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@xxxxxxxxxx> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Cc: Stanislav Fomichev <sdf@xxxxxxxxxxx> Cc: Willem de Bruijn <willemb@xxxxxxxxxx> Reported-by: Matt Moeller <moeller.matt@xxxxxxxxx> Closes: https://issuetracker.google.com/384636719 [Treble - GKI partner internal] Signed-off-by: Maciej Żenczykowski <maze@xxxxxxxxxx> 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
