[PATCH net v6 1/2] flow_dissector: do not dissect PPPoE PFC frames

[PATCH net v6 1/2] flow_dissector: do not dissect PPPoE PFC frames

[Qingfang Deng]

RFC 2516 Section 7 states that Protocol Field Compression (PFC) is NOT
RECOMMENDED for PPPoE. In practice, pppd does not support negotiating
PFC for PPPoE sessions, and the flow dissector driver has assumed an
uncompressed frame until the blamed commit.

During the review process of that commit [1], support for PFC is
suggested. However, having a compressed (1-byte) protocol field means
the subsequent PPP payload is shifted by one byte, causing 4-byte
misalignment for the network header and an unaligned access exception
on some architectures.

The exception can be reproduced by sending a PPPoE PFC frame to an
ethernet interface of a MIPS board, with RPS enabled, even if no PPPoE
session is active on that interface:

$ 0   : 00000000 80c40000 00000000 85144817
$ 4   : 00000008 00000100 80a75758 81dc9bb8
$ 8   : 00000010 8087ae2c 0000003d 00000000
$12   : 000000e0 00000039 00000000 00000000
$16   : 85043240 80a75758 81dc9bb8 00006488
$20   : 0000002f 00000007 85144810 80a70000
$24   : 81d1bda0 00000000
$28   : 81dc8000 81dc9aa8 00000000 805ead08
Hi    : 00009d51
Lo    : 2163358a
epc   : 805e91f0 __skb_flow_dissect+0x1b0/0x1b50
ra    : 805ead08 __skb_get_hash_net+0x74/0x12c
Status: 11000403        KERNEL EXL IE
Cause : 40800010 (ExcCode 04)
BadVA : 85144817
PrId  : 0001992f (MIPS 1004Kc)
Call Trace:
[<805e91f0>] __skb_flow_dissect+0x1b0/0x1b50
[<805ead08>] __skb_get_hash_net+0x74/0x12c
[<805ef330>] get_rps_cpu+0x1b8/0x3fc
[<805fca70>] netif_receive_skb_list_internal+0x324/0x364
[<805fd120>] napi_complete_done+0x68/0x2a4
[<8058de5c>] mtk_napi_rx+0x228/0xfec
[<805fd398>] __napi_poll+0x3c/0x1c4
[<805fd754>] napi_threaded_poll_loop+0x234/0x29c
[<805fd848>] napi_threaded_poll+0x8c/0xb0
[<80053544>] kthread+0x104/0x12c
[<80002bd8>] ret_from_kernel_thread+0x14/0x1c

Code: 02d51821  1060045b  00000000 <8c640000> 3084000f  2c820005  144001a2  00042080  8e220000

To reduce the attack surface and maintain performance, do not process
PPPoE PFC frames.

[1] https://lore.kernel.org/r/20220630231016.GA392@debian.home
Fixes: 46126db9c861 ("flow_dissector: Add PPPoE dissectors")
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
Changes in v6: document that PFC is rejected by ppp_proto_is_valid().
 Link to v5: https://lore.kernel.org/r/20260414021353.23471-1-qingfang.deng@linux.dev

 net/core/flow_dissector.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 1b61bb25ba0e..c471559db4d9 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1374,16 +1374,13 @@ bool __skb_flow_dissect(const struct net *net,
 			break;
 		}
 
-		/* least significant bit of the most significant octet
-		 * indicates if protocol field was compressed
+		/* PFC (compressed 1-byte protocol) frames are not processed.
+		 * A compressed protocol field has the least significant bit of
+		 * the most significant octet set, which will fail the following
+		 * ppp_proto_is_valid(), returning FLOW_DISSECT_RET_OUT_BAD.
 		 */
 		ppp_proto = ntohs(hdr->proto);
-		if (ppp_proto & 0x0100) {
-			ppp_proto = ppp_proto >> 8;
-			nhoff += PPPOE_SES_HLEN - 1;
-		} else {
-			nhoff += PPPOE_SES_HLEN;
-		}
+		nhoff += PPPOE_SES_HLEN;
 
 		if (ppp_proto == PPP_IP) {
 			proto = htons(ETH_P_IP);
-- 
2.43.0

[PATCH net v6 2/2] pppoe: drop PFC frames

[Qingfang Deng]

RFC 2516 Section 7 states that Protocol Field Compression (PFC) is NOT
RECOMMENDED for PPPoE. In practice, pppd does not support negotiating
PFC for PPPoE sessions, and the current PPPoE driver assumes an
uncompressed (2-byte) protocol field. However, the generic PPP layer
function ppp_input() is not aware of the negotiation result, and still
accepts PFC frames.

