On 7/9/20 12:53 PM, Kirill A. Shutemov wrote:
> VMA with VM_GROWSDOWN or VM_GROWSUP flag set can change their size under
> mmap_read_lock(). It can lead to race with __do_munmap():
>
> Thread A Thread B
> __do_munmap()
> detach_vmas_to_be_unmapped()
> mmap_write_downgrade()
> expand_downwards()
> vma->vm_start = address;
> // The VMA now overlaps with
> // VMAs detached by the Thread A
> // page fault populates expanded part
> // of the VMA
> unmap_region()
> // Zaps pagetables partly
> // populated by Thread B
>
> Similar race exists for expand_upwards().
>
> The fix is to avoid downgrading mmap_lock in __do_munmap() if detached
> VMAs are next to VM_GROWSDOWN or VM_GROWSUP VMA.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.20
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Thanks!
> ---
> mm/mmap.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 59a4682ebf3f..71df4b36b42a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2620,7 +2620,7 @@ static void unmap_region(struct mm_struct *mm,
> * Create a list of vma's touched by the unmap, removing them from the mm's
> * vma list as we go..
> */
> -static void
> +static bool
> detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
> struct vm_area_struct *prev, unsigned long end)
> {
> @@ -2645,6 +2645,17 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
>
> /* Kill the cache */
> vmacache_invalidate(mm);
> +
> + /*
> + * Do not downgrade mmap_sem if we are next to VM_GROWSDOWN or
> + * VM_GROWSUP VMA. Such VMAs can change their size under
> + * down_read(mmap_sem) and collide with the VMA we are about to unmap.
> + */
> + if (vma && (vma->vm_flags & VM_GROWSDOWN))
> + return false;
> + if (prev && (prev->vm_flags & VM_GROWSUP))
> + return false;
> + return true;
> }
>
> /*
> @@ -2825,7 +2836,8 @@ int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
> }
>
> /* Detach vmas from rbtree */
> - detach_vmas_to_be_unmapped(mm, vma, prev, end);
> + if (!detach_vmas_to_be_unmapped(mm, vma, prev, end))
> + downgrade = false;
>
> if (downgrade)
> mmap_write_downgrade(mm);
>
