From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta1.migadu.com (out-182.mta1.migadu.com [95.215.58.182]) (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 A4DC01E5702 for ; Tue, 7 Jul 2026 02:05:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783389947; cv=none; b=gEVDB1w2NnqrZauMgT5+y+jU3vlJf6BiS/3lLbWWQ6l/uww/lRq+ll+BD2gWQpOlYz6/CkTT4tqEoD3XJYIojw+EYDl6C+Y4MUcDpqvyTIzYViM0n5imO7B/71kSA/3rzp0zSHi3mLOisGcy9vgkA3W0a7KnvqGpmqnnF1aEW0Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783389947; c=relaxed/simple; bh=CxakYhcTC8qBau/fxdIwtdXstC43wajWnY+nfADNZWY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WxUjV/ZIhziMhGuA4VApTmk0bjTyQ2Oo6bGZWsAL1zdtn7UzLajMVKEkOXnK4Wtj+ALKnGNC8GC6jDZ1xO/Y0MfauyhzfIU/CIkbSdgFLmCXSLWym9z20ZQNS0rWPbgd4HoO+cj49yLaI4H4WB8FkCZ746/PgpC8uRWWqjeJyA0= 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=EdF7Qsgn; arc=none smtp.client-ip=95.215.58.182 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="EdF7Qsgn" Message-ID: <45dd60a1-231d-4837-9280-786c5d0dbc72@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783389933; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Ynz9q6tQQnN+EtjSzOMxhstbYYQHChNYPvl8qjxy068=; b=EdF7QsgnX8LnqtSvbL07KGokgaang2C56R+9LPeRuc7ovgzc8HjGNt4KMWjtLFUdQsa068 RerDrPK10VyvpwN/sVoQmYxp5wskwwXafzgU4VCrNn4/tKpyqLUuNuLC6AfY6i9Atz2MY0 baoTVPAple0Mljx05j92PxXypJwcwvE= Date: Tue, 7 Jul 2026 10:05:18 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH net v3] ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF To: Norbert Szetei , netdev@vger.kernel.org Cc: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Yue Haibing , Guillaume Nault , Kees Cook , Taegu Ha , linux-ppp@vger.kernel.org, linux-kernel@vger.kernel.org References: X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qingfang Deng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/7/6 17:01, Norbert Szetei wrote: > pppol2tp_recv() runs in the L2TP UDP-encap softirq RX path: > > l2tp_udp_encap_recv() -> l2tp_recv_common() -> pppol2tp_recv() > -> ppp_input(&po->chan) > > It runs under rcu_read_lock() holding only an l2tp_session reference and > takes NO reference on the internal PPP channel (struct channel, > chan->ppp) that ppp_input() dereferences. > > The pppox socket is SOCK_RCU_FREE, so 'po' and the embedded ppp_channel > are RCU-safe. But the internal struct channel is a separate allocation > that ppp_release_channel() frees with a plain kfree(): > > close(data socket) -> pppol2tp_release() -> pppox_unbind_sock() > -> ppp_unregister_channel() -> ppp_release_channel() -> kfree(pch) > > For a channel that is bound (PPPIOCGCHAN) but not attached to a ppp unit > (no PPPIOCCONNECT, pch->ppp == NULL) and not bridged, teardown skips > both ppp_disconnect_channel()'s synchronize_net() and > ppp_unbridge_channels()'s synchronize_rcu(), so the kfree() has no grace > period. rcu_read_lock() in pppol2tp_recv() does not protect against a > plain kfree(), so an in-flight ppp_input() on one CPU can dereference > the channel just freed by close() on another CPU. > > The bug is reachable by an unprivileged user. > > Defer the channel free to an RCU callback via call_rcu() so the grace > period fences any in-flight ppp_input(). The disconnect and unbridge > teardown paths already fence with synchronize_net()/synchronize_rcu(); > call_rcu() does the same here without stalling the close() path. > > Fixes: ee40fb2e1eb5 ("l2tp: protect sock pointer of struct pppol2tp_session with RCU") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Norbert Szetei > --- > v3: > - Added rcu_barrier() at the end of ppp_cleanup() to ensure all > ppp_release_channel_free() callbacks complete before the module's > text segment is unloaded (Documentation/RCU/rcubarrier.rst). > v2: https://lore.kernel.org/linux-ppp/D9C0245B-608B-4884-8A09-F55BA4A9F948@doyensec.com/ > - Moved skb_queue_purge() to a dedicated RCU callback to prevent leaking > skbs added by an in-flight ppp_input() during the grace period (Sebastian). > - Retained call_rcu() to avoid introducing synchronous multi-millisecond > latency into the teardown path. > v1: https://lore.kernel.org/netdev/C954A7EA-AA98-4E3C-80B5-42C34B3183A3@doyensec.com/ > > drivers/net/ppp/ppp_generic.c | 18 +++++++++++++++--- > 1 file changed, 15 insertions(+), 3 deletions(-) > > diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c > index 57c68efa5ff8..717c1d3aa953 100644 > --- a/drivers/net/ppp/ppp_generic.c > +++ b/drivers/net/ppp/ppp_generic.c > @@ -184,6 +184,7 @@ struct channel { > struct list_head clist; /* link in list of channels per unit */ > spinlock_t upl; /* protects `ppp' and 'bridge' */ > struct channel __rcu *bridge; /* "bridged" ppp channel */ > + struct rcu_head rcu; /* for RCU-deferred free of the channel */ > #ifdef CONFIG_PPP_MULTILINK > u8 avail; /* flag used in multilink stuff */ > u8 had_frag; /* >= 1 fragments have been sent */ > @@ -3562,6 +3563,18 @@ ppp_disconnect_channel(struct channel *pch) > return err; > } > > +/* Purge after the grace period: a late ppp_input() may still queue an > + * skb on pch->file.rq before the last RCU reader drains. > + */ > +static void ppp_release_channel_free(struct rcu_head *rcu) > +{ > + struct channel *pch = container_of(rcu, struct channel, rcu); > + > + skb_queue_purge(&pch->file.xq); > + skb_queue_purge(&pch->file.rq); > + kfree(pch); > +} > + > /* > * Drop a reference to a ppp channel and free its memory if the refcount reaches > * zero. > @@ -3581,9 +3594,7 @@ static void ppp_release_channel(struct channel *pch) > pr_err("ppp: destroying undead channel %p !\n", pch); > return; > } > - skb_queue_purge(&pch->file.xq); > - skb_queue_purge(&pch->file.rq); > - kfree(pch); > + call_rcu(&pch->rcu, ppp_release_channel_free); > } > > static void __exit ppp_cleanup(void) > @@ -3596,6 +3607,7 @@ static void __exit ppp_cleanup(void) > device_destroy(&ppp_class, MKDEV(PPP_MAJOR, 0)); > class_unregister(&ppp_class); > unregister_pernet_device(&ppp_net_ops); > + rcu_barrier(); /* wait for RCU callbacks before module unload */ > } > > /* Reviewed-by: Qingfang Deng