Re: mm: use-after-free in collapse_huge_page

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Aug 29, 2016 at 05:35:48PM +0200, Andrea Arcangeli wrote:
> Hello Kirill,
> 
> On Mon, Aug 29, 2016 at 03:42:33PM +0300, Kirill A. Shutemov wrote:
> > @@ -898,13 +899,13 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> >  		/* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
> >  		if (ret & VM_FAULT_RETRY) {
> >  			down_read(&mm->mmap_sem);
> > -			if (hugepage_vma_revalidate(mm, address)) {
> > +			if (hugepage_vma_revalidate(mm, address, &vma)) {
> >  				/* vma is no longer available, don't continue to swapin */
> >  				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> >  				return false;
> >  			}
> >  			/* check if the pmd is still valid */
> > -			if (mm_find_pmd(mm, address) != pmd)
> > +			if (mm_find_pmd(mm, address) != pmd || vma != fe.vma)
> >  				return false;
> >  		}
> >  		if (ret & VM_FAULT_ERROR) {
> 
> You check if the vma changed if the mmap_sem was released by the
> VM_FAULT_RETRY case but not below:
> 
> 	/*
> 	 * Prevent all access to pagetables with the exception of
> 	 * gup_fast later handled by the ptep_clear_flush and the VM
> > @@ -994,7 +995,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> >  	 * handled by the anon_vma lock + PG_lock.
> >  	 */
> >  	down_write(&mm->mmap_sem);
> > -	result = hugepage_vma_revalidate(mm, address);
> > +	result = hugepage_vma_revalidate(mm, address, &vma);
> >  	if (result)
> >  		goto out;
> >  	/* check if the pmd is still valid */
> 	if (mm_find_pmd(mm, address) != pmd)
> 		goto out;
> 
> Here you go ahead without care if the vma has changed as long as the
> "vma" pointer was updated to the new one, and the pmd is still present
> and stable (present and not huge) and all vma details matched as
> before.
> 
> Either we care that the vma changed in both places or we don't in
> either of the two places.
> 
> The idea was that even if the vma changed it doesn't matter because
> it's still good to proceed for a collapse if all revalidation check
> pass.
> 
> What we failed at, was in refreshing the pointer of the vma to the new
> one after the vma revalidation passed, so that the code that goes
> ahead uses the right vma pointer and not the stale one we got
> initially.
> 
> Now it may give a perception that it is safer to check fa.vma != vma
> but in reality it is not, because the vma may be freed and reallocated
> in exactly the same address...
> 
> So I think the vma != fe.vma check shall be removed because no matter
> what the safety of the vma revalidate cannot come from checking if the
> pointer has not changed and it must come from something else.

[ Finally back to this. ]

Here's updated version.

>From 14d748bd8a7eb003efc10b1e5d5b8a644e7181b1 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Mon, 29 Aug 2016 15:32:50 +0300
Subject: [PATCH] khugepaged: fix use-after-free in collapse_huge_page()

hugepage_vma_revalidate() tries to re-check if we still should try to
collapse small pages into huge one after the re-acquiring mmap_sem.

The problem Dmitry Vyukov reported[1] is that the vma found by
hugepage_vma_revalidate() can be suitable for huge pages, but not the
same vma we had before dropping mmap_sem. And dereferencing original vma
can lead to fun results..

Let's use vma hugepage_vma_revalidate() found instead of assuming it's
the same as what we had before the lock was dropped.

[1] http://lkml.kernel.org/r/CACT4Y+Z3gigBvhca9kRJFcjX0G70V_nRhbwKBU+yGoESBDKi9Q@mail.gmail.com

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
---
 mm/khugepaged.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f401e9dfcc0c..728d7790dc2d 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -838,7 +838,8 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
  * value (scan code).
  */
 
-static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address)
+static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
+		struct vm_area_struct **vmap)
 {
 	struct vm_area_struct *vma;
 	unsigned long hstart, hend;
@@ -846,7 +847,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address)
 	if (unlikely(khugepaged_test_exit(mm)))
 		return SCAN_ANY_PROCESS;
 
-	vma = find_vma(mm, address);
+	*vmap = vma = find_vma(mm, address);
 	if (!vma)
 		return SCAN_VMA_NULL;
 
@@ -898,7 +899,7 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 		/* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
 		if (ret & VM_FAULT_RETRY) {
 			down_read(&mm->mmap_sem);
-			if (hugepage_vma_revalidate(mm, address)) {
+			if (hugepage_vma_revalidate(mm, address, &fe.vma)) {
 				/* vma is no longer available, don't continue to swapin */
 				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
 				return false;
@@ -923,7 +924,6 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 static void collapse_huge_page(struct mm_struct *mm,
 				   unsigned long address,
 				   struct page **hpage,
-				   struct vm_area_struct *vma,
 				   int node, int referenced)
 {
 	pmd_t *pmd, _pmd;
@@ -933,6 +933,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	spinlock_t *pmd_ptl, *pte_ptl;
 	int isolated = 0, result = 0;
 	struct mem_cgroup *memcg;
+	struct vm_area_struct *vma;
 	unsigned long mmun_start;	/* For mmu_notifiers */
 	unsigned long mmun_end;		/* For mmu_notifiers */
 	gfp_t gfp;
@@ -961,7 +962,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	}
 
 	down_read(&mm->mmap_sem);
-	result = hugepage_vma_revalidate(mm, address);
+	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result) {
 		mem_cgroup_cancel_charge(new_page, memcg, true);
 		up_read(&mm->mmap_sem);
@@ -994,7 +995,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	down_write(&mm->mmap_sem);
-	result = hugepage_vma_revalidate(mm, address);
+	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result)
 		goto out;
 	/* check if the pmd is still valid */
@@ -1202,7 +1203,7 @@ out_unmap:
 	if (ret) {
 		node = khugepaged_find_target_node();
 		/* collapse_huge_page will return with the mmap_sem released */
-		collapse_huge_page(mm, address, hpage, vma, node, referenced);
+		collapse_huge_page(mm, address, hpage, node, referenced);
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
-- 
 Kirill A. Shutemov