[PATCH 6.12 039/265] bpf: Fix stack-out-of-bounds write in devmap

Greg Kroah-Hartman
2026-03-12T20:13:44Z

From: Kohei Enju <kohei@enjuk.jp>

[ Upstream commit b7bf516c3ecd9a2aae2dc2635178ab87b734fef1 ]

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.
When there are too many upper devices, return -EOVERFLOW and abort the
redirect.

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")
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
Link: https://lore.kernel.org/r/20260225053506.4738-1-kohei@enjuk.jp
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/devmap.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 3aa002a47a966..39b7efa396b8e 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -588,18 +588,22 @@ 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.
- * Returns the number of ifindexes added.
+ * least 'max' elements.
+ * Returns the number of ifindexes added, or -EOVERFLOW if there are too
+ * many upper devices.
*/
-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)
+ return -EOVERFLOW;
indexes[n++] = upper->ifindex;
}
+
return n;
}

@@ -615,7 +619,11 @@ 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);
+ if (num_excluded < 0)
+ return num_excluded;
+
excluded_devices[num_excluded++] = dev_rx->ifindex;
}

@@ -733,7 +741,11 @@ 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);
+ if (num_excluded < 0)
+ return num_excluded;
+
excluded_devices[num_excluded++] = dev->ifindex;
}

--
2.51.0