linux-kernel - Re: [PATCH] vma: Detect infinite loop in vma tree

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Message-ID: <3g7wnnw4glxouzv7biiwcxpdwk6s37eosdfl6wb3bjhwltbzx6@qkusx2ktwt7i>
Date: Thu, 31 Oct 2024 13:30:07 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] vma: Detect infinite loop in vma tree

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241031 13:22]:
> On Thu, Oct 31, 2024 at 01:13:28PM -0400, Liam R. Howlett wrote:
> > * Vlastimil Babka <vbabka@...e.cz> [241031 13:07]:
> > > On 10/31/24 18:01, Liam R. Howlett wrote:
> > > > From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> > > >
> > > > There have been no reported infinite loops in the tree, but checking the
> > > > detection of an infinite loop during validation is simple enough.  Add
> > > > the detection to the validate_mm() function so that error reports are
> > > > clear and don't just report stalls.
> > > >
> > > > This does not protect against internal maple tree issues, but it does
> > > > detect too many vmas being returned from the tree.
> > > >
> > > > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > > > Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > > Cc: Vlastimil Babka <vbabka@...e.cz>
> > > > Cc: Jann Horn <jannh@...gle.com>
> > > > ---
> > > >  mm/vma.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/vma.c b/mm/vma.c
> > > > index 68138e8c153e..60ed8cc187ad 100644
> > > > --- a/mm/vma.c
> > > > +++ b/mm/vma.c
> > > > @@ -615,7 +615,8 @@ void validate_mm(struct mm_struct *mm)
> > > >  			anon_vma_unlock_read(anon_vma);
> > > >  		}
> > > >  #endif
> > > > -		i++;
> > > > +		if (++i > mm->map_count)
> > > > +			break;
> > >
> > > Would it make sense to allow some slack so that the error below can
> > > distinguish better between off-by-one/few error from a complete corruption?
> > >
> > > And in that case assign some special value to "i" (-1?) to make it clear
> > > this was triggered?
> >
> > Yes, probably.  10 would be plenty.  In recent memory I cannot think of
> > an example that we exceeded 7 munmap()'s in a single operation -
> > although it is easily possible to do.
> >
> > I like the idea of -1 too, at least someone would come to inspect where
> > it came from at that point.
> 
> Hm this feels a little arbitrary though... I mean can we race with
> map_count at this point or is everything locked down such that no munmaps()
> can happen?
> 
> Otherwise it feels a bit whack-a-mole.
> 
> I agree with Vlastimil though it'd be nice to sort of differentiate, but if
> we _absolutely can only iterate mm->map_count times_ here, it might be
> worth letting a few more go, then in the next bit of code...

this is done under the write lock, otherwise it would vary already.

> 
> >
> > >
> > > >  	}
> > > >  	if (i != mm->map_count) {
> > > >  		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);
> > >
> 
> ...here which does indeed imply that i literally cannot be anything but
> mm->map_count, I mean I guess we already get to see here how far off we
> are.
> 
> So yeah something like letting it go 10 more times (maybe like #define
> UNUSUALLY_BAD_CORRUPTION_COUNT 10 or something I don't know naming is hard)
> just so we can pick that out would be nice.
> 
> But I do like the general idea here though!

This may not ever produce anything - if we are in the maple tree code
and never reaching a leaf to return anything then we will still loop
forever.  But it is something that is easy enough to detect and stop -
which may make a syzbot report a bit easier to swallow.