From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0B3AF4B7A26; Thu, 17 Sep 2026 15:25:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789658742; cv=none; b=WoSNGFFbtEO8ypr4fac3QMfgAmYhgOCzCheEO/T9FJCZG0B7t5bDnNFlKKlDfwjPlw3FYrw1OLZZI3hIcjUOXPZHSfU2+WjE9fHfWg8DVa2ChHQhteIXmw+tL8yizYwFGFsns5b4/0ETq4tF+GAdHWdhQBnI68LpWS+wDHbcg4E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789658742; c=relaxed/simple; bh=y3FdZUVFG/GhNRJhiEYZx9SFGGkRW7JwT3+STFpQDXo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=sQc53KCYfJbcc0o+xsfs/rdcdF9jnR+FGXj0+7WjH8YvGTIBq64pPWEzeoJk1sT4WbZXJAQN7+TGjF1uG/x+nsxfeEs3ApF5WiMpLOTnKqsvbM/1sgNtGhM5hgp68NHXWXoh/OvdNjF+cLMprMYD/LFoV0P7n6wQ4dy/e5RkD3o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=qwyKI+HS; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="qwyKI+HS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4E431F00893; Thu, 17 Sep 2026 15:25:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789658737; bh=bn1nk9aouYNQtdYE33nWpiRmeylvv6olnlweWQYGqJc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=qwyKI+HSQ25OWDOhQZhEdMopJ7kJmVeGcEmiU7s9VUe6hWA5S+Ty04uCF+aYCtQW+ dDkKcJCy/EgPI6MaZ5WY8v9leHw0QWouYkifm7QwxrNasJoxFt7sxsyglTLsv/oBIs YU1lC0Y13t2Wc5ODitSuGmbq1NhOQZn2WbCqqJ9Q= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Eric Dumazet , Vlatko Kosturjak , Jakub Kicinski , Sasha Levin Subject: [PATCH 7.2 005/733] ppp_async: drop the errored frame instead of resetting its headroom Date: Thu, 17 Sep 2026 16:05:13 +0100 Message-ID: <20260917151350.759675044@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260917151350.597953846@linuxfoundation.org> References: <20260917151350.597953846@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Vlatko Kosturjak [ Upstream commit 8dc5d98a16fa23c00999aecf10018c9f69fa5bf4 ] ppp_receive_nonmp_frame() prepends a two-byte direction tag before running the pass/active BPF filters: *(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_INBOUND_TAG); Nothing on the receive path guarantees those two bytes of headroom. The frame-error path in ppp_async's process_input_packet() resets a reused skb's headroom to zero while claiming to restore it to a freshly allocated state - but a fresh skb from dev_alloc_skb() carries NET_SKB_PAD: err: if (skb) { /* make skb appear as freshly allocated */ skb_trim(skb, 0); skb_reserve(skb, - skb_headroom(skb)); } ap->rpkt still points at that skb, so the next frame is reassembled into it with no headroom at all. A peer that sends a bad-FCS frame followed by one beginning ff 03 then leaves a single byte of headroom by the time the filter tag is pushed, which lands one byte below skb->head: skbuff: skb_under_panic: len:49 put:2 head:ffff888003c10000 data:ffff888003c0ffff tail:0x30 end:0x640 dev: kernel BUG at net/core/skbuff.c:214! RIP: 0010:skb_panic+0x13e/0x230 Call Trace: skb_push+0xbd/0x100 ppp_receive_nonmp_frame+0x48a/0x1d10 ppp_input+0x4e9/0x2f80 ppp_async_process+0x2a/0xe0 tasklet_action_common+0x20f/0x8a0 handle_softirqs+0x18e/0x590 Kernel panic - not syncing: Fatal exception in interrupt Zeroing the headroom violates the NET_SKB_PAD guarantee that dev_alloc_skb() gives the rest of the receive path. Besides the filter panic above, when CCP compression is enabled ppp_decompress_frame() hands skb->data - 2 to ->decompress()/->incomp(), which then reads out of bounds before skb->head for the same reason. Rather than restore the headroom, drop the errored frame - as ppp_synctty already does on its error path - and clear ap->rpkt so the next frame is reassembled into a fresh skb with proper headroom. This is simpler and fixes both the filter under-panic and the CCP out-of-bounds read. The original V1 of this patch made room in ppp_receive_nonmp_frame() with skb_cow_head(); Eric pointed out that fixing the root cause in the transport is the right approach. Found by fuzzing the PPP receive path with a mutating peer on a pty; it is an interesting (remote) DoS: root configures PPP, the peer supplies two crashing frames. The reproducer (repro-ppp-skb.c, unchanged from v1) panics in about a second, and returns cleanly with this applied. Fixes: 6722e78c9005 ("[PPP]: handle misaligned accesses") Suggested-by: Eric Dumazet Signed-off-by: Vlatko Kosturjak Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/apkR6ZU+tqP2C3Fl@griffin.linux.hr Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- drivers/net/ppp/ppp_async.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c index 93a7b0f6c4e7e..a5db4f78b5fc8 100644 --- a/drivers/net/ppp/ppp_async.c +++ b/drivers/net/ppp/ppp_async.c @@ -812,11 +812,8 @@ process_input_packet(struct asyncppp *ap) err: /* frame had an error, remember that, reset SC_TOSS & SC_ESCAPE */ ap->state = SC_PREV_ERROR; - if (skb) { - /* make skb appear as freshly allocated */ - skb_trim(skb, 0); - skb_reserve(skb, - skb_headroom(skb)); - } + kfree_skb(skb); + ap->rpkt = NULL; } /* Called when the tty driver has data for us. Runs parallel with the -- 2.53.0