From: Jiayuan Chen <jiayuan.chen@linux.dev>
To: Emil Tsalapatis <emil@etsalapatis.com>, bpf@vger.kernel.org
Cc: ast@kernel.org, andrii@kernel.org, eddyz87@gmail.com,
memxor@gmail.com, daniel@iogearbox.net, netdev@vger.kernel.org,
Nicholas Carlini <nicholas@carlini.com>
Subject: Re: [PATCH bpf 01/11] bpf: Fix bounds check for skb-backed dynptrs
Date: Wed, 16 Sep 2026 19:52:29 +0800 [thread overview]
Message-ID: <a6b0f8c6-3564-498d-bef7-2a528333c0a3@linux.dev> (raw)
In-Reply-To: <20260916050830.8774-2-emil@etsalapatis.com>
On 9/16/26 1:08 PM, Emil Tsalapatis wrote:
> The skb_pointer_if_linear() function checks whether a
> memory region of length len starting at offset off into
> the skb is in the linear area, and returns a pointer to
> the region if so. The check currently subtracts between
> skb_headlen and offset of the check, and since skb_headlen
> is unsigned the subtraction can underflow. This causes the
> bounds check to spuriously pass and generate an arbitrary
> pointer of the form *(skb->data + off).
>
> The only user of this helper is currently skb-backed BPF
> dynptr code. Returning the wrong pointer leads to the
> dynptr erroneously being backed with invalid memory.
>
> Ensure the subtraction cannot underflow, and fail the check if
> it would. Use u64 arithmetic to also prevent overflow when
> calculating (skb_headlen(skb) - off) since off is unsigned.
>
> Fixes: 6f5a630d7c57 ("bpf, net: Introduce skb_pointer_if_linear().")
> Reported-by: Nicholas Carlini <nicholas@carlini.com>
> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>
> While the code for this is in skbuff.h, the only consumer is
> BPF-related, as is the selftest that validates it in the next
> patch. So I think it makes sense to route through BPF. If there
> are any objections I will split the patch off and resend to net.
>
> include/linux/skbuff.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 421f6fc45..d0c1463db 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4372,7 +4372,8 @@ skb_header_pointer_careful(const struct sk_buff *skb, int offset,
> static inline void * __must_check
> skb_pointer_if_linear(const struct sk_buff *skb, int offset, int len)
> {
> - if (likely(skb_headlen(skb) - offset >= len))
> + if (likely((u64)offset <= skb_headlen(skb) &&
trailing whitespace after "&&"
next prev parent reply other threads:[~2026-09-16 11:52 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-16 5:08 [PATCH bpf 00/11] skb/arena bugfixes Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 01/11] bpf: Fix bounds check for skb-backed dynptrs Emil Tsalapatis
2026-09-16 11:52 ` Jiayuan Chen [this message]
2026-09-16 23:08 ` Jakub Kicinski
2026-09-16 5:08 ` [PATCH bpf 02/11] selftests/bpf: Test dynptr slices past end of skb Emil Tsalapatis
2026-09-16 5:18 ` sashiko-bot
2026-09-16 5:08 ` [PATCH bpf 03/11] bpf: Fix bpf_sock context code generation Emil Tsalapatis
2026-09-16 12:21 ` Jiayuan Chen
2026-09-16 5:08 ` [PATCH bpf 04/11] selftests/bpf: Add selftests for rx_queue_mapping context access Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 05/11] bpf: Reject pkt arguments in mutating subprogs Emil Tsalapatis
2026-09-16 5:57 ` Amery Hung
2026-09-16 5:58 ` Amery Hung
2026-09-16 18:41 ` Emil Tsalapatis
2026-09-16 19:26 ` Amery Hung
2026-09-16 5:08 ` [PATCH bpf 06/11] selftests/bpf: Test rejection of pkt args to " Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 07/11] bpf: Prevent variable arena/non-arena register contents Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 08/11] selftests/bpf: Test for mixed arena/nonarena code paths Emil Tsalapatis
2026-09-16 5:17 ` sashiko-bot
2026-09-16 5:08 ` [PATCH bpf 09/11] bpf: Track whether dynptr type is known Emil Tsalapatis
2026-09-16 5:22 ` sashiko-bot
2026-09-16 18:41 ` Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 10/11] bpf: Track skb memory invalidation by packet-backed dynptrs Emil Tsalapatis
2026-09-16 5:08 ` [PATCH bpf 11/11] selftests/bpf: Test dynptr slice invalidation on skb clobber Emil Tsalapatis
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