SKB_LL_OFF fail to
read when these offsets extend into frags.

This has been observed with iwlwifi and reproduced with tun with
IFF_NAPI_FRAGS. The below straightforward socket filter on UDP port,
applied to a RAW socket, will silently miss matching packets.

    const int offset_proto = offsetof(struct ip6_hdr, ip6_nxt);
    const int offset_dport = sizeof(struct ip6_hdr) + offsetof(struct udphdr, dest);
    struct sock_filter filter_code[] = {
            BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_AD_OFF + SKF_AD_PKTTYPE),
            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, PACKET_HOST, 0, 4),
            BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_NET_OFF + offset_proto),
            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 2),
            BPF_STMT(BPF_LD  + BPF_H   + BPF_ABS, SKF_NET_OFF + offset_dport),

This is unexpected behavior. Socket filter programs should be
consistent regardless of environment. Silent misses are
particularly concerning as hard to detect.

Use skb_copy_bits for offsets outside linear, same as done for
non-SKF_(LL|NET) offsets.

Offset is always positive after subtracting the reference threshold
SKB_(LL|NET)_OFF, so is always >= skb_(mac|network)_offset. The sum of
the two is an offset against skb->data, and may be negative, but it
cannot point before skb->head, as skb_(mac|network)_offset would too.

This appears to go back to when frag support was introduced to
sk_run_filter in linux-2.4.4, before the introduction of git.

The amount of code change and 8/16/32 bit duplication are unfortunate.
But any attempt I made to be smarter saved very few LoC while
complicating the code.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://lore.kernel.org/netdev/20250122200402.3461154-1-maze@google.com/
Link: https://elixir.bootlin.com/linux/2.4.4/source/net/core/filter.c#L244
Reported-by: Matt Moeller <moeller.matt@gmail.com>
Co-developed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>

---

v1->v2
  - introduce bfp_skb_load_helper_convert_offset to avoid open coding
---
 include/linux/filter.h |  3 --
 kernel/bpf/core.c      | 21 -----------
 net/core/filter.c      | 80 +++++++++++++++++++++++-------------------
 3 files changed, 44 insertions(+), 60 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index f5cf4d35d83e..708ac7e0cd36 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1496,9 +1496,6 @@ static inline u16 bpf_anc_helper(const struct sock_filter *ftest)
 	}
 }
 
-void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb,
-					   int k, unsigned int size);
-
 static inline int bpf_tell_extensions(void)
 {
 	return SKF_AD_MAX;
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ba6b6118cf50..0e836b5ac9a0 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -68,27 +68,6 @@
 struct bpf_mem_alloc bpf_global_ma;
 bool bpf_global_ma_set;
 
-/* No hurry in this branch
- *
- * Exported for the bpf jit load helper.
- */
-void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, unsigned int size)
-{
-	u8 *ptr = NULL;
-
-	if (k >= SKF_NET_OFF) {
-		ptr = skb_network_header(skb) + k - SKF_NET_OFF;
-	} else if (k >= SKF_LL_OFF) {
-		if (unlikely(!skb_mac_header_was_set(skb)))
-			return NULL;
-		ptr = skb_mac_header(skb) + k - SKF_LL_OFF;
-	}
-	if (ptr >= skb->head && ptr + size <= skb_tail_pointer(skb))
-		return ptr;
-
-	return NULL;
-}
-
 /* tell bpf programs that include vmlinux.h kernel's PAGE_SIZE */
 enum page_size_enum {
 	__PAGE_SIZE = PAGE_SIZE
diff --git a/net/core/filter.c b/net/core/filter.c
index bc6828761a47..79cab4d78dc3 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -218,24 +218,36 @@ BPF_CALL_3(bpf_skb_get_nlattr_nest, struct sk_buff *, skb, u32, a, u32, x)
 	return 0;
 }
 
