netdev - Re: [PATCH bpf v5 1/3] bpf: don't return EINVAL from {get,set}sockopt when optlen > PAGE

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Date:   Wed, 17 Jun 2020 10:59:29 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     sdf@...gle.com
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net,
        David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH bpf v5 1/3] bpf: don't return EINVAL from
 {get,set}sockopt when optlen > PAGE_SIZE

On Wed, Jun 17, 2020 at 10:45:08AM -0700, sdf@...gle.com wrote:
> On 06/17, Alexei Starovoitov wrote:
> > On Tue, Jun 16, 2020 at 06:04:14PM -0700, Stanislav Fomichev wrote:
> > > Attaching to these hooks can break iptables because its optval is
> > > usually quite big, or at least bigger than the current PAGE_SIZE limit.
> > > David also mentioned some SCTP options can be big (around 256k).
> > >
> > > For such optvals we expose only the first PAGE_SIZE bytes to
> > > the BPF program. BPF program has two options:
> > > 1. Set ctx->optlen to 0 to indicate that the BPF's optval
> > >    should be ignored and the kernel should use original userspace
> > >    value.
> > > 2. Set ctx->optlen to something that's smaller than the PAGE_SIZE.
> > >
> > > v5:
> > > * use ctx->optlen == 0 with trimmed buffer (Alexei Starovoitov)
> > > * update the docs accordingly
> > >
> > > v4:
> > > * use temporary buffer to avoid optval == optval_end == NULL;
> > >   this removes the corner case in the verifier that might assume
> > >   non-zero PTR_TO_PACKET/PTR_TO_PACKET_END.
> > >
> > > v3:
> > > * don't increase the limit, bypass the argument
> > >
> > > v2:
> > > * proper comments formatting (Jakub Kicinski)
> > >
> > > Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> > > Cc: David Laight <David.Laight@...LAB.COM>
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > >  kernel/bpf/cgroup.c | 53 ++++++++++++++++++++++++++++-----------------
> > >  1 file changed, 33 insertions(+), 20 deletions(-)
> > >
> > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > > index 4d76f16524cc..ac53102e244a 100644
> > > --- a/kernel/bpf/cgroup.c
> > > +++ b/kernel/bpf/cgroup.c
> > > @@ -1276,16 +1276,23 @@ static bool
> > __cgroup_bpf_prog_array_is_empty(struct cgroup *cgrp,
> > >
> > >  static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int
> > max_optlen)
> > >  {
> > > -	if (unlikely(max_optlen > PAGE_SIZE) || max_optlen < 0)
> > > +	if (unlikely(max_optlen < 0))
> > >  		return -EINVAL;
> > >
> > > +	if (unlikely(max_optlen > PAGE_SIZE)) {
> > > +		/* We don't expose optvals that are greater than PAGE_SIZE
> > > +		 * to the BPF program.
> > > +		 */
> > > +		max_optlen = PAGE_SIZE;
> > > +	}
> > > +
> > >  	ctx->optval = kzalloc(max_optlen, GFP_USER);
> > >  	if (!ctx->optval)
> > >  		return -ENOMEM;
> > >
> > >  	ctx->optval_end = ctx->optval + max_optlen;
> > >
> > > -	return 0;
> > > +	return max_optlen;
> > >  }
> > >
> > >  static void sockopt_free_buf(struct bpf_sockopt_kern *ctx)
> > > @@ -1319,13 +1326,13 @@ int __cgroup_bpf_run_filter_setsockopt(struct
> > sock *sk, int *level,
> > >  	 */
> > >  	max_optlen = max_t(int, 16, *optlen);
> > >
> > > -	ret = sockopt_alloc_buf(&ctx, max_optlen);
> > > -	if (ret)
> > > -		return ret;
> > > +	max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
> > > +	if (max_optlen < 0)
> > > +		return max_optlen;
> > >
> > >  	ctx.optlen = *optlen;
> > >
> > > -	if (copy_from_user(ctx.optval, optval, *optlen) != 0) {
> > > +	if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) !=
> > 0) {
> > >  		ret = -EFAULT;
> > >  		goto out;
> > >  	}
> > > @@ -1353,8 +1360,14 @@ int __cgroup_bpf_run_filter_setsockopt(struct
> > sock *sk, int *level,
> > >  		/* export any potential modifications */
> > >  		*level = ctx.level;
> > >  		*optname = ctx.optname;
> > > -		*optlen = ctx.optlen;
> > > -		*kernel_optval = ctx.optval;
> > > +
> > > +		/* optlen == 0 from BPF indicates that we should
> > > +		 * use original userspace data.
> > > +		 */
> > > +		if (ctx.optlen != 0) {
> > > +			*optlen = ctx.optlen;
> 
> > I think it should be:
> > *optlen = min(ctx.optlen, max_optlen);
> We do have the following (existing) check above:
> 	} else if (ctx.optlen > max_optlen || ctx.optlen < -1) {
> 		/* optlen is out of bounds */
> 		ret = -EFAULT;
> 	} else {
> 
> So we shouldn't need any min here? Or am I missing something?

ahh. you're right.
Applied to bpf tree.

> > Otherwise when bpf prog doesn't adjust ctx.oplen the kernel will see
> > 4k only in kernel_optval whereas optlen will be > 4k.
> > I suspect iptables sockopt should have crashed at this point.
> > How did you test it?
> The selftests that I've attached in the series. The test is passing
> two pages and for IP_TOS we bypass the value via optlen=0 and
> for IP_FREEBIND we trim the buffer to 1 byte. I think this should
> cover this check here.
> 
> One thing I didn't really test is getsockopt when the kernel
> returns really large buffer (iptables). Right now, the test
> gets 4 bytes (trimmed) from the kernel. I think that's the only
> place that I didn't properly test. I wonder whether I should
> do a real iptables-like setsockopt/getsockopt :-/

would be nice :)