linux-kernel - Re: Question about to KMSAN: uninit-value in can

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Message-Id: <20251117173012.230731-1-activprithvi@gmail.com>
Date: Mon, 17 Nov 2025 23:00:12 +0530
From: Prithvi Tambewagh <activprithvi@...il.com>
To: mkl@...gutronix.de,
	socketcan@...tkopp.net
Cc: Prithvi Tambewagh <activprithvi@...il.com>,
	linux-can@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: Question about to KMSAN: uninit-value in can_receive

Hello,

The call trace suggests that the bug appears to be due to effect of change
in headroom by pskb_header_expand(). The new headroom remains uninitialized 
and when can_receive tries accessing can_skb_prv(skb)->skbcnt, indirectly 
skb->head is accessed which causes KMSAN uninitialized value read bug.

To fix this bug, I think we can call can_dropped_invalid_skb() in can_rcv() 
just before calling can_receive(). Further, we can add a condition for these 
sk_buff with uninitialized headroom to initialize the skb, the way it had 
been done in the patch for an earlier packet injection case in a similar 
KMSAN bug:
https://lore.kernel.org/linux-can/20191207183418.28868-1-socketcan@hartkopp.net/

However, I am not getting on what basis can I filter the sk_buff so that 
only those with an uninitialized headroom will be initialized via this path. 
Is this the correct approach?

Thank you,
Prithvi