Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Dan Williams
2018-01-04T06:32:11Z

On Wed, Jan 3, 2018 at 9:01 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
> "Williams, Dan J" <dan.j.williams@intel.com> writes:
>
>
>
>> Note that these are "a human looked at static analysis reports and
>> could not rationalize that these are false positives". Specific domain
>> knowledge about these paths may find that some of them are indeed false
>> positives.
>>
>> The change to m_start in kernel/user_namespace.c is interesting because
>> that's an example where the nospec_load() approach by itself would need
>> to barrier speculation twice whereas if_nospec can do it once for the
>> whole block.
>
>
> This user_namespace.c change is very convoluted for what it is trying to
> do.

Sorry this was my rebase on top of commit d5e7b3c5f51f "userns: Don't
read extents twice in m_start" the original change from Elena was
simpler. Part of the complexity arises from converting the common
kernel pattern of

if (<invalid condition>)
return NULL;
do_stuff;

...to:

if (<valid conidtion>) {
barrier();
do_stuff;
}

> It simplifies to a one liner that just adds osb() after pos >=
> extents. AKA:
>
> if (pos >= extents)
> return NULL;
> + osb();
>
> Is the intent to hide which branch branch we take based on extents,
> after the pos check?

The intent is to prevent speculative execution from triggering any
reads when 'pos' is invalid.

> I suspect this implies that using a user namespace and a crafted uid
> map you can hit this in stat, on the fast path.
>
> At which point I suspect we will be better off extending struct
> user_namespace by a few pointers, so there is no union and remove the
> need for blocking speculation entirely.

How does this help prevent a speculative read with an invalid 'pos'
reading arbitrary kernel addresses?

>
>> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
>> index 246d4d4ce5c7..aa0be8cef2d4 100644
>> --- a/kernel/user_namespace.c
>> +++ b/kernel/user_namespace.c
>> @@ -648,15 +648,18 @@ static void *m_start(struct seq_file *seq, loff_t *ppos,
>> {
>> loff_t pos = *ppos;
>> unsigned extents = map->nr_extents;
>> - smp_rmb();
>>
>> - if (pos >= extents)
>> - return NULL;
>> + /* paired with smp_wmb in map_write */
>> + smp_rmb();
>>
>> - if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
>> - return &map->extent[pos];
>> + if (pos < extents) {
>> + osb();
>> + if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
>> + return &map->extent[pos];
>> + return &map->forward[pos];
>> + }
>>
>> - return &map->forward[pos];
>> + return NULL;
>> }
>>
>> static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
>
>
>
>> diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
>> index 8ca9915befc8..7f83abdea255 100644
>> --- a/net/mpls/af_mpls.c
>> +++ b/net/mpls/af_mpls.c
>> @@ -81,6 +81,8 @@ static struct mpls_route *mpls_route_input_rcu(struct net *net, unsigned index)
>> if (index < net->mpls.platform_labels) {
>> struct mpls_route __rcu **platform_label =
>> rcu_dereference(net->mpls.platform_label);
>> +
>> + osb();
>> rt = rcu_dereference(platform_label[index]);
>> }
>> return rt;
>
> Ouch! This adds a barrier in the middle of an rcu lookup, on the
> fast path for routing mpls packets. Which if memory serves will
> noticably slow down software processing of mpls packets.
>
> Why does osb() fall after the branch for validity? So that we allow
> speculation up until then?

It falls there so that the cpu only issues reads with known good 'index' values.

> I suspect it would be better to have those barriers in the tun/tap
> interfaces where userspace can inject packets and thus time them. Then
> the code could still speculate and go fast for remote packets.
>
> Or does the speculation stomping have to be immediately at the place
> where we use data from userspace to perform a table lookup?

The speculation stomping barrier has to be between where we validate
the input and when we may speculate on invalid input. So, yes, moving
the user controllable input validation earlier and out of the fast
path would be preferred. Think of this patch purely as a static
analysis warning that something might need to be done to resolve the
report.