If a peer with a broken implementation or an attacker sends a frame with
a compressed (1-byte) protocol field, the subsequent PPP payload is
shifted by one byte. This causes the network header to be 4-byte
misaligned, which may trigger unaligned access exceptions on some
architectures.

To reduce the attack surface, drop PPPoE PFC frames. Introduce
ppp_skb_is_compressed_proto() helper function to be used in both
ppp_generic.c and pppoe.c to avoid open-coding.

Fixes: 7fb1b8ca8fa1 ("ppp: Move PFC decompression to PPP generic layer")
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
---
Changes in v6: none
 Link to v5: https://lore.kernel.org/r/20260414021353.23471-2-qingfang.deng@linux.dev

 drivers/net/ppp/ppp_generic.c |  2 +-
 drivers/net/ppp/pppoe.c       |  8 +++++++-
 include/linux/ppp_defs.h      | 16 ++++++++++++++++
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index b0d3bc49c685..57c68efa5ff8 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2245,7 +2245,7 @@ ppp_do_recv(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
  */
 static void __ppp_decompress_proto(struct sk_buff *skb)
 {
-	if (skb->data[0] & 0x01)
+	if (ppp_skb_is_compressed_proto(skb))
 		*(u8 *)skb_push(skb, 1) = 0x00;
 }
 
diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
index d546a7af0d54..bdd61c504a1c 100644
--- a/drivers/net/ppp/pppoe.c
+++ b/drivers/net/ppp/pppoe.c
@@ -393,7 +393,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (skb_mac_header_len(skb) < ETH_HLEN)
 		goto drop;
 
-	if (!pskb_may_pull(skb, sizeof(struct pppoe_hdr)))
+	if (!pskb_may_pull(skb, PPPOE_SES_HLEN))
 		goto drop;
 
 	ph = pppoe_hdr(skb);
@@ -403,6 +403,12 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (skb->len < len)
 		goto drop;
 
+	/* skb->data points to the PPP protocol header after skb_pull_rcsum.
+	 * Drop PFC frames.
+	 */
+	if (ppp_skb_is_compressed_proto(skb))
+		goto drop;
+
 	if (pskb_trim_rcsum(skb, len))
 		goto drop;
 
diff --git a/include/linux/ppp_defs.h b/include/linux/ppp_defs.h
index b7e57fdbd413..b1d1f46d7d3b 100644
--- a/include/linux/ppp_defs.h
+++ b/include/linux/ppp_defs.h
@@ -8,6 +8,7 @@
 #define _PPP_DEFS_H_
 
 #include <linux/crc-ccitt.h>
+#include <linux/skbuff.h>
 #include <uapi/linux/ppp_defs.h>
 
 #define PPP_FCS(fcs, c) crc_ccitt_byte(fcs, c)
@@ -25,4 +26,19 @@ static inline bool ppp_proto_is_valid(u16 proto)
 	return !!((proto & 0x0101) == 0x0001);
 }
 
+/**
+ * ppp_skb_is_compressed_proto - checks if PPP protocol in a skb is compressed
+ * @skb: skb to check
+ *
+ * Check if the PPP protocol field is compressed (the least significant
+ * bit of the most significant octet is 1). skb->data must point to the PPP
+ * protocol header.
+ *
+ * Return: Whether the PPP protocol field is compressed.
+ */
+static inline bool ppp_skb_is_compressed_proto(const struct sk_buff *skb)
+{
+	return unlikely(skb->data[0] & 0x01);
+}
+
 #endif /* _PPP_DEFS_H_ */
-- 
2.43.0

Re: [PATCH net v6 1/2] flow_dissector: do not dissect PPPoE PFC frames

[Qingfang Deng]

The patch state is "Changes Requested" in patchwork but I haven't 
received any feedback. Was it set by mistake?

Re: [PATCH net v6 1/2] flow_dissector: do not dissect PPPoE PFC frames

[Jakub Kicinski]

On Wed, 15 Apr 2026 21:42:09 +0800 Qingfang Deng wrote:
> The patch state is "Changes Requested" in patchwork but I haven't 
> received any feedback. Was it set by mistake?

Fixed, thanks for the report.