On 7/9/20 3:53 AM, 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.

Thanks for catching this. The fix makes sense to me. Reviewed-by: Yang Shi yang.shi@linux.alibaba.com

Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Reported-by: Jann Horn jannh@google.com Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") Cc: stable@vger.kernel.org # 4.20 Cc: Yang Shi yang.shi@linux.alibaba.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Oleg Nesterov oleg@redhat.com Cc: Matthew Wilcox willy@infradead.org

---

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);