From 5f1e29cb4c60b62e67c30a209b982b01cabb6f6f Mon Sep 17 00:00:00 2001 From: Xiao Guangrong Date: Fri, 21 Sep 2012 10:57:21 +1000 Subject: [PATCH] mm: mmu_notifier: fix inconsistent memory between secondary MMU and host There is a bug in set_pte_at_notify() which always sets the pte to the new page before releasing the old page in the secondary MMU. At this time, the process will access on the new page, but the secondary MMU still access on the old page, the memory is inconsistent between them The below scenario shows the bug more clearly: at the beginning: *p = 0, and p is write-protected by KSM or shared with parent process CPU 0 CPU 1 write 1 to p to trigger COW, set_pte_at_notify will be called: *pte = new_page + W; /* The W bit of pte is set */ *p = 1; /* pte is valid, so no #PF */ return back to secondary MMU, then the secondary MMU read p, but get: *p == 0; /* * !!!!!! * the host has already set p to 1, but the secondary * MMU still get the old value 0 */ call mmu_notifier_change_pte to release old page in secondary MMU We can fix it by release old page first, then set the pte to the new page. Note, the new page will be firstly used in secondary MMU before it is mapped into the page table of the process, but this is safe because it is protected by the page table lock, there is no race to change the pte Signed-off-by: Xiao Guangrong Cc: Avi Kivity Cc: Marcelo Tosatti Cc: Andrea Arcangeli Cc: Hugh Dickins Signed-off-by: Andrew Morton --- include/linux/mmu_notifier.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h index 1d1b1e13f79f..8c7435a0d2c6 100644 --- a/include/linux/mmu_notifier.h +++ b/include/linux/mmu_notifier.h @@ -317,8 +317,8 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm) unsigned long ___address = __address; \ pte_t ___pte = __pte; \ \ - set_pte_at(___mm, ___address, __ptep, ___pte); \ mmu_notifier_change_pte(___mm, ___address, ___pte); \ + set_pte_at(___mm, ___address, __ptep, ___pte); \ }) #else /* CONFIG_MMU_NOTIFIER */ -- 2.39.5