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Message-ID: <CANP3RGc7-ZkQ2xGtkW00+A+zcWpm4WdMEbSn=UjotBQHzO+f7w@mail.gmail.com>
Date: Wed, 22 Jan 2025 12:28:00 -0800
From: Maciej Żenczykowski <zenczykowski@...il.com>
To: Maciej Żenczykowski <zenczykowski@...il.com>,
Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>
Cc: Linux Network Development Mailing List <netdev@...r.kernel.org>, "David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, BPF Mailing List <bpf@...r.kernel.org>,
Stanislav Fomichev <sdf@...ichev.me>, Willem de Bruijn <willemb@...gle.com>,
Matt Moeller <moeller.matt@...il.com>
Subject: Re: [PATCH bpf] bpf: fix classic bpf reads from negative offset
outside of linear skb portion
On Wed, Jan 22, 2025 at 12:16 PM Maciej Żenczykowski
<zenczykowski@...il.com> wrote:
>
> On Wed, Jan 22, 2025 at 12:04 PM Maciej Żenczykowski <maze@...gle.com> 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@...nel.org>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Stanislav Fomichev <sdf@...ichev.me>
> > Cc: Willem de Bruijn <willemb@...gle.com>
> > Reported-by: Matt Moeller <moeller.matt@...il.com>
> > Closes: https://issuetracker.google.com/384636719 [Treble - GKI partner internal]
> > Signed-off-by: Maciej Żenczykowski <maze@...gle.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
> >
>
> 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)
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