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Miller" , Jakub Kicinski , Jesper Dangaard Brouer , syzbot+af9492708df9797198d6@syzkaller.appspotmail.com, Eric Dumazet , Paolo Abeni , bpf@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH bpf] xdp: use flags field to disambiguate broadcast redirect Message-ID: References: <20240418071840.156411-1-toke@redhat.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240418071840.156411-1-toke@redhat.com> On Thu, Apr 18, 2024 at 09:18:39AM +0200, Toke Høiland-Jørgensen wrote: > When redirecting a packet using XDP, the bpf_redirect_map() helper will set > up the redirect destination information in struct bpf_redirect_info (using > the __bpf_xdp_redirect_map() helper function), and the xdp_do_redirect() > function will read this information after the XDP program returns and pass > the frame on to the right redirect destination. > > When using the BPF_F_BROADCAST flag to do multicast redirect to a whole > map, __bpf_xdp_redirect_map() sets the 'map' pointer in struct > bpf_redirect_info to point to the destination map to be broadcast. And > xdp_do_redirect() reacts to the value of this map pointer to decide whether > it's dealing with a broadcast or a single-value redirect. However, if the > destination map is being destroyed before xdp_do_redirect() is called, the > map pointer will be cleared out (by bpf_clear_redirect_map()) without > waiting for any XDP programs to stop running. This causes xdp_do_redirect() > to think that the redirect was to a single target, but the target pointer > is also NULL (since broadcast redirects don't have a single target), so > this causes a crash when a NULL pointer is passed to dev_map_enqueue(). > > To fix this, change xdp_do_redirect() to react directly to the presence of > the BPF_F_BROADCAST flag in the 'flags' value in struct bpf_redirect_info > to disambiguate between a single-target and a broadcast redirect. And only > read the 'map' pointer if the broadcast flag is set, aborting if that has > been cleared out in the meantime. This prevents the crash, while keeping > the atomic (cmpxchg-based) clearing of the map pointer itself, and without > adding any more checks in the non-broadcast fast path. > > Fixes: e624d4ed4aa8 ("xdp: Extend xdp_redirect_map with broadcast support") > Reported-and-tested-by: syzbot+af9492708df9797198d6@syzkaller.appspotmail.com > Signed-off-by: Toke Høiland-Jørgensen > --- > net/core/filter.c | 42 ++++++++++++++++++++++++++++++++---------- > 1 file changed, 32 insertions(+), 10 deletions(-) > > diff --git a/net/core/filter.c b/net/core/filter.c > index 786d792ac816..8120c3dddf5e 100644 > --- a/net/core/filter.c > +++ b/net/core/filter.c > @@ -4363,10 +4363,12 @@ static __always_inline int __xdp_do_redirect_frame(struct bpf_redirect_info *ri, > enum bpf_map_type map_type = ri->map_type; > void *fwd = ri->tgt_value; > u32 map_id = ri->map_id; > + u32 flags = ri->flags; > struct bpf_map *map; > int err; > > ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */ > + ri->flags = 0; > ri->map_type = BPF_MAP_TYPE_UNSPEC; > > if (unlikely(!xdpf)) { > @@ -4378,11 +4380,20 @@ static __always_inline int __xdp_do_redirect_frame(struct bpf_redirect_info *ri, > case BPF_MAP_TYPE_DEVMAP: > fallthrough; > case BPF_MAP_TYPE_DEVMAP_HASH: > - map = READ_ONCE(ri->map); > - if (unlikely(map)) { > + if (unlikely(flags & BPF_F_BROADCAST)) { > + map = READ_ONCE(ri->map); > + > + /* The map pointer is cleared when the map is being torn > + * down by bpf_clear_redirect_map() > + */ > + if (unlikely(!map)) { > + err = -ENOENT; > + break; > + } > + > WRITE_ONCE(ri->map, NULL); > err = dev_map_enqueue_multi(xdpf, dev, map, > - ri->flags & BPF_F_EXCLUDE_INGRESS); > + flags & BPF_F_EXCLUDE_INGRESS); > } else { > err = dev_map_enqueue(fwd, xdpf, dev); > } > @@ -4445,9 +4456,9 @@ EXPORT_SYMBOL_GPL(xdp_do_redirect_frame); > static int xdp_do_generic_redirect_map(struct net_device *dev, > struct sk_buff *skb, > struct xdp_buff *xdp, > - struct bpf_prog *xdp_prog, > - void *fwd, > - enum bpf_map_type map_type, u32 map_id) > + struct bpf_prog *xdp_prog, void *fwd, > + enum bpf_map_type map_type, u32 map_id, > + u32 flags) > { > struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info); > struct bpf_map *map; > @@ -4457,11 +4468,20 @@ static int xdp_do_generic_redirect_map(struct net_device *dev, > case BPF_MAP_TYPE_DEVMAP: > fallthrough; > case BPF_MAP_TYPE_DEVMAP_HASH: > - map = READ_ONCE(ri->map); > - if (unlikely(map)) { > + if (unlikely(flags & BPF_F_BROADCAST)) { > + map = READ_ONCE(ri->map); > + > + /* The map pointer is cleared when the map is being torn > + * down by bpf_clear_redirect_map() > + */ > + if (unlikely(!map)) { > + err = -ENOENT; > + break; > + } > + > WRITE_ONCE(ri->map, NULL); > err = dev_map_redirect_multi(dev, skb, xdp_prog, map, > - ri->flags & BPF_F_EXCLUDE_INGRESS); > + flags & BPF_F_EXCLUDE_INGRESS); > } else { > err = dev_map_generic_redirect(fwd, skb, xdp_prog); > } > @@ -4498,9 +4518,11 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb, > enum bpf_map_type map_type = ri->map_type; > void *fwd = ri->tgt_value; > u32 map_id = ri->map_id; > + u32 flags = ri->flags; > int err; > > ri->map_id = 0; /* Valid map id idr range: [1,INT_MAX[ */ > + ri->flags = 0; > ri->map_type = BPF_MAP_TYPE_UNSPEC; > > if (map_type == BPF_MAP_TYPE_UNSPEC && map_id == INT_MAX) { > @@ -4520,7 +4542,7 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb, > return 0; > } > > - return xdp_do_generic_redirect_map(dev, skb, xdp, xdp_prog, fwd, map_type, map_id); > + return xdp_do_generic_redirect_map(dev, skb, xdp, xdp_prog, fwd, map_type, map_id, flags); > err: > _trace_xdp_redirect_err(dev, xdp_prog, ri->tgt_index, err); > return err; > -- > 2.44.0 > Reviewed-by: Hangbin Liu