From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.katalix.com (mail.katalix.com [3.9.82.81]) by smtp.subspace.kernel.org (Postfix) with ESMTP id EC9A0171E60; Wed, 3 Jul 2024 12:39:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=3.9.82.81 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720010368; cv=none; b=BOY2CigUgKZaIVhS6TaJoofiBirXj3l8UC5l4ZvRbnW94ikO2OurHrHfLy3h52pyac+GkEc1y4J6hseINISKZoIc3/xQyC0L2OwQ9e8RTi3MzJk84erB2xaUl5U7UNxVe2T7HxTodEFkolI9ybsBp2w0IVLFjg/1HPaMrQectVM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720010368; c=relaxed/simple; bh=4KS0NOSTe916DVS2ghPWA1bcCCr8Xm00pZgHtyhX8Kc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tCOo2xYLbwJgP/j7Zn7GvcnZPn/Kx/FqnyJRortqE01nD5dnJv6srrq0AO/8gEdQI5dOD3mDwiKv5rnFk9WzbRA8ppVVsbvxmZ6hXLctlIhSqXF54WkTlntIkIz2kxo+Jg5OVSIrcY+umXXCKFNEGMyk6M2fU44SWJqZbHvj3to= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=katalix.com; spf=pass smtp.mailfrom=katalix.com; dkim=pass (2048-bit key) header.d=katalix.com header.i=@katalix.com header.b=GnZFnWrg; arc=none smtp.client-ip=3.9.82.81 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=katalix.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=katalix.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=katalix.com header.i=@katalix.com header.b="GnZFnWrg" Received: from localhost (unknown [IPv6:2a02:8012:909b:0:1e90:7398:2278:75e2]) (Authenticated sender: tom) by mail.katalix.com (Postfix) with ESMTPSA id 11FDB7D76D; Wed, 3 Jul 2024 13:39:26 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=katalix.com; s=mail; t=1720010366; bh=4KS0NOSTe916DVS2ghPWA1bcCCr8Xm00pZgHtyhX8Kc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Disposition:In-Reply-To:From; z=Date:=20Wed,=203=20Jul=202024=2013:39:25=20+0100|From:=20Tom=20Pa rkin=20|To:=20Hillf=20Danton=20|Cc:=20syzbot=20,=0D=0A=09linux-kernel@vger.kernel.org,=20netdev@vger.ker nel.org,=0D=0A=09James=20Chapman=20,=0D=0A=0 9syzkaller-bugs@googlegroups.com|Subject:=20Re:=20[syzbot]=20[net? ]=20KASAN:=20slab-use-after-free=20Write=20in=0D=0A=20l2tp_session _delete|Message-ID:=20|References:=2 0=0D=0A=20<20240703115113.2928-1-hda nton@sina.com>|MIME-Version:=201.0|Content-Disposition:=20inline|I n-Reply-To:=20<20240703115113.2928-1-hdanton@sina.com>; b=GnZFnWrgLUYiTr5NbJYMtRCj0TOpeqxDrAybnef27jT4/ntNTFLn3lnCdCKA8pSHN ToSADbbWTU7JvvcPiW54YKlE9mORbjbR0E7urB0jxhjr2Et1UPI6+1YErEeG6SEo5O bqwzsKFoLOXDaXGDU85xoa/Iju+zix6wx4jFA5QIi2dNxEyAD4z4bzmyBE6RVfdguK k4K22t7PEUtY3w30DAINd6ZA0vjLtoY26cSAY+HbO7sQxRToX3T4dOnpkP2ZWjDtim QeCj4o3sMHt+HmznJ0UyNOsrPVpYxJQj6Ikpu60HvjimxFVIS5px2HkRNkqT3//OWq mmLLJ2gtUfALQ== Date: Wed, 3 Jul 2024 13:39:25 +0100 From: Tom Parkin To: Hillf Danton Cc: syzbot , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, James Chapman , syzkaller-bugs@googlegroups.com Subject: Re: [syzbot] [net?] KASAN: slab-use-after-free Write in l2tp_session_delete Message-ID: References: <20240703115113.2928-1-hdanton@sina.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="OWXArNVFVWFs/V+O" Content-Disposition: inline In-Reply-To: <20240703115113.2928-1-hdanton@sina.com> --OWXArNVFVWFs/V+O Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hi Hillf, On Wed, Jul 03, 2024 at 19:51:13 +0800, Hillf Danton wrote: > On Wed, 3 Jul 2024 12:24:49 +0100 Tom Parkin > >=20 > > [-- Attachment #1.1: Type: text/plain, Size: 379 bytes --] > >=20 > > On Tue, Jun 25, 2024 at 06:25:23 -0700, syzbot wrote: > > > syzbot found the following issue on: > > >=20 > > > HEAD commit: 185d72112b95 net: xilinx: axienet: Enable multicast b= y def.. > > > git tree: net-next > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=3D1062bd469= 80000 > >=20 > > #syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-ne= xt.git 185d72112b95 > >=20 > > [-- Attachment #1.2: 0001-l2tp-fix-possible-UAF-when-cleaning-up-tunnel= s.patch --] > > [-- Type: text/x-diff, Size: 3275 bytes --] > >=20 > > From 31321b7742266c4e58355076c19d8d490fa005d2 Mon Sep 17 00:00:00 2001 > > From: James Chapman > > Date: Tue, 2 Jul 2024 12:49:07 +0100 > > Subject: [PATCH] l2tp: fix possible UAF when cleaning up tunnels > >=20 > > syzbot reported a UAF caused by a race when the L2TP work queue closes a > > tunnel at the same time as a userspace thread closes a session in that > > tunnel. > >=20 > > Tunnel cleanup is handled by a work queue which iterates through the > > sessions contained within a tunnel, and closes them in turn. > >=20 > > Meanwhile, a userspace thread may arbitrarily close a session via > > either netlink command or by closing the pppox socket in the case of > > l2tp_ppp. > >=20 > > The