Re: [PATCH net] qede: fix off-by-one in BD ring consumption on build_skb failure

[Jamie Bainbridge]

On Fri, 3 Jul 2026 at 09:52, Jamie Bainbridge
<jamie.bainbridge@xxxxxxxxx> wrote:
>
> On Wed, 1 Jul 2026 at 02:47, Shigeru Yoshida <syoshida@xxxxxxxxxx> wrote:
> >
> > qede_rx_build_skb() and qede_tpa_rx_build_skb() do not check for a
> > NULL return from qede_build_skb(). When it returns NULL under memory
> > pressure, the functions still consume a BD from the ring before
> > returning NULL. The callers then recycle additional BDs, resulting in
> > one extra BD being consumed (off-by-one). This desynchronizes the BD
> > ring, which can corrupt DMA page reference counts and lead to SLUB
> > freelist corruption.
>
> Good catch.
>
> Reviewed-by: Jamie Bainbridge <jamie.bainbridge@xxxxxxxxx>

Sorry for the double mail.

I believe the Fixes: should be against the problematic original code:

Fixes: 8a8633978b842 ("qede: Add build_skb() support.")

because that is what you are fixing.

Jamie

> > Commit 4e910dbe3650 ("qede: confirm skb is allocated before using")
> > added a NULL check inside qede_build_skb() to prevent a NULL pointer
> > dereference, but did not address the missing NULL checks in the
> > callers, making this off-by-one reachable.
> >
> > Fix this by adding NULL checks for the return value of
> > qede_build_skb() in both qede_rx_build_skb() and
> > qede_tpa_rx_build_skb(), returning NULL immediately before any BD ring
> > manipulation.
> >
> > Fixes: 4e910dbe3650 ("qede: confirm skb is allocated before using")
> > Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
> > ---
> >  drivers/net/ethernet/qlogic/qede/qede_fp.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c b/drivers/net/ethernet/qlogic/qede/qede_fp.c
> > index 33e18bb69774..c11e0d8f98aa 100644
> > --- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
> > +++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
> > @@ -765,6 +765,9 @@ qede_tpa_rx_build_skb(struct qede_dev *edev,
> >         struct sk_buff *skb;
> >
> >         skb = qede_build_skb(rxq, bd, len, pad);
> > +       if (unlikely(!skb))
> > +               return NULL;
> > +
> >         bd->page_offset += rxq->rx_buf_seg_size;
> >
> >         if (bd->page_offset == PAGE_SIZE) {
> > @@ -812,6 +815,8 @@ qede_rx_build_skb(struct qede_dev *edev,
> >         }
> >
> >         skb = qede_build_skb(rxq, bd, len, pad);
> > +       if (unlikely(!skb))
> > +               return NULL;
> >
> >         if (unlikely(qede_realloc_rx_buffer(rxq, bd))) {
> >                 /* Incr page ref count to reuse on allocation failure so
> > --
> > 2.54.0
> >