[PATCH bpf] bpf: fix classic bpf reads from negative offset outside of linear skb portion

["Maciej Żenczykowski" <maze@google.com>]

We're received reports of cBPF code failing to accept DHCP packets.
"BPF filter for DHCP not working (android14-6.1-lts + android-14.0.0_r74)"

The relevant Android code is at:
  https://cs.android.com/android/platform/superproject/main/+/main:packages/modules/NetworkStack/jni/network_stack_utils_jni.cpp;l=95;drc=9df50aef1fd163215dcba759045706253a5624f5
which uses a lot of macros from:
  https://cs.android.com/android/platform/superproject/main/+/main:packages/modules/Connectivity/bpf/headers/include/bpf/BpfClassic.h;drc=c58cfb7c7da257010346bd2d6dcca1c0acdc8321

This is widely used and does work on the vast majority of drivers,
but is exposing a core kernel cBPF bug related to driver skb layout.

Root cause is iwlwifi driver, specifically on (at least):
  Dell 7212: Intel Dual Band Wireless AC 8265
  Dell 7220: Intel Wireless AC 9560
  Dell 7230: Intel Wi-Fi 6E AX211
delivers frames where the UDP destination port is not in the skb linear
portion, while the cBPF code is using SKF_NET_OFF relative addressing.

simplified from above, effectively:
  BPF_STMT(BPF_LDX | BPF_B | BPF_MSH, SKF_NET_OFF)
  BPF_STMT(BPF_LD  | BPF_H | BPF_IND, SKF_NET_OFF + 2)
  BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, 68, 1, 0)
  BPF_STMT(BPF_RET | BPF_K, 0)
  BPF_STMT(BPF_RET | BPF_K, 0xFFFFFFFF)
fails to match udp dport=68 packets.

Specifically the 3rd cBPF instruction fails to match the condition:
  if (ptr >= skb->head && ptr + size <= skb_tail_pointer(skb))
within bpf_internal_load_pointer_neg_helper() and thus returns NULL,
which results in reading -EFAULT.

This is because bpf_skb_load_helper_{8,16,32} don't include the
"data past headlen do skb_copy_bits()" logic from the non-negative
offset branch in the negative offset branch.

Note: I don't know sparc assembly, so this doesn't fix sparc...
ideally we should just delete bpf_internal_load_pointer_neg_helper()
This seems to have always been broken (but not pre-git era, since
obviously there was no eBPF helpers back then), but stuff older
than 5.4 is no longer LTS supported anyway, so using 5.4 as fixes tag.

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Willem de Bruijn <willemb@google.com>
Reported-by: Matt Moeller <moeller.matt@gmail.com>
Closes: https://issuetracker.google.com/384636719 [Treble - GKI partner internal]
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Fixes: 219d54332a09 ("Linux 5.4")
---
 include/linux/filter.h |  2 ++
 kernel/bpf/core.c      | 14 +++++++++
 net/core/filter.c      | 69 +++++++++++++++++-------------------------
 3 files changed, 43 insertions(+), 42 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index a3ea46281595..c24d8e338ce4 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1479,6 +1479,8 @@ 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);
 
+int bpf_internal_neg_helper(const struct sk_buff *skb, int k);
+
 static inline int bpf_tell_extensions(void)
 {
 	return SKF_AD_MAX;
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index da729cbbaeb9..994988dabb97 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -89,6 +89,20 @@ void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, uns
 	return NULL;
 }
 
+int bpf_internal_neg_helper(const struct sk_buff *skb, int k)
+{
+	if (k >= 0)
+		return k;
+	if (k >= SKF_NET_OFF)
+		return skb->network_header + k - SKF_NET_OFF;
+	if (k >= SKF_LL_OFF) {
+		if (unlikely(!skb_mac_header_was_set(skb)))
+			return -1;
+		return skb->mac_header + k - SKF_LL_OFF;
+	}
+	return -1;
+}
+
 /* 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 e56a0be31678..609ef7df71ce 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -221,21 +221,16 @@ 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;
-	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;
-	}
+	u8 tmp;
 
-	return -EFAULT;
+	offset = bpf_internal_neg_helper(skb, offset);
+	if (unlikely(offset < 0))
+		return -EFAULT;
+	if (headlen - offset >= sizeof(u8))
+		return *(u8 *)(data + offset);
+	if (skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return -EFAULT;
+	return tmp;
 }
 
 BPF_CALL_2(bpf_skb_load_helper_8_no_cache, const struct sk_buff *, skb,
@@ -248,21 +243,16 @@ 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;
-	const int len = sizeof(tmp);
+	__be16 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);
-	}
-
-	return -EFAULT;
+	offset = bpf_internal_neg_helper(skb, offset);
+	if (unlikely(offset < 0))
+		return -EFAULT;
+	if (headlen - offset >= sizeof(__be16))
+		return get_unaligned_be16(data + offset);
+	if (skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return -EFAULT;
+	return be16_to_cpu(tmp);
 }
 
 BPF_CALL_2(bpf_skb_load_helper_16_no_cache, const struct sk_buff *, skb,
@@ -275,21 +265,16 @@ 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;
-	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);
-	}
+	__be32 tmp;
 
-	return -EFAULT;
+	offset = bpf_internal_neg_helper(skb, offset);
+	if (unlikely(offset < 0))
+		return -EFAULT;
+	if (headlen - offset >= sizeof(__be32))
+		return get_unaligned_be32(data + offset);
+	if (skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
+		return -EFAULT;
+	return be32_to_cpu(tmp);
 }
 
 BPF_CALL_2(bpf_skb_load_helper_32_no_cache, const struct sk_buff *, skb,
-- 
2.48.1.262.g85cc9f2d1e-goog