Re: [PATCH 4/5] bpf: sockmap, sockhash: return file descriptors from privileged lookup
From: Jakub Sitnicki <jakub@cloudflare.com>
Date: 2020-03-17 15:18:27
Also in:
bpf, lkml
On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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
+ read_unlock_bh(&sk->sk_callback_lock); + + fd_install(fd, file); + *(u32 *)value = fd; + return 0; + default: return -ENOSPC; }