[PATCH net] ppp: linearize bridged packets when required

PPP channel bridging is commonly used for PPPoE relaying or forwarding
from PPPoE to PPPoL2TP. Both destination channel types handle non-linear
packets, but other channels, such as PPP Async, require linear packets.

Ethernet drivers can pass non-linear packets to PPPoE. If one is bridged
to PPP Async, ppp_async_encode() accesses the entire packet through
skb->data, assuming that it is linear. This can read beyond the linear
buffer and transmit unrelated memory over the serial interface.

The normal PPP transmit path in ppp_push() linearizes packets before
passing them to channels without direct_xmit. Apply the same rule in
ppp_channel_bridge_input(), and drop the packet if linearization fails.

Fixes: 4cf476ced45d ("ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260908072135.877364-1-qingfang.deng@linux.dev
Signed-off-by: Qingfang Deng <redacted>
---
 drivers/net/ppp/ppp_generic.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 1a610a18893b..a84618e24d61 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c

@@ -2299,6 +2299,11 @@ static bool ppp_channel_bridge_input(struct channel *pch, struct sk_buff *skb)
 		goto outl;
 	}
 
+	if (!pchb->chan->direct_xmit && skb_linearize(skb)) {
+		kfree_skb(skb);
+		goto outl;
+	}
+
 	skb_scrub_packet(skb, !net_eq(pch->chan_net, pchb->chan_net));
 	if (!pchb->chan->ops->start_xmit(pchb->chan, skb))
 		kfree_skb(skb);

-- 
2.43.0