On Wed, Jan 22, 2025 at 12:04 PM Maciej Żenczykowski <maze@xxxxxxxxxx> wrote: > > 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: 2nd of course > 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 > Note: this is currently only compile and boot tested. Which doesn't prove all that much ;-) - Maciej
