[patch 02/15] mm/mmap.c: close race between munmap() and expand_upwards()/downwards()

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From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: mm/mmap.c: close race between munmap() and expand_upwards()/downwards()

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.

[akpm@xxxxxxxxxxxxxxxxxxxx: s/mmap_sem/mmap_lock/ in comment]
Link: http://lkml.kernel.org/r/20200709105309.42495-1-kirill.shutemov@xxxxxxxxxxxxxxx
Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.20+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/mmap.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/mm/mmap.c~mm-close-race-between-munmap-and-expand_upwards-downwards
+++ a/mm/mmap.c
@@ -2620,7 +2620,7 @@ static void unmap_region(struct mm_struc
  * 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_str
 
 	/* Kill the cache */
 	vmacache_invalidate(mm);
+
+	/*
+	 * Do not downgrade mmap_lock if we are next to VM_GROWSDOWN or
+	 * VM_GROWSUP VMA. Such VMAs can change their size under
+	 * down_read(mmap_lock) 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, un
 	}
 
 	/* 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);
_




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