netdev - Re: [PATCH bpf-next 1/2] bpf, test_run: Fix user-memory-access vulnerability for LIVE

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Message-ID: <87ms2s57sp.fsf@toke.dk>
Date: Mon, 05 Jan 2026 17:43:50 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: KaFai Wan <kafai.wan@...ux.dev>, ast@...nel.org, daniel@...earbox.net,
 andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com,
 song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com,
 kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, horms@...nel.org, hawk@...nel.org, shuah@...nel.org,
 aleksander.lobakin@...el.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Cc: Yinhao Hu <dddddd@...t.edu.cn>, Kaiyan Mei <M202472210@...t.edu.cn>,
 Dongliang Mu <dzm91@...t.edu.cn>
Subject: Re: [PATCH bpf-next 1/2] bpf, test_run: Fix user-memory-access
 vulnerability for LIVE_FRAMES

KaFai Wan <kafai.wan@...ux.dev> writes:

> On Mon, 2026-01-05 at 11:46 +0100, Toke Høiland-Jørgensen wrote:
>> KaFai Wan <kafai.wan@...ux.dev> writes:
>> 
>> > This fix reverts to the original version and ensures data_hard_start
>> > correctly points to the xdp_frame structure, eliminating the security
>> > risk.
>> 
>> This is wrong. We should just be checking the meta_len on input to
>> account for the size of xdp_frame. I'll send a patch.
>
> Current version the actual limit of the max input meta_len for live frames is 
> XDP_PACKET_HEADROOM - sizeof(struct xdp_frame), not
> XDP_PACKET_HEADROOM.

By "current version", you mean the patch I sent[0], right?

If so, that was deliberate: the stack limits the maximum data_meta size
to XDP_PACKET_HEADROOM - sizeof(struct xdp_frame), so there's no reason
not to do the same for bpf_prog_run(). And some chance that diverging
here will end up surfacing other bugs down the line.

-Toke

[0] https://lore.kernel.org/r/20260105114747.1358750-1-toke@redhat.com