linux-kernel - Re: [PATCH] net: skb: guard kmalloc_reserve() against oversized allocations

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Message-ID: <34a4e8db-b0b8-4ad8-b8a2-bdfd69cc8b00@lunn.ch>
Date: Sat, 20 Sep 2025 17:38:37 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Kriish Sharma <kriish.sharma2006@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, horms@...nel.org, willemb@...gle.com,
	kerneljasonxing@...il.com, martin.lau@...nel.org, mhal@...x.co,
	almasrymina@...gle.com, ebiggers@...gle.com,
	aleksander.lobakin@...el.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
	syzbot+5a2250fd91b28106c37b@...kaller.appspotmail.com
Subject: Re: [PATCH] net: skb: guard kmalloc_reserve() against oversized
 allocations

On Sat, Sep 20, 2025 at 11:37:23AM +0000, Kriish Sharma wrote:
> Add an explicit size check in kmalloc_reserve() to reject requests
> larger than KMALLOC_MAX_SIZE before they reach the allocator.
> 
> syzbot reported warnings triggered by attempts to allocate buffers
> with an object size exceeding KMALLOC_MAX_SIZE. While the existing
> code relies on kmalloc() failure and a comment states that truncation
> is "harmless", in practice this causes high-order allocation warnings
> and noisy kernel logs that interfere with testing.
> 
> This patch introduces an early guard in kmalloc_reserve() that returns
> NULL if obj_size exceeds KMALLOC_MAX_SIZE. This ensures impossible
> requests fail fast and silently, avoiding allocator warnings while
> keeping the intended semantics unchanged.

What is the value of KMALLOC_MAX_SIZE? Do we actually want a
warn_once() here because it indicates a driver bug, or missing
validation of user space parameters?

	   Andrew