Re: [PATCH] IB/core: Add mitigation for Spectre V1

[Ira Weiny <ira.weiny@intel.com>]

On Tue, Jul 30, 2019 at 06:52:12PM -0500, Gustavo A. R. Silva wrote:
> 
> 
> On 7/30/19 3:24 PM, Tony Luck wrote:
> > Some processors may mispredict an array bounds check and
> > speculatively access memory that they should not. With
> > a user supplied array index we like to play things safe
> > by masking the value with the array size before it is
> > used as an index.
> > 
> > Signed-off-by: Tony Luck <tony.luck@intel.com>
> > ---
> > 
> > [I don't have h/w, so just compile tested]
> > 
> >  drivers/infiniband/core/user_mad.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
> > index 9f8a48016b41..fdce254e4f65 100644
> > --- a/drivers/infiniband/core/user_mad.c
> > +++ b/drivers/infiniband/core/user_mad.c
> > @@ -49,6 +49,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/semaphore.h>
> >  #include <linux/slab.h>
> > +#include <linux/nospec.h>
> >  
> >  #include <linux/uaccess.h>
> >  
> > @@ -888,6 +889,7 @@ static int ib_umad_unreg_agent(struct ib_umad_file *file, u32 __user *arg)
> >  	mutex_lock(&file->port->file_mutex);
> >  	mutex_lock(&file->mutex);
> >  
> > +	id = array_index_nospec(id, IB_UMAD_MAX_AGENTS);
> 
> This is wrong. This prevents the below condition id >= IB_UMAD_MAX_AGENTS
> from ever being true. And I don't think this is what you want.

Ah Yea...  FWIW this would probably never be hit.

Tony; split the check?

	if (id >= IB_UMAD_MAX_AGENTS) {
		ret = -EINVAL;
		goto out;
	}

	id = array_index_nospec(id, IB_UMAD_MAX_AGENTS);

	if (!__get_agent(file, id)) {
		ret = -EINVAL;
		goto out;
	}

Ira

> 
> >  	if (id >= IB_UMAD_MAX_AGENTS || !__get_agent(file, id)) {
> >  		ret = -EINVAL;
> >  		goto out;
> > 
> 
> --
> Gustavo