* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241031 13:22]:
> On Thu, Oct 31, 2024 at 01:13:28PM -0400, Liam R. Howlett wrote:
> > * 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.
>
> 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.
