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

[Michal Hocko <mhocko@suse.com>]

On Tue 22-02-22 19:36:10, Dan Carpenter wrote:
> On Tue, Feb 22, 2022 at 04:15:45PM +0000, Schaufler, Casey wrote:
> > > -----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.
> > 
> 
> He meant Smatch.  It's a really common mistake that I did not anticipate
> in 2002.

Right. My bad.

> I can probably silence the spectre warnings for __init functions.  TBH,
> I don't really understand spectre at all so I mostly ignore those
> warnings.  :/

AFAIU the spectre gadget would need to be called repeatedly to be usable
for a side channel attack. I might be really missing some scenario but
to me it seems that __init functions cannot really be used for that.
-- 
Michal Hocko
SUSE Labs