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Message-ID: <CANP3RGe_cspCzYe_scA37Hgr0GNbASmN94RRnPRUT1YiBcn=eg@mail.gmail.com>
Date: Wed, 22 Jan 2025 12:16:18 -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 12:04 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 ;-)
- Maciej
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