Re: KMSAN: uninit-value in mpol_rebind_mm

[Alexander Potapenko]

On Thu, Jan 3, 2019 at 9:42 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Thu, Jan 3, 2019 at 9:36 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> >
> > On 12/31/18 8:51 AM, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following crash on:
> > >
> > > HEAD commit:    79fc24ff6184 kmsan: highmem: use kmsan_clear_page() in cop..
> > > git tree:       kmsan
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=13c48b67400000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=901dd030b2cc57e7
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=b19c2dc2c990ea657a71
> > > compiler:       clang version 8.0.0 (trunk 349734)
> > >
> > > Unfortunately, I don't have any reproducer for this crash yet.
> > >
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+b19c2dc2c990ea657a71@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > ==================================================================
> > > BUG: KMSAN: uninit-value in mpol_rebind_policy mm/mempolicy.c:353 [inline]
> > > BUG: KMSAN: uninit-value in mpol_rebind_mm+0x249/0x370 mm/mempolicy.c:384
> >
> > The report doesn't seem to indicate where the uninit value resides in
> > the mempolicy object.
>
> Yes, it doesn't and it's not trivial to do. The tool reports uses of
> unint _values_. Values don't necessary reside in memory. It can be a
> register, that come from another register that was calculated as a sum
> of two other values, which may come from a function argument, etc.
>
> > I'll have to guess. mm/mempolicy.c:353 contains:
> >
> >         if (!mpol_store_user_nodemask(pol) &&
> >             nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
> >
> > "mpol_store_user_nodemask(pol)" is testing pol->flags, which I couldn't
> > see being uninitialized after leaving mpol_new(). So I'll guess it's
> > actually about accessing pol->w.cpuset_mems_allowed on line 354.
> >
> > For w.cpuset_mems_allowed to be not initialized and the nodes_equal()
> > reachable for a mempolicy where mpol_set_nodemask() is called in
> > do_mbind(), it seems the only possibility is a MPOL_PREFERRED policy
> > with empty set of nodes, i.e. MPOL_LOCAL equivalent. Let's see if the
> > patch below helps. This code is a maze to me. Note the uninit access
> > should be benign, rebinding this kind of policy is always a no-op.
If I'm reading mempolicy.c right, `pol->flags & MPOL_F_LOCAL` doesn't
imply `pol->mode == MPOL_PREFERRED`, shouldn't we check for both here?

> > ----8<----
> > From ff0ca29da6bc2572d7b267daa77ced6083e3f02d Mon Sep 17 00:00:00 2001
> > From: Vlastimil Babka <vbabka@xxxxxxx>
> > Date: Thu, 3 Jan 2019 09:31:59 +0100
> > Subject: [PATCH] mm, mempolicy: fix uninit memory access
> >
> > ---
> >  mm/mempolicy.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index d4496d9d34f5..a0b7487b9112 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -350,7 +350,7 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask)
> >  {
> >         if (!pol)
> >                 return;
> > -       if (!mpol_store_user_nodemask(pol) &&
> > +       if (!mpol_store_user_nodemask(pol) && !(pol->flags & MPOL_F_LOCAL) &&
> >             nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
> >                 return;
> >
> > --
> > 2.19.2
> >
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-- 
Alexander Potapenko
Software Engineer

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