The patch titled
Subject: mm/mmap.c: close race between munmap() and expand_upwards()/downwards()
has been added to the -mm tree. Its filename is
mm-close-race-between-munmap-and-expand_upwards-downwards.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-close-race-between-munmap-and-expand_upwards-downwards.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-close-race-between-munmap-and-expand_upwards-downwards.patch
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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.
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>
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
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_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, 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);
_
Patches currently in -mm which might be from kirill.shutemov@xxxxxxxxxxxxxxx are
mm-close-race-between-munmap-and-expand_upwards-downwards.patch
