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

[Liam R. Howlett]

* Vlastimil Babka <vbabka@xxxxxxx> [241031 13:07]:
> On 10/31/24 18:01, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > ---
> >  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.

> 
> >  	}
> >  	if (i != mm->map_count) {
> >  		pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i);
>