race condition may occur when l2tp_tunnel_closeall walks the list > > of sessions in the tunnel and deletes each one. Currently this is > > implemented using list_for_each_safe, but because the list spinlock is > > dropped in the loop body it's possible for other threads to manipulate > > the list during list_for_each_safe's list walk. This can lead to the > > list iterator being corrupted, leading to list_for_each_safe spinning. > > One sequence of events which may lead to this is as follows: > >=20 > > * A tunnel is created, containing two sessions A and B. > > * A thread closes the tunnel, triggering tunnel cleanup via the work > > queue. > > * l2tp_tunnel_closeall runs in the context of the work queue. It > > removes session A from the tunnel session list, then drops the list > > lock. At this point the list_for_each_safe temporary variable is > > pointing to the other session on the list, which is session B, and > > the list can be manipulated by other threads since the list lock has > > been released. > > * Userspace closes session B, which removes the session from its parent > > tunnel via l2tp_session_delete. Since l2tp_tunnel_closeall has > > released the tunnel list lock, l2tp_session_delete is able to call > > list_del_init on the session B list node. > > * Back on the work queue, l2tp_tunnel_closeall resumes execution and > > will now spin forever on the same list entry until the underlying > > session structure is freed, at which point UAF occurs. > >=20 > > The solution is to iterate over the tunnel's session list using > > list_first_entry_not_null to avoid the possibility of the list > > iterator pointing at a list item which may be removed during the walk. > >=20 > > --- > > net/l2tp/l2tp_core.c | 9 +++++---- > > 1 file changed, 5 insertions(+), 4 deletions(-) > >=20 > > diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c > > index 64f446f0930b..afa180b7b428 100644 > > --- a/net/l2tp/l2tp_core.c > > +++ b/net/l2tp/l2tp_core.c > > @@ -1290,13 +1290,14 @@ static void l2tp_session_unhash(struct l2tp_ses= sion *session) > > static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel) > > { > > struct l2tp_session *session; > > - struct list_head *pos; > > - struct list_head *tmp; > > =20 > > spin_lock_bh(&tunnel->list_lock); > > tunnel->acpt_newsess =3D false; > > - list_for_each_safe(pos, tmp, &tunnel->session_list) { > > - session =3D list_entry(pos, struct l2tp_session, list); > > + for (;;) { > > + session =3D list_first_entry_or_null(&tunnel->session_list, > > + struct l2tp_session, list); > > + if (!session) > > + break; >=20 > WTF difference could this patch make wrt closing the race above? > Sorry if the commit message isn't clear :-( The specific UAF that syzbot hits is due to the list node that the list_for_each_safe temporary variable points to being modified while the list_for_each_safe walk is in process. This is possible due to l2tp_tunnel_closeall dropping the spin lock that protects the list mid way through the list_for_each_safe loop. This opens the door for another thread to call list_del_init on the node that list_for_each_safe is planning to process next, causing l2tp_tunnel_closeall to repeatedly iterate on that node forever. In the context of l2tp_ppp, this eventually leads to UAF because the session structure itself is freed when the pppol2tp socket is destroyed and the pppol2tp sk_destruct handler unrefs the session structure to zero. So to avoid the UAF, the list can safely be processed using a loop which accesses the first entry in the tunnel session list under spin lock protection, removing that entry, then dropping the lock to call l2tp_session_delete. FWIW, I note that syzbot has bisected this UAF to d18d3f0a24fc ("l2tp: replace hlist with simple list for per-tunnel session list") which changes l2tp_tunnel_closeall from a similar pattern of repeatedly grabbing the list head under spin lock projection, to the current code in net-next. > > list_del_init(&session->list); > > spin_unlock_bh(&tunnel->list_lock); > > l2tp_session_delete(session); --OWXArNVFVWFs/V+O Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAABCgAdFiEEsUkgyDzMwrj81nq0lIwGZQq6i9AFAmaFRnkACgkQlIwGZQq6 i9D4MQgAlIrC9CzRSg+P0UVnqtS/PLhm/PeRB4C6jo0u0H9ikWTDgNC0oM1Rxc18 jxt1yHHMUxdwKIdZGsY2i07WaA9ZAgpGfZk3w07GFDpa1Mub449igzOBPK9XwaYa J6gjgMMQmiQ3b3ag0VZkhreMuN3uPUERi2r2gxWfsqQq7Z8AZmsUkbWirM+VxnLE 1jwVrQmqxFvJB0S9nIq81srJcpr/LlE6KtzB7g1RXGBJtY/veJ58yRH66i9I51KA Y5AHKm0vVgTlFVzb1XKio97dH0pPBFwleJkYGEFNDn+MN06br7b/XHzUidBJGVZ+ oNOJrb+SfoRbisG2s5ElFcDdi7WckQ== =XdNU -----END PGP SIGNATURE----- --OWXArNVFVWFs/V+O--