Re: mm: BUG in expand_downwards

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Jan 28, 2016 at 4:47 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> Hello,
>
> On Wed, Jan 27, 2016 at 10:11:44PM +0100, Dmitry Vyukov wrote:
>> Sorry, I meant only the second once. The mm bug.
>> I guess you need at least CONFIG_DEBUG_VM.  Run it in a tight parallel
>> loop with CPU oversubscription (e.g. 32 parallel processes on 2 cores)
>> for  at least an hour.
>
> Does this help for the mm bug?

Yes, it seems to fix the issue.
I will also run fuzzer with this patch and report if I see it again.

> From 0cc410ae59800444ca929e3dc48e4f1580a95be6 Mon Sep 17 00:00:00 2001
> From: Andrea Arcangeli <aarcange@redhat.com>
> Date: Thu, 28 Jan 2016 16:34:44 +0100
> Subject: [PATCH 1/1] mm: validate_mm browse_rb SMP race condition
>
> The mmap_sem for reading in validate_mm called from expand_stack is
> not enough to prevent the argumented rbtree rb_subtree_gap information
> to change from under us because expand_stack may be running from other
> threads concurrently which will hold the mmap_sem for reading too.
>
> The argumented rbtree is updated with vma_gap_update under the
> page_table_lock so use it in browse_rb() too to avoid false positives.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>  mm/mmap.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f384def..8389e03 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -389,8 +389,9 @@ static long vma_compute_subtree_gap(struct vm_area_struct *vma)
>  }
>
>  #ifdef CONFIG_DEBUG_VM_RB
> -static int browse_rb(struct rb_root *root)
> +static int browse_rb(struct mm_struct *mm)
>  {
> +       struct rb_root *root = &mm->mm_rb;
>         int i = 0, j, bug = 0;
>         struct rb_node *nd, *pn = NULL;
>         unsigned long prev = 0, pend = 0;
> @@ -413,12 +414,14 @@ static int browse_rb(struct rb_root *root)
>                                   vma->vm_start, vma->vm_end);
>                         bug = 1;
>                 }
> +               spin_lock(&mm->page_table_lock);
>                 if (vma->rb_subtree_gap != vma_compute_subtree_gap(vma)) {
>                         pr_emerg("free gap %lx, correct %lx\n",
>                                vma->rb_subtree_gap,
>                                vma_compute_subtree_gap(vma));
>                         bug = 1;
>                 }
> +               spin_unlock(&mm->page_table_lock);
>                 i++;
>                 pn = nd;
>                 prev = vma->vm_start;
> @@ -474,7 +477,7 @@ static void validate_mm(struct mm_struct *mm)
>                           mm->highest_vm_end, highest_address);
>                 bug = 1;
>         }
> -       i = browse_rb(&mm->mm_rb);
> +       i = browse_rb(mm);
>         if (i != mm->map_count) {
>                 if (i != -1)
>                         pr_emerg("map_count %d rb %d\n", mm->map_count, i);

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