From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (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 E0E7C18A6CF for ; Tue, 14 Apr 2026 02:14:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776132854; cv=none; b=WnH4bolqR1EmQkIBznhLYb3bsJ9hRyosuiCIRt6ziu3ppZ2LR4FmMurYeFEpqLRcz4daWlLILz22pIw/lgTuO84dsUKrlrBFIHik14nGj9Q9rqtm/o09UrBnq2BfuOOB2SoRv7CBJBi90xsZwdy5rauOEqD6O3dSu6gqj5pFLTU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776132854; c=relaxed/simple; bh=723IGzpIiYRTW1HKqyAyT8h2Yx8UuC2XxwBYyg+HGus=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=OyermsB2b3ybJKJiHUrJFmwBrPBqT13vBUIAayVfAsJY6quPks5wTs0ggYoYLW6UKhqBbsGCWiWk+6YEhvzzx260ss+xHJOn07wFteewFW9EzSOrcZOdtl8G3pUBkph8HS0rxyC3v8/NI2Flv2ujboY/e/u4743akj5oAhID5wE= 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=tYcgF+jI; arc=none smtp.client-ip=91.218.175.178 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="tYcgF+jI" 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=1776132850; 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=4Wbmy/a9gqKJwNpYShcTiMCxp21HIntu0hhOBpqeZJw=; b=tYcgF+jIwU/OfWlrR+lhZjYoCfL60SrUYHBMMoo/EB1hlt4p7CrTxB90vhG1DpQMqRXMBt WiPUsFpMPgzTb7X67kcRhewSdrXPrWSBFnfXYLj5HhuQvsQAJE6NcUjN9dNqP1r0OZ1Zzu mKYboVeKEoINrJ+BeCyRvUhiAtJZYqE= From: Qingfang Deng To: linux-ppp@vger.kernel.org, Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Qingfang Deng , Breno Leitao , Sebastian Andrzej Siewior , Kees Cook , Kuniyuki Iwashima , Guillaume Nault , Eric Woudstra , Simon Horman , 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 v5 2/2] pppoe: drop PFC frames Date: Tue, 14 Apr 2026 10:13:49 +0800 Message-ID: <20260414021353.23471-2-qingfang.deng@linux.dev> In-Reply-To: <20260414021353.23471-1-qingfang.deng@linux.dev> References: <20260414021353.23471-1-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 Reviewed-by: Simon Horman --- Changes in v5: add Reviewed-by tag from Simon Link to v4: https://lore.kernel.org/netdev/20260410033627.93786-2-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 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 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 +#include #include #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