[PATCH net] ppp: linearize bridged packets when required

[PATCH net] ppp: linearize bridged packets when required

[Qingfang Deng]

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 <qingfang.deng@linux.dev>
---
 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

Re: [PATCH net] ppp: linearize bridged packets when required

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net] ppp: linearize bridged packets when required

[Qingfang Deng]

On 2026/10/21 10:39, netdev-bot+sinfo@kernel.org wrote:
> Hi!
>
> This is an automated message. This series looks like a fix, but its
> commit messages seem to be missing some information:
>
>   - Whether the issue was actually triggered, or is only theoretical
>     (e.g. found by code inspection). If it was triggered please include
>     the symptoms, like the stack trace or error messages.

This is reproduced in a VM, with a TAP interface bridged to a PTY. When 
IFF_NAPI_FRAGS is set, a non-linear skb is passed to PPPoE and then to 
ppp_async_send() without linearization. The packet sent to the PTY then 
contains garbage data.

>
> Please do not repost the series just to address the above. Instead,
> reply to this email with the missing information, so that reviewers
> can take it into account. If the series needs another revision for
> other reasons, please include the information in the commit messages
> then.
>
> The evaluation is done by an LLM so it may be wrong, if you think
> that is the case please reply and explain.