Re: [RFC PATCH stable-4.4] coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping

[Ajay Kaher <akaher@vmware.com>]

> From: Andrea Arcangeli <aarcange@redhat.com>
>
> Upstream 04f5866e41fb70690e28397487d8bd8eea7d712a commit.
>
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> Hi,
> this is based on the backport I have done for out 4.4 based distribution
> kernel. Please double check that I haven't missed anything before
> applying to the stable tree. I have also CCed Joel for the binder part
> which is not in the current upstream anymore but I believe it needs the
> check as well.
>
> Review feedback welcome.
>
> drivers/android/binder.c |  6 ++++++
> fs/proc/task_mmu.c       | 18 ++++++++++++++++++
> fs/userfaultfd.c         | 10 ++++++++--
> include/linux/mm.h       | 21 +++++++++++++++++++++
> mm/huge_memory.c         |  2 +-
> mm/mmap.c                |  7 ++++++-
> 6 files changed, 60 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index 260ce0e60187..1fb1cddbd19a 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -570,6 +570,12 @@ static int binder_update_page_range(struct binder_proc *proc, int allocate,
> 
> 	if (mm) {
> 		down_write(&mm->mmap_sem);
> +		if (!mmget_still_valid(mm)) {
> +			if (allocate == 0)
> +				goto free_range;

Please cross check, free_range: should not end-up with modifications in vma.
 
> +			goto err_no_vma;
> +		}
> +
> 		vma = proc->vma;
> 		if (vma && mm != proc->vma_vm_mm) {
> 			pr_err("%d: vma mm and task mm mismatch\n",