Re: [PATCH 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Wed, May 17, 2023 at 02:37:41PM -0400, Peter Xu wrote:
> On Wed, May 17, 2023 at 06:20:55PM +0100, Lorenzo Stoakes wrote:
> > On Wed, May 17, 2023 at 11:04:07AM -0400, Peter Xu wrote:
> > > It seems vma merging with uffd paths is broken with either
> > > register/unregister, where right now we can feed wrong parameters to
> > > vma_merge() and it's found by recent patch which moved asserts upwards in
> > > vma_merge() by Lorenzo Stoakes:
> > >
> > > https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> > >
> > > The problem is in the current code base we didn't fixup "prev" for the case
> > > where "start" address can be within the "prev" vma section.  In that case
> > > we should have "prev" points to the current vma rather than the previous
> > > one when feeding to vma_merge().
> >
> > This doesn't seem quite correct, perhaps - "where start is contained within vma
> > but not clamped to its start. We need to convert this into case 4 which permits
> > subdivision of prev by assigning vma to prev. As we loop, each subsequent VMA
> > will be clamped to the start."
>
> I think it covers more than case 4 - it can also be case 0 where no merge
> will happen?

Ugh please let's not call a case that doesn't merge by a number :P but sure of
course it might also not merge.

>
> >
> > >
> > > This patch will eliminate the report and make sure vma_merge() calls will
> > > become legal again.
> > >
> > > One thing to mention is that the "Fixes: 29417d292bd0" below is there only
> > > to help explain where the warning can start to trigger, the real commit to
> > > fix should be 69dbe6daf104.  Commit 29417d292bd0 helps us to identify the
> > > issue, but unfortunately we may want to keep it in Fixes too just to ease
> > > kernel backporters for easier tracking.
> > >
> > > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > > Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> > > Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> > > Reported-by: Mark Rutland <mark.rutland@arm.com>
> > > Fixes: 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
> > > Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")
> > > Closes: https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> > > Cc: linux-stable <stable@vger.kernel.org>
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > >  fs/userfaultfd.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > >
> > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > > index 0fd96d6e39ce..17c8c345dac4 100644
> > > --- a/fs/userfaultfd.c
> > > +++ b/fs/userfaultfd.c
> > > @@ -1459,6 +1459,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > >
> > >  	vma_iter_set(&vmi, start);
> > >  	prev = vma_prev(&vmi);
> > > +	if (vma->vm_start < start)
> > > +		prev = vma;
> > >
> > >  	ret = 0;
> > >  	for_each_vma_range(vmi, vma, end) {
> > > @@ -1625,6 +1627,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> > >
> > >  	vma_iter_set(&vmi, start);
> > >  	prev = vma_prev(&vmi);
> > > +	if (vma->vm_start < start)
> > > +		prev = vma;
> > > +
> > >  	ret = 0;
> > >  	for_each_vma_range(vmi, vma, end) {
> > >  		cond_resched();
> > > --
> > > 2.39.1
> > >
> >
> > Other than that looks good:-
> >
> > Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
>
> Thanks to both on the quick reviews!

No problem!

>
> --
> Peter Xu
>