netdev - Re: [PATCH net v2] tcp: drop secpath at the same time as we currently drop dst

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Message-ID: <Z7PC1JoBvgFL9JAU@hog>
Date: Tue, 18 Feb 2025 00:14:28 +0100
From: Sabrina Dubroca <sd@...asysnail.net>
To: Paul Moore <paul@...l-moore.com>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Kuniyuki Iwashima <kuniyu@...zon.com>,
	David Ahern <dsahern@...nel.org>,
	linux-security-module@...r.kernel.org, Xiumei Mu <xmu@...hat.com>
Subject: Re: [PATCH net v2] tcp: drop secpath at the same time as we
 currently drop dst

2025-02-17, 17:35:32 -0500, Paul Moore wrote:
> On Mon, Feb 17, 2025 at 5:23 AM Sabrina Dubroca <sd@...asysnail.net> wrote:
> >
> > Xiumei reported hitting the WARN in xfrm6_tunnel_net_exit while
> > running tests that boil down to:
> >  - create a pair of netns
> >  - run a basic TCP test over ipcomp6
> >  - delete the pair of netns
> >
> > The xfrm_state found on spi_byaddr was not deleted at the time we
> > delete the netns, because we still have a reference on it. This
> > lingering reference comes from a secpath (which holds a ref on the
> > xfrm_state), which is still attached to an skb. This skb is not
> > leaked, it ends up on sk_receive_queue and then gets defer-free'd by
> > skb_attempt_defer_free.
> >
> > The problem happens when we defer freeing an skb (push it on one CPU's
> > defer_list), and don't flush that list before the netns is deleted. In
> > that case, we still have a reference on the xfrm_state that we don't
> > expect at this point.
> >
> > We already drop the skb's dst in the TCP receive path when it's no
> > longer needed, so let's also drop the secpath. At this point,
> > tcp_filter has already called into the LSM hooks that may require the
> > secpath, so it should not be needed anymore.
> 
> I don't recall seeing any follow up in the v1 patchset regarding
> IP_CMSG_PASSSEC/security_socket_getpeersec_dgram(), can you confirm
> that the secpath is preserved for that code path?
> 
> https://lore.kernel.org/linux-security-module/CAHC9VhQZ+k1J0UidJ-bgdBGBuVX9M18tQ+a+fuqXQM_L-PFvzA@mail.gmail.com

Sorry, I thought we'd addressed this in the v1 discussion with Eric.

IP_CMSG_PASSSEC is not blocked for TCP sockets, but it will only
process skbs that came from the error queue (ip_recv_error ->
ip_cmsg_recv -> ip_cmsg_recv_offset -> ip_cmsg_recv_security ->
security_socket_getpeersec_dgram), which don't go through those code
paths at all. So AFAICT IP_CMSG_PASSSEC for TCP isn't affected by
dropping the secpath early.

-- 
Sabrina