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SubjectRe: [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup
On Wed, 11 Mar 2020 at 23:27, John Fastabend <john.fastabend@gmail.com> wrote:
>
> 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);
> > + read_unlock_bh(&sk->sk_callback_lock);
> > +
> > + fd_install(fd, file);
> > + *(u32 *)value = fd;
> > + return 0;
> > +
>
> Hi Lorenz, Can you say something about what happens if the sk
> is deleted from the map or the sock is closed/unhashed ideally
> in the commit message so we have it for later reference. I guess
> because we are in an rcu block here the sk will be OK and psock
> reference will exist until after the rcu block at least because
> of call_rcu(). If the psock is destroyed from another path then
> the fd will still point at the sock. correct?

This is how I understand it:
* sk is protected by rcu_read_lock (as you point out)
* sk->sk_callback_lock protects against sk->sk_socket being
modified by sock_orphan, sock_graft, etc. via sk_set_socket
* get_file increments the refcount on the file

I'm not sure how the psock figures into this, maybe you can
elaborate a little?

--
Lorenz Bauer | Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com

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