Re: [PATCH] mm: Close race between munmap() and expand_upwards()/downwards()

[Yang Shi]

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....@linux.alibaba.com>


Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
Reported-by: Jann Horn <ja...@google.com>
Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
Cc: <sta...@vger.kernel.org> # 4.20
Cc: Yang Shi <yang....@linux.alibaba.com>
Cc: Vlastimil Babka <vba...@suse.cz>
Cc: Oleg Nesterov <o...@redhat.com>
Cc: Matthew Wilcox <wi...@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);