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

["Schaufler, Casey" <casey.schaufler@intel.com>]

> -----Original Message-----
> From: Michal Hocko <mhocko@suse.com>
> Sent: Monday, February 21, 2022 11:48 PM
> To: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: linux-mm@kvack.org; linux-kernel@vger.kernel.org; Baolin Wang
> <baolin.wang@linux.alibaba.com>; Zhenguo Yao
> <yaozhenguo1@gmail.com>; Liu Yuntao <liuyuntao10@huawei.com>; Dan
> Carpenter <dan.carpenter@oracle.com>; Andrew Morton <akpm@linux-
> foundation.org>
> Subject: Re: [PATCH v2] hugetlb: clean up potential spectre issue warnings
> 
> 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.

Why is Smack getting called out? The relationship is not obvious.

> 
> 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