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From: John Fastabend <john.fastabend@gmail.com>
To: Jakub Sitnicki <jakub@cloudflare.com>, Lorenz Bauer <lmb@cloudflare.com>
Cc: John Fastabend <john.fastabend@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	kernel-team@cloudflare.com, netdev@vger.kernel.org,
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup
Date: Tue, 17 Mar 2020 11:16:17 -0700	[thread overview]
Message-ID: <5e7113f16e7c6_278b2b1b264c65b445@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <87imj3xb5t.fsf@cloudflare.com>

Jakub Sitnicki wrote:
> On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
> > Allow callers with CAP_NET_ADMIN to retrieve file descriptors from a
> > sockmap and sockhash. O_CLOEXEC is enforced on all fds.
> >
> > Without this, it's difficult to resize or otherwise rebuild existing
> > sockmap or sockhashes.
> >
> > Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
> > Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> > ---
> >  net/core/sock_map.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > index 03e04426cd21..3228936aa31e 100644
> > --- a/net/core/sock_map.c
> > +++ b/net/core/sock_map.c
> > @@ -347,12 +347,31 @@ static void *sock_map_lookup(struct bpf_map *map, void *key)
> >  static int __sock_map_copy_value(struct bpf_map *map, struct sock *sk,
> >  				 void *value)
> >  {
> > +	struct file *file;
> > +	int fd;
> > +
> >  	switch (map->value_size) {
> >  	case sizeof(u64):
> >  		sock_gen_cookie(sk);
> >  		*(u64 *)value = atomic64_read(&sk->sk_cookie);
> >  		return 0;
> >
> > +	case sizeof(u32):
> > +		if (!capable(CAP_NET_ADMIN))
> > +			return -EPERM;
> > +
> > +		fd = get_unused_fd_flags(O_CLOEXEC);
> > +		if (unlikely(fd < 0))
> > +			return fd;
> > +
> > +		read_lock_bh(&sk->sk_callback_lock);
> > +		file = get_file(sk->sk_socket->file);
> 
> I think this deserves a second look.
> 
> We don't lock the sock, so what if tcp_close orphans it before we enter
> this critical section? Looks like sk->sk_socket might be NULL.
> 
> I'd find a test that tries to trigger the race helpful, like:
> 
>   thread A: loop in lookup FD from map
>   thread B: loop in insert FD into map, close FD

Agreed, this was essentially my question above as well.

When the psock is created we call sock_hold() and will only do a sock_put()
after an rcu grace period when its removed. So at least if you have the
sock here it should have a sk_refcnt. (Note the user data is set to NULL
so if you do reference psock you need to check its non-null.)

Is that enough to ensure sk_socket? Seems not to me, tcp_close for example
will still happen and call sock_orphan(sk) based on my admittddly quick
look.

Further, even if you do check sk->sk_socket is non-null what does it mean
to return a file with a socket that is closed, deleted from the sock_map
and psock removed? At this point is it just a dangling reference?

Still a bit confused as well what would or should happen when the sock is closed
after you have the file reference? I could probably dig up what exactly
would happen but I think we need it in the commiit message so we understand
it. I also didn't dig up the details here but if the receiver of the
fd crashes or otherwise disappears this hopefully all get cleaned up?

> 
> > +		read_unlock_bh(&sk->sk_callback_lock);
> > +
> > +		fd_install(fd, file);
> > +		*(u32 *)value = fd;
> > +		return 0;
> > +
> >  	default:
> >  		return -ENOSPC;
> >  	}

  reply	other threads:[~2020-03-17 18:16 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200310174711.7490-1-lmb@cloudflare.com>
2020-03-10 17:47 ` [PATCH 1/5] bpf: add map_copy_value hook Lorenz Bauer
2020-03-10 17:47 ` [PATCH 2/5] bpf: convert queue and stack map to map_copy_value Lorenz Bauer
2020-03-11 14:00   ` Jakub Sitnicki
2020-03-11 22:31     ` John Fastabend
2020-03-10 17:47 ` [PATCH 3/5] bpf: convert sock map and hash " Lorenz Bauer
2020-03-11 13:55   ` Jakub Sitnicki
2020-03-10 17:47 ` [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup Lorenz Bauer
2020-03-11 23:27   ` John Fastabend
2020-03-17 10:17     ` Lorenz Bauer
2020-03-17 15:18   ` Jakub Sitnicki
2020-03-17 18:16     ` John Fastabend [this message]
2020-03-10 17:47 ` [PATCH 5/5] bpf: sockmap, sockhash: test looking up fds Lorenz Bauer
2020-03-11 13:52   ` Jakub Sitnicki
2020-03-11 17:24     ` Lorenz Bauer

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