Re: [PATCH v2 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Andrew Morton]

On Wed, 17 May 2023 15:09:15 -0400 Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> 
> It's possible that "start" is contained within vma but not clamped to its
> start.  We need to convert this into either "cannot merge" case or "can
> merge" case 4 which permits subdivision of prev by assigning vma to
> prev. As we loop, each subsequent VMA will be clamped to the start.
> 
> 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@xxxxxxxxx>
> Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
> Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Fixes: 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
> Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")

I don't know how -stable maintainers are to handle more than a single
Fixes: target, given that Fixes: means "kernels which have that patch
need this one".  Can we narrow this down to a single commit for this
purpose?