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mm/mmap.c: close race between munmap() and expand_upwards()/downwards()

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commit 246c320a8cfe0b11d81a4af38fa9985ef0cc9a4c upstream.

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.

[[email protected]: s/mmap_sem/mmap_lock/ in comment]

Fixes: dd2283f260 ("mm: mmap: zap pages with read mmap_sem in munmap")
Reported-by: Jann Horn <[email protected]>
Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: <[email protected]>	[4.20+]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Kirill A. Shutemov 2020-07-23 21:15:11 -07:00 • committed by Greg Kroah-Hartman
commit 549bfc1427
1 changed files with 14 additions and 2 deletions
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16
mm/mmap.c
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@ -2622,7 +2622,7 @@ 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)
{
@ -2647,6 +2647,17 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
/* Kill the cache */
vmacache_invalidate(mm);
/*
* Do not downgrade mmap_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;
}
/*
@ -2827,7 +2838,8 @@ 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)
downgrade_write(&mm->mmap_sem);