netdev - Re: [PATCH v1 net] af_unix: Call kfree_skb() for dead unix_(sk)->oob

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Message-ID: <20240203091459.9066-1-kuniyu@amazon.com>
Date: Sat, 3 Feb 2024 01:14:59 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <kuni1840@...il.com>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<syzbot+fa3ef895554bdbfd1183@...kaller.appspotmail.com>
Subject: Re: [PATCH v1 net] af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.

From: Eric Dumazet <edumazet@...gle.com>
Date: Sat, 3 Feb 2024 10:01:04 +0100
> On Sat, Feb 3, 2024 at 9:33 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > syzbot reported a warning [0] in __unix_gc() with a repro, which
> > creates a socketpair and sends one socket's fd to itself using the
> > peer.
> >
> >   socketpair(AF_UNIX, SOCK_STREAM, 0, [3, 4]) = 0
> >   sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\360", iov_len=1}],
> >           msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET,
> >                                       cmsg_type=SCM_RIGHTS, cmsg_data=[3]}],
> >           msg_controllen=24, msg_flags=0}, MSG_OOB|MSG_PROBE|MSG_DONTWAIT|MSG_ZEROCOPY) = 1
> >
> > This forms a self-cyclic reference that GC should finally untangle
> > but does not due to lack of MSG_OOB handling, resulting in memory
> > leak.
> >
> > Recently, commit 11498715f266 ("af_unix: Remove io_uring code for
> > GC.") removed io_uring's dead code in GC and revealed the problem.
> >
> > The code was executed at the final stage of GC and unconditionally
> > moved all GC candidates from gc_candidates to gc_inflight_list.
> > That papered over the reported problem by always making the following
> > WARN_ON_ONCE(!list_empty(&gc_candidates)) false.
> >
> > The problem has been there since commit 2aab4b969002 ("af_unix: fix
> > struct pid leaks in OOB support") added full scm support for MSG_OOB
> > while fixing another bug.
> >
> > To fix this problem, we must call kfree_skb() for unix_sk(sk)->oob_skb
> > if the socket still exists in gc_candidates after purging collected skb.
> >
> > Note that the leaked socket remained being linked to a global list, so
> > kmemleak also could not detect it.  We need to check /proc/net/protocol
> > to notice the unfreed socket.
> >
> > [
> > Reported-by: syzbot+fa3ef895554bdbfd1183@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=fa3ef895554bdbfd1183
> > Fixes: 2aab4b969002 ("af_unix: fix struct pid leaks in OOB support")
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > ---
> > Given the commit disabling SCM_RIGHTS w/ io_uring was backporeted to
> > stable trees, we can backport this patch without commit 11498715f266,
> > so targeting net tree.
> > ---
> >  net/unix/garbage.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/net/unix/garbage.c b/net/unix/garbage.c
> > index 2405f0f9af31..61f313d4a5dd 100644
> > --- a/net/unix/garbage.c
> > +++ b/net/unix/garbage.c
> > @@ -314,6 +314,15 @@ void unix_gc(void)
> >         /* Here we are. Hitlist is filled. Die. */
> >         __skb_queue_purge(&hitlist);
> >
> > +       list_for_each_entry_safe(u, next, &gc_candidates, link) {
> > +               struct sk_buff *skb = u->oob_skb;
> > +
> > +               if (skb) {
> > +                       u->oob_skb = NULL;
> > +                       kfree_skb(skb);
> > +               }
> > +       }
> > +
> 
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> Note there is already a 'struct sk_buff *skb;" variable in scope.
> 
> This could be rewritten
> 
> list_for_each_entry_safe(u, next, &gc_candidates, link) {
>         kfree_skb(u->oob_skb);
>         u->oob_skb = NULL;
> }

I wrote that in the inital fix but noticed that this
kfree_skb() triggers fput(), and later in unix_release_sock()
we will call the duplicate kfree_skb().

        if (u->oob_skb) {
                kfree_skb(u->oob_skb);
		u->oob_skb = NULL;
        }

So, we need to set NULL before kfree_skb() in __unix_gc().


> 
> Also we probably can send this later:

Yes, I changed as such in a new GC implementation, this needs
respin for MSG_OOB support though...
https://lore.kernel.org/netdev/20240203030058.60750-14-kuniyu@amazon.com/

Thanks!

> 
> diff --git a/net/unix/garbage.c b/net/unix/garbage.c
> index 2405f0f9af31c0ccefe2aa404002cfab8583c090..02466224445c9ec9b1259468d30c89fc5e905a6b
> 100644
> --- a/net/unix/garbage.c
> +++ b/net/unix/garbage.c
> @@ -283,7 +283,7 @@ void unix_gc(void)
>          * inflight counters for these as well, and remove the skbuffs
>          * which are creating the cycle(s).
>          */
> -       skb_queue_head_init(&hitlist);
> +       __skb_queue_head_init(&hitlist);
>         list_for_each_entry(u, &gc_candidates, link)
>                 scan_children(&u->sk, inc_inflight, &hitlist);