Re: [PATCH net] vsock/virtio: fix skb overhead overflow on 32-bit builds

[David Laight <david.laight.linux@gmail.com>]

On Mon, 25 May 2026 11:57:45 +0200
Stefano Garzarella <sgarzare@redhat.com> wrote:

> On Sat, May 23, 2026 at 05:35:57PM +0100, David Laight wrote:
> >On Sat, 23 May 2026 02:20:29 +0000
> >patchwork-bot+netdevbpf@kernel.org wrote:
> >  
> >> Hello:
> >>
> >> This patch was applied to netdev/net.git (main)
> >> by Jakub Kicinski <kuba@kernel.org>:  
> >
> >Did anyone else notice that is isn't a bug?
> >
> >There is no way that a 'count of bytes of kernel memory' can overflow
> >the size of 'long'.  
> 
> It's more of an estimate than an actual calculation of memory usage if 
> we queue the incoming packet. In theory, an overflow could occur if the 
> user sets `buf_alloc` to 4GB. In practice, though, I think you're right: 
> the memory should run out before we get to that check.

The calculation is:

	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0); 

skb_queue_len() will be the number of items on the queue.
SKB_TRUESIZE(0) is the memory taken up by a zero length skb (basically sizeof(skb)).

Unless you either corrupt the queue length or manage to allocate skb that use
less than the minimum about of memory that product can't overflow 'unsigned long'.

The later calculations might wrap - but the multiply can't.

-- David

> 
> Thanks,
> Stefano
> 
> >
> >-- David
> >  
> >>
> >> On Thu, 21 May 2026 14:47:32 +0200 you wrote:  
> >> > From: Stefano Garzarella <sgarzare@redhat.com>
> >> >
> >> > On 32-bit architectures, both skb_queue_len() and SKB_TRUESIZE(0) evaluate
> >> > to 32-bit values. The multiplication can overflow before being assigned to
> >> > the u64 skb_overhead variable, making the skb overhead check ineffective.
> >> >
> >> > Cast skb_queue_len() to u64 so the multiplication is always performed in
> >> > 64-bit arithmetic.
> >> >
> >> > [...]  
> >>
> >> Here is the summary with links:
> >>   - [net] vsock/virtio: fix skb overhead overflow on 32-bit builds
> >>     https://git.kernel.org/netdev/net/c/4157501b9a8f
> >>
> >> You are awesome, thank you!  
> >  
>