netdev - Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN: uninit-value in can

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Message-ID: <63c20aae-e014-44f9-a201-99e0e7abadcb@hartkopp.net>
Date: Sat, 3 Jan 2026 13:20:34 +0100
From: Oliver Hartkopp <socketcan@...tkopp.net>
To: Jakub Kicinski <kuba@...nel.org>, Prithvi <activprithvi@...il.com>
Cc: andrii@...nel.org, mkl@...gutronix.de, linux-can@...r.kernel.org,
 linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
 netdev@...r.kernel.org
Subject: Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN:
 uninit-value in can_receive

Hello Jakub,

thanks for stepping in!

On 02.01.26 21:04, Jakub Kicinski wrote:

> You're asking the wrong person, IIUC Andrii is tangentially involved
> in XDP (via bpf links?):
> 
(..)
> 
> Without looking too deeply - XDP has historically left the new space
> uninitialized after push, expecting programs to immediately write the
> headers in that space. syzbot had run into this in the past but I can't
> find any references to past threads quickly :(

To identify Andrii I mainly looked into the code with 'git blame' that 
led to this problematic call chain:

   pskb_expand_head+0x226/0x1a60 net/core/skbuff.c:2275
   netif_skb_check_for_xdp net/core/dev.c:5081 [inline]
   netif_receive_generic_xdp net/core/dev.c:5112 [inline]
   do_xdp_generic+0x9e3/0x15a0 net/core/dev.c:5180

Having in mind that the syzkaller refers to 
6.13.0-rc7-syzkaller-00039-gc3812b15000c I wonder if we can leave this 
report as-is, as the problem might be solved in the meantime??

In any case I wonder, if we should add some code to re-check if the 
headroom of the CAN-related skbs is still consistent and not changed in 
size by other players. And maybe add some WARN_ON_ONCE() before dropping 
the skb then.

When the skb headroom is not safe to be used we need to be able to 
identify and solve it.

Best regards,
Oliver