Re: [PATCH v2 bpf] bpf: devmap: fix stack-out-of-bounds write in get_upper_ifindexes()

[Kohei Enju <kohei@enjuk.jp>]

On 02/23 12:32, Toke Høiland-Jørgensen wrote:
> Kohei Enju <kohei@enjuk.jp> writes:
> 
> > get_upper_ifindexes() iterates over all upper devices and writes their
> > indices into an array without checking bounds.
> >
> > Also the callers assume that the max number of upper devices is
> > MAX_NEST_DEV and allocate excluded_devices[1+MAX_NEST_DEV] on the stack,
> > but that assumption is not correct and the number of upper devices could
> > be larger than MAX_NEST_DEV (e.g., many macvlans), causing a
> > stack-out-of-bounds write.
> >
> > Add a max parameter to get_upper_ifindexes() to avoid the issue.
> >
> > To reproduce, create more than MAX_NEST_DEV(8) macvlans on a device with
> > an XDP program attached using BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS.
> > Then send a packet to the device to trigger the XDP redirect path.
> >
> > Reported-by: syzbot+10cc7f13760b31bd2e61@syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/all/698c4ce3.050a0220.340abe.000b.GAE@google.com/T/
> > Fixes: aeea1b86f936 ("bpf, devmap: Exclude XDP broadcast to master device")
> > Signed-off-by: Kohei Enju <kohei@enjuk.jp>
> > ---
> > Changes:
> >   v2:
> >     - fix formatting, accounting that max-line-length is 100
> >   v1: https://lore.kernel.org/bpf/20260216201428.65641-1-kohei@enjuk.jp/
> > ---
> >  kernel/bpf/devmap.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> > index 2625601de76e..c8c49d86a403 100644
> > --- a/kernel/bpf/devmap.c
> > +++ b/kernel/bpf/devmap.c
> > @@ -588,16 +588,18 @@ static inline bool is_ifindex_excluded(int *excluded, int num_excluded, int ifin
> >  }
> >  
> >  /* Get ifindex of each upper device. 'indexes' must be able to hold at
> > - * least MAX_NEST_DEV elements.
> > + * least 'max' elements.
> >   * Returns the number of ifindexes added.
> >   */
> > -static int get_upper_ifindexes(struct net_device *dev, int *indexes)

> > +static int get_upper_ifindexes(struct net_device *dev, int *indexes, int max)
> >  {
> >  	struct net_device *upper;
> >  	struct list_head *iter;
> >  	int n = 0;
> >  
> >  	netdev_for_each_upper_dev_rcu(dev, upper, iter) {
> > +		if (n >= max)
> > +			break;
> 
> Hmm, how about returning an error here, and aborting the redirect
> entirely? This silent truncation to an arbitrary number seems a bit
> surprising, and very hard to debug. With an error, there will at least
> be an error tracepoint to catch the failure.
> 
> -Toke

Hi, Toke. 

Thanks for the review.
Indeed I agree that silent truncation is surprising and hard to debug.
I'll send a v3 that returns -EOVERFLOW and aborts the redirect when
there are too many upper devices.