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Message-ID: <20250403140846.1268564-2-willemdebruijn.kernel@gmail.com>
Date: Thu, 3 Apr 2025 10:07:45 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: bpf@...r.kernel.org
Cc: netdev@...r.kernel.org,
ast@...nel.org,
daniel@...earbox.net,
john.fastabend@...il.com,
Willem de Bruijn <willemb@...gle.com>,
Matt Moeller <moeller.matt@...il.com>,
Maciej Żenczykowski <maze@...gle.com>
Subject: [PATCH bpf 1/2] bpf: support SKF_NET_OFF and SKF_LL_OFF on skb frags
From: Willem de Bruijn <willemb@...gle.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@...il.com>
Co-developed-by: Maciej Żenczykowski <maze@...gle.com>
Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
include/linux/filter.h | 3 --
kernel/bpf/core.c | 21 ------------
net/core/filter.c | 75 +++++++++++++++++++++++-------------------
3 files changed, 42 insertions(+), 57 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..b232b70dd10d 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -221,21 +221,24 @@ BPF_CALL_3(bpf_skb_get_nlattr_nest, struct sk_buff *, skb, u32, a, u32, x)
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;
+ if (offset < 0) {
+ if (offset >= SKF_NET_OFF)
+ offset += skb_network_offset(skb) - SKF_NET_OFF;
+ else if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
+ offset += skb_mac_offset(skb) - SKF_LL_OFF;
+ else
+ 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 +251,24 @@ 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);
+ if (offset < 0) {
+ if (offset >= SKF_NET_OFF)
+ offset += skb_network_offset(skb) - SKF_NET_OFF;
+ else if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
+ offset += skb_mac_offset(skb) - SKF_LL_OFF;
+ else
+ 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 +281,24 @@ 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);
+ if (offset < 0) {
+ if (offset >= SKF_NET_OFF)
+ offset += skb_network_offset(skb) - SKF_NET_OFF;
+ else if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
+ offset += skb_mac_offset(skb) - SKF_LL_OFF;
+ else
+ 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,
--
2.49.0.472.ge94155a9ec-goog
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