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@xxxxxxxxxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.20
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
---
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);
