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

[Kohei Enju <kohei@enjuk.jp>]

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;
 		indexes[n++] = upper->ifindex;
 	}
 	return n;
@@ -615,7 +617,8 @@ int dev_map_enqueue_multi(struct xdp_frame *xdpf, struct net_device *dev_rx,
 	int err;
 
 	if (exclude_ingress) {
-		num_excluded = get_upper_ifindexes(dev_rx, excluded_devices);
+		num_excluded = get_upper_ifindexes(dev_rx, excluded_devices,
+						   ARRAY_SIZE(excluded_devices) - 1);
 		excluded_devices[num_excluded++] = dev_rx->ifindex;
 	}
 
@@ -733,7 +736,8 @@ int dev_map_redirect_multi(struct net_device *dev, struct sk_buff *skb,
 	int err;
 
 	if (exclude_ingress) {
-		num_excluded = get_upper_ifindexes(dev, excluded_devices);
+		num_excluded = get_upper_ifindexes(dev, excluded_devices,
+						   ARRAY_SIZE(excluded_devices) - 1);
 		excluded_devices[num_excluded++] = dev->ifindex;
 	}
 
-- 
2.51.0