+static int bpf_skb_load_helper_convert_offset(const struct sk_buff *skb, int offset)
+{
+	if (likely(offset >= 0))
+		return offset;
+
+	if (offset >= SKF_NET_OFF)
+		return offset - SKF_NET_OFF + skb_network_offset(skb);
+
+	if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
+		return offset - SKF_LL_OFF + skb_mac_offset(skb);
+
+	return INT_MIN;
+}
+
 BPF_CALL_4(bpf_skb_load_helper_8, const struct sk_buff *, skb, const void *,
 	   data, int, headlen, int, offset)
 {
-	u8 tmp, *ptr;
+	u8 tmp;
 	const int len = sizeof(tmp);
 
-	if (offset >= 0) {
-		if (headlen - offset >= len)
-			return *(u8 *)(data + offset);
-		if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
-			return tmp;
-	} else {
-		ptr = bpf_internal_load_pointer_neg_helper(skb, offset, len);
-		if (likely(ptr))
-			return *(u8 *)ptr;
-	}
+	offset = bpf_skb_load_helper_convert_offset(skb, offset);
+	if (offset == INT_MIN)
+		return -EFAULT;
 
-	return -EFAULT;
+	if (headlen - offset >= len)
+		return *(u8 *)(data + offset);
+	if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return tmp;
+	else
+		return -EFAULT;
 }
 
 BPF_CALL_2(bpf_skb_load_helper_8_no_cache, const struct sk_buff *, skb,
@@ -248,21 +260,19 @@ BPF_CALL_2(bpf_skb_load_helper_8_no_cache, const struct sk_buff *, skb,
 BPF_CALL_4(bpf_skb_load_helper_16, const struct sk_buff *, skb, const void *,
 	   data, int, headlen, int, offset)
 {
-	__be16 tmp, *ptr;
+	__be16 tmp;
 	const int len = sizeof(tmp);
 
-	if (offset >= 0) {
-		if (headlen - offset >= len)
-			return get_unaligned_be16(data + offset);
-		if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
-			return be16_to_cpu(tmp);
-	} else {
-		ptr = bpf_internal_load_pointer_neg_helper(skb, offset, len);
-		if (likely(ptr))
-			return get_unaligned_be16(ptr);
-	}
+	offset = bpf_skb_load_helper_convert_offset(skb, offset);
+	if (offset == INT_MIN)
+		return -EFAULT;
 
-	return -EFAULT;
+	if (headlen - offset >= len)
+		return get_unaligned_be16(data + offset);
+	if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return be16_to_cpu(tmp);
+	else
+		return -EFAULT;
 }
 
 BPF_CALL_2(bpf_skb_load_helper_16_no_cache, const struct sk_buff *, skb,
@@ -275,21 +285,19 @@ BPF_CALL_2(bpf_skb_load_helper_16_no_cache, const struct sk_buff *, skb,
 BPF_CALL_4(bpf_skb_load_helper_32, const struct sk_buff *, skb, const void *,
 	   data, int, headlen, int, offset)
 {
-	__be32 tmp, *ptr;
+	__be32 tmp;
 	const int len = sizeof(tmp);
 
-	if (likely(offset >= 0)) {
-		if (headlen - offset >= len)
-			return get_unaligned_be32(data + offset);
-		if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
-			return be32_to_cpu(tmp);
-	} else {
-		ptr = bpf_internal_load_pointer_neg_helper(skb, offset, len);
-		if (likely(ptr))
-			return get_unaligned_be32(ptr);
-	}
+	offset = bpf_skb_load_helper_convert_offset(skb, offset);
+	if (offset == INT_MIN)
+		return -EFAULT;
 
-	return -EFAULT;
+	if (headlen - offset >= len)
+		return get_unaligned_be32(data + offset);
+	if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return be32_to_cpu(tmp);
+	else
+		return -EFAULT;
 }
 
 BPF_CALL_2(bpf_skb_load_helper_32_no_cache, const struct sk_buff *, skb,
-- 
2.49.0.504.g3bcea36a83-goog