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 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."

>
> 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>