Re: [PATCH] bpf: fix reference count leak in bpf_prog_test_run_xdp()

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> writes:

> syzbot is reporting
>
>   unregister_netdevice: waiting for sit0 to become free. Usage count = 2
>
> problem. A debug printk() patch found that a refcount is obtained at
> xdp_convert_md_to_buff() from bpf_prog_test_run_xdp().
>
> According to commit ec94670fcb3b ("bpf: Support specifying ingress via
> xdp_md context in BPF_PROG_TEST_RUN"), the refcount obtained by
> xdp_convert_md_to_buff() will be released by xdp_convert_buff_to_md().
>
> Therefore, we can consider that the error handling path introduced by
> commit 1c1949982524 ("bpf: introduce frags support to
> bpf_prog_test_run_xdp()") forgot to call xdp_convert_buff_to_md().
>
> Reported-by: syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=881d65229ca4f9ae8c84
> Fixes: 1c1949982524 ("bpf: introduce frags support to bpf_prog_test_run_xdp()")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> Since syzbot has no reproducer for this problem, I can't test this patch.
>
>  net/bpf/test_run.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 655efac6f133..9a16293ba14b 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1355,13 +1355,13 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  
>  			if (sinfo->nr_frags == MAX_SKB_FRAGS) {
>  				ret = -ENOMEM;
> -				goto out;
> +				goto out_put_dev;
>  			}
>  
>  			page = alloc_page(GFP_KERNEL);
>  			if (!page) {
>  				ret = -ENOMEM;
> -				goto out;
> +				goto out_put_dev;
>  			}
>  
>  			frag = &sinfo->frags[sinfo->nr_frags++];
> @@ -1373,7 +1373,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  			if (copy_from_user(page_address(page), data_in + size,
>  					   data_len)) {
>  				ret = -EFAULT;
> -				goto out;
> +				goto out_put_dev;
>  			}
>  			sinfo->xdp_frags_size += data_len;
>  			size += data_len;
> @@ -1388,6 +1388,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  		ret = bpf_test_run_xdp_live(prog, &xdp, repeat, batch_size, &duration);
>  	else
>  		ret = bpf_test_run(prog, &xdp, repeat, &retval, &duration, true);
> +out_put_dev:
>  	/* We convert the xdp_buff back to an xdp_md before checking the return
>  	 * code so the reference count of any held netdevice will be decremented
>  	 * even if the test run failed.

Hmm, this will end up call bpf_ctx_finish() in the error path, which I'm
not sure we want?

Could we just move the xdp_convert_md_to_buff() call to after the frags
have been copied? Not sure there's technically any dependency there,
even though it does look a little off?

-Toke