From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 97E0430AACB; Fri, 10 Apr 2026 03:36:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775792219; cv=none; b=psqu0vWU/gNwIIQddeQuluahjV/gVOljf7OaZNavjiAwSt40PjQoJEjtwxwzVc6ktXR/liM7yqHOBcXg1KVItZ7GZsXH6ixRJFjoCyY6pWYM2vLDYbIkc5oRMwacSHjbn7Hxl68XO80OkO2d8xFMMHmKdfkVpZys7RZ8zb4/7Ro= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775792219; c=relaxed/simple; bh=qOvRNxr5Cs+kIujf5Omcxxoc0SIvL5nuVH71WgX7zAQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=tvTIwv17/SVyxlWldZBt050XuC1uIy+W98fU4nBGyow1Z3VJza/SKpIFHZdazTLs7mRXYJ4ZY9vk1daVjV7sAGghTYpIf5oEPO9CouhYJwABUxnV+nuubobZtwzQz8Cgkal+G9upUtOZbfqlZzXCP+7hFOWiRHk5eCp56yNz3Vs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=OPUBEwGz; arc=none smtp.client-ip=91.218.175.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="OPUBEwGz" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1775792215; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QAJ9K4xL9oDCb2SK19yKakNMeycMiF95YjwaYyavShE=; b=OPUBEwGzY2co+p0AEcBKlsjfkOhx0y6rAbqhwI5SI7d2Yzrp+zBVHIdNpqUM1CsMF7zwgO KfWQCchMFxI11y9sAdp0JQ00Er7Osfn42U19WWQfxnf9pr25iTYftgHwybXa9cMveKmHI1 z1lBoy8WtH0fKIq3DfVWYSt2PLIPhKw= From: Qingfang Deng To: linux-ppp@vger.kernel.org, Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Qingfang Deng , Guillaume Nault , Breno Leitao , Kuniyuki Iwashima , Kees Cook , Sebastian Andrzej Siewior , Eric Woudstra , Sam Protsenko , netdev@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Paul Mackerras , Jaco Kroon , James Carlson , Wojciech Drewek , Marcin Szycik Subject: [PATCH net v4 2/2] pppoe: drop PFC frames Date: Fri, 10 Apr 2026 11:36:21 +0800 Message-ID: <20260410033627.93786-2-qingfang.deng@linux.dev> In-Reply-To: <20260410033627.93786-1-qingfang.deng@linux.dev> References: <20260410033627.93786-1-qingfang.deng@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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 --- Changes in v4: Update Fixes tag as suggested by AI review Link to v3: https://lore.kernel.org/r/20260409031107.616630-2-qingfang.deng@linux.dev Changes in v3: Fix kdoc warning Link to v2: https://lore.kernel.org/r/20260408024245.312732-1-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 b097d1b38ac9..853da966ad46 100644 --- a/drivers/net/ppp/ppp_generic.c +++ b/drivers/net/ppp/ppp_generic.c @@ -2242,7 +2242,7 @@ 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 1ac61c273b28..4cd10c908711 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 45c0947fa404..6e9587ce651c 100644 --- a/include/linux/ppp_defs.h +++ b/include/linux/ppp_defs.h @@ -8,8 +8,24 @@ #define _PPP_DEFS_H_ #include +#include #include #define PPP_FCS(fcs, c) crc_ccitt_byte(fcs, c) +/** + * 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