netdev - Re: [PATCH bpf] xsk: add overflow check for u64 division, stored into u32

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Date:   Mon, 25 May 2020 11:30:59 -0700
From:   Jonathan Lemon <bsd@...com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     magnus.karlsson@...el.com, jonathan.lemon@...il.com,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org,
        Björn Töpel <bjorn.topel@...el.com>,
        maximmi@...lanox.com,
        Minh Bùi Quang <minhquangbui99@...il.com>
Subject: Re: [PATCH bpf] xsk: add overflow check for u64 division, stored
 into u32

On Mon, May 25, 2020 at 10:03:59AM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> The npgs member of struct xdp_umem is an u32 entity, and stores the
> number of pages the UMEM consumes. The calculation of npgs
> 
>   npgs = size / PAGE_SIZE
> 
> can overflow.
> 
> To avoid overflow scenarios, the division is now first stored in a
> u64, and the result is verified to fit into 32b.
> 
> An alternative would be storing the npgs as a u64, however, this
> wastes memory and is an unrealisticly large packet area.
> 
> Link: https://lore.kernel.org/bpf/CACtPs=GGvV-_Yj6rbpzTVnopgi5nhMoCcTkSkYrJHGQHJWFZMQ@mail.gmail.com/
> Fixes: c0c77d8fb787 ("xsk: add user memory registration support sockopt")
> Reported-by: "Minh Bùi Quang" <minhquangbui99@...il.com>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>

Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>