Re: [PATCH v2] hugetlb: clean up potential spectre issue warnings

[Michal Hocko <mhocko@suse.com>]

On Mon 21-02-22 12:24:25, Mike Kravetz wrote:
> On 2/21/22 00:42, Michal Hocko wrote:
> > On Fri 18-02-22 13:29:46, Mike Kravetz wrote:
> > [...]
> >> @@ -4161,7 +4162,7 @@ static int __init hugepages_setup(char *s)
> >>  			}
> >>  			if (tmp >= nr_online_nodes)
> >>  				goto invalid;
> >> -			node = tmp;
> >> +			node = array_index_nospec(tmp, nr_online_nodes);
> >>  			p += count + 1;
> >>  			/* Parse hugepages */
> >>  			if (sscanf(p, "%lu%n", &tmp, &count) != 1)
> >> @@ -6889,9 +6890,9 @@ static int __init cmdline_parse_hugetlb_cma(char *p)
> >>  			break;
> >>  
> >>  		if (s[count] == ':') {
> >> -			nid = tmp;
> >> -			if (nid < 0 || nid >= MAX_NUMNODES)
> >> +			if (tmp >= MAX_NUMNODES)
> >>  				break;
> >> +			nid = array_index_nospec(tmp, MAX_NUMNODES);
> >>  
> >>  			s += count + 1;
> >>  			tmp = memparse(s, &s);
> > 
> > This is an early boot code, how is this supposed to be used as a side
> > channel?
> 
> I do not have an evil hacker mind, but I can not think of a way this one time
> use of a user specified index could be an issue.  It does add noise to the
> BUILD REGRESSION emails sent to Andrew.

Maybe Smack can be taught to ignore __init and other early boot
functions.

I do not have any strong objections to using array_index_nospec because
it won't do any harm. Except that it makes a security measure a normal
comodity so any future changes to array_index_nospec and its users will
have to consult additional callers. Whether that is something we should
deeply care about, I don't know.

At minimum make sure to be explicit that this can hardly be a Spectre
gadget as it is a _one_ time early boot call. If there is a scenario
where this could be really abused then it should be mentioned
explicitly.
-- 
Michal Hocko
SUSE Labs