netdev - Re: [PATCH bpf] bpf: don't leak memory in bpf getsockopt when optlen == 0

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Date:   Mon, 11 Jan 2021 14:38:02 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf] bpf: don't leak memory in bpf getsockopt when optlen
 == 0

On Mon, Jan 11, 2021 at 2:32 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Mon, Jan 11, 2021 at 11:47:38AM -0800, Stanislav Fomichev wrote:
> > optlen == 0 indicates that the kernel should ignore BPF buffer
> > and use the original one from the user. We, however, forget
> > to free the temporary buffer that we've allocated for BPF.
> >
> > Reported-by: Martin KaFai Lau <kafai@...com>
> > Fixes: d8fe449a9c51 ("bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE")
> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > ---
> >  kernel/bpf/cgroup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index 6ec088a96302..09179ab72c03 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -1395,7 +1395,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> >       }
> >
> >  out:
> > -     if (ret)
> > +     if (*kernel_optval == NULL)
> It seems fragile to depend on the caller to init *kernel_optval to NULL.
We can manually reset it to NULL when we enter
__cgroup_bpf_run_filter_setsockopt,
I didn't bother since there is only one existing caller.

But you patch also LGTM, I don't really have a preference.

> How about something like:
>
> diff --git i/kernel/bpf/cgroup.c w/kernel/bpf/cgroup.c
> index 6ec088a96302..8d94c004e781 100644
> --- i/kernel/bpf/cgroup.c
> +++ w/kernel/bpf/cgroup.c
> @@ -1358,7 +1358,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
>
>         if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) != 0) {
>                 ret = -EFAULT;
> -               goto out;
> +               goto err_out;
>         }
>
>         lock_sock(sk);
> @@ -1368,7 +1368,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
>
>         if (!ret) {
>                 ret = -EPERM;
> -               goto out;
> +               goto err_out;
>         }
>
>         if (ctx.optlen == -1) {
> @@ -1379,7 +1379,6 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
>                 ret = -EFAULT;
>         } else {
>                 /* optlen within bounds, run kernel handler */
> -               ret = 0;
>
>                 /* export any potential modifications */
>                 *level = ctx.level;
> @@ -1391,12 +1390,15 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
>                 if (ctx.optlen != 0) {
>                         *optlen = ctx.optlen;
>                         *kernel_optval = ctx.optval;
> +               } else {
> +                       sockopt_free_buf(&ctx);
>                 }
> +
> +               return 0;
>         }
>
> -out:
> -       if (ret)
> -               sockopt_free_buf(&ctx);
> +err_out:
> +       sockopt_free_buf(&ctx);
>         return ret;
>  }