Re: [PATCH] vma: Detect infinite loop in vma tree

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Oct 31, 2024 at 01:13:28PM -0400, Liam R. Howlett wrote:
> * Vlastimil Babka <vbabka@suse.cz> [241031 13:07]:
> > On 10/31/24 18:01, Liam R. Howlett wrote:
> > > From: "Liam R. Howlett" <Liam.Howlett@Oracle.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@Oracle.com>
> > > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > Cc: Vlastimil Babka <vbabka@suse.cz>
> > > Cc: Jann Horn <jannh@google.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...

>
> >
> > >  	}
> > >  	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!