linux-kernel - Re: [PATCH 5.10 26/76] sfc: Check null pointer of rx_queue->page

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Date:   Sat, 1 Jan 2022 11:54:29 +0000
From:   Martin Habets <habetsm.xilinx@...il.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jiasheng Jiang <jiasheng@...as.ac.cn>,
        Jakub Kicinski <kuba@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.10 26/76] sfc: Check null pointer of rx_queue->page_ring

On Wed, Dec 29, 2021 at 12:17:30PM +0100, Pavel Machek wrote:
> hi!
> 
> > From: Jiasheng Jiang <jiasheng@...as.ac.cn>
> > 
> > [ Upstream commit bdf1b5c3884f6a0dc91b0dbdb8c3b7d205f449e0 ]
> > 
> > Because of the possible failure of the kcalloc, it should be better to
> > set rx_queue->page_ptr_mask to 0 when it happens in order to maintain
> > the consistency.
> 
> This is confusing/wrong, or at least not a complete fix.
> 
> > +++ b/drivers/net/ethernet/sfc/rx_common.c
> > @@ -150,7 +150,10 @@ static void efx_init_rx_recycle_ring(struct efx_rx_queue *rx_queue)
> >  					    efx->rx_bufs_per_page);
> >  	rx_queue->page_ring = kcalloc(page_ring_size,
> >  				      sizeof(*rx_queue->page_ring), GFP_KERNEL);
> > -	rx_queue->page_ptr_mask = page_ring_size - 1;
> > +	if (!rx_queue->page_ring)
> > +		rx_queue->page_ptr_mask = 0;
> > +	else
> > +		rx_queue->page_ptr_mask = page_ring_size - 1;
> >  }
> >  
> 
> So we have !rx_queue->page_ring. But in efx_reuse_page, we do
> 
>         index = rx_queue->page_remove & rx_queue->page_ptr_mask;
> 	page = rx_queue->page_ring[index];
> 
> So index is now zero, but we'll derefernce null pointer anyway.

Good point. I've posted a patch for this to netdev.
https://lore.kernel.org/netdev/164103678041.26263.8354809911405746465.stgit@palantir17.mph.net/

Thanks,
Martin