linux-kernel - Re: [PATCH] gve: Fix a possible invalid memory access

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Date:   Sun, 24 Oct 2021 11:36:41 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     jeroendb@...gle.com, Catherine Sullivan <csully@...gle.com>,
        awogbemila@...gle.com, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Bailey Forrest <bcf@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        John Fraker <jfraker@...gle.com>, yangchun@...gle.com,
        xliutaox@...gle.com, Sagi Shahar <sagis@...gle.com>,
        Luigi Rizzo <lrizzo@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] gve: Fix a possible invalid memory access

On Sun, Oct 24, 2021 at 10:58 AM Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
>
> Le 24/10/2021 à 15:51, Willem de Bruijn a écrit :
> > On Sun, Oct 24, 2021 at 7:52 AM Christophe JAILLET
> > <christophe.jaillet@...adoo.fr> wrote:
> >>
> >> It is spurious to allocate a bitmap for 'num_qpls' bits and record the
> >> size of this bitmap with another value.
> >>
> >> 'qpl_map_size' is used in 'drivers/net/ethernet/google/gve/gve.h' with
> >> 'find_[first|next]_zero_bit()'.
> >> So, it looks that memory after the allocated 'qpl_id_map' could be
> >> scanned.
> >
> > find_first_zero_bit takes a length argument in bits:
> >
> >      /**
> >       * find_first_zero_bit - find the first cleared bit in a memory region
> >       * @addr: The address to start the search at
> >       * @size: The maximum number of bits to search
> >
> > qpl_map_size is passed to find_first_zero_bit.
> >
> > It does seem roundabout to compute first the number of longs needed to
> > hold num_qpl bits
> >
> >      BITS_TO_LONGS(num_qpls)
> >
> > then again compute the number of bits in this buffer
> >
> >      * sizeof(unsigned long) * BITS_PER_BYTE
> >
> > Which will simply be num_qpls again.
> >
> > But, removing BITS_PER_BYTE does not arrive at the right number.
>
> (* embarrassed *)
>
> So obvious.
> Thank you for taking time for the explanation on a so badly broken patch.
>
> I apologize for the noise and the waste of time :(

No worries, it happens. Thanks for reviewing code.

>
> BTW, why not just have 'priv->qpl_cfg.qpl_map_size = num_qpls;'?

Yes, that seems more straightforward to me too.