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

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Date:   Mon, 11 Jan 2021 15:10:04 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Stanislav Fomichev <sdf@...gle.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 02:38:02PM -0800, Stanislav Fomichev wrote:
> 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,
It feels weird to reset the caller value at the beginning while this is not
intended to be an _init() like function, so I avoided it.

but yeah, I am fine on this way also and won't oppose it strongly ;)

> 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;
> >  }