On 01/22, Maciej Żenczykowski wrote: > On Wed, Jan 22, 2025 at 12:28 PM Maciej Żenczykowski > <zenczykowski@xxxxxxxxx> wrote: > > > > On Wed, Jan 22, 2025 at 12:16 PM Maciej Żenczykowski > > <zenczykowski@xxxxxxxxx> wrote: > > > > > > 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 ;-) > > > > Furthermore even after cherrypicking this (or a similar style fix) > > into older LTS: > > - sparc jit is (presumably, maybe only 32-bit) still broken, as the > > assembly uses the old function > > - same for mips jit on 5.4/5.10/5.15 > > - same for powerpc jit on 5.4/5.10 > > > > (but I would guess we don't care) > > and Willem points out that: > > "skb->network_header is an offset against head, while the > get_unaligned_be16 and skb_copy_bits take an offset relative to data. > > I did check yesterday that skb_copy_bits can take negative offsets, in > case a protocol header (e.g., SKF_LL_OFF) is smaller than skb->data." > > which makes me think this approach is possibly (likely?) incorrect? > skb offsets always leave me confused... Same for me WRT skb offsets... However, it should be easy to write a minimal reproducer to trigger the issues in a local vm: create tap device, attach egress bpf (classic) classifier, use tap's new zerocopy api (tap_get_user SOCK_ZEROCOPY) to create fragmented skb. And attach it as a selftest O:-)
