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Commit 68827280 authored by Huang Ying's avatar Huang Ying Committed by Linus Torvalds
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mm, gup: prevent pmd checking race in follow_pmd_mask() 

mmap_sem will be read locked when calling follow_pmd_mask().  But this
cannot prevent PMD from being changed for all cases when PTL is
unlocked, for example, from pmd_trans_huge() to pmd_none() via
MADV_DONTNEED.  So it is possible for the pmd_present() check in
follow_pmd_mask() to encounter an invalid PMD.  This may cause an
incorrect VM_BUG_ON() or an infinite loop.  Fix this by reading the PMD
entry into a local variable with READ_ONCE() and checking the local
variable and pmd_none() in the retry loop.

As Kirill pointed out, with PTL unlocked, the *pmd may be changed under
us, so reading it directly again and again may incur weird bugs.  So
although using *pmd directly other than for pmd_present() checking may
be safe, it is still better to replace them to read *pmd once and check
the local variable multiple times.

When PTL unlocked, replace all *pmd with local variable was suggested by
Kirill.

Link: http://lkml.kernel.org/r/20180419083514.1365-1-ying.huang@intel.com


Signed-off-by: default avatar"Huang, Ying" <ying.huang@intel.com>
Reviewed-by: default avatarZi Yan <zi.yan@cs.rutgers.edu>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7854207f

mm/gup.c

+27 −11
Original line number Diff line number Diff line
@@ -212,53 +212,69 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, 
				    unsigned long address, pud_t *pudp,

				    unsigned int flags, unsigned int *page_mask)

{

	pmd_t *pmd;

	pmd_t *pmd, pmdval;

	spinlock_t *ptl;

	struct page *page;

	struct mm_struct *mm = vma->vm_mm;



	pmd = pmd_offset(pudp, address);

	if (pmd_none(*pmd))

	/*

	 * The READ_ONCE() will stabilize the pmdval in a register or

	 * on the stack so that it will stop changing under the code.

	 */

	pmdval = READ_ONCE(*pmd);

	if (pmd_none(pmdval))

		return no_page_table(vma, flags);

	if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) {

	if (pmd_huge(pmdval) && vma->vm_flags & VM_HUGETLB) {

		page = follow_huge_pmd(mm, address, pmd, flags);

		if (page)

			return page;

		return no_page_table(vma, flags);

	}

	if (is_hugepd(__hugepd(pmd_val(*pmd)))) {

	if (is_hugepd(__hugepd(pmd_val(pmdval)))) {

		page = follow_huge_pd(vma, address,

				      __hugepd(pmd_val(*pmd)), flags,

				      __hugepd(pmd_val(pmdval)), flags,

				      PMD_SHIFT);

		if (page)

			return page;

		return no_page_table(vma, flags);

	}

retry:

	if (!pmd_present(*pmd)) {

	if (!pmd_present(pmdval)) {

		if (likely(!(flags & FOLL_MIGRATION)))

			return no_page_table(vma, flags);

		VM_BUG_ON(thp_migration_supported() &&

				  !is_pmd_migration_entry(*pmd));

		if (is_pmd_migration_entry(*pmd))

				  !is_pmd_migration_entry(pmdval));

		if (is_pmd_migration_entry(pmdval))

			pmd_migration_entry_wait(mm, pmd);

		pmdval = READ_ONCE(*pmd);

		/*

		 * MADV_DONTNEED may convert the pmd to null because

		 * mmap_sem is held in read mode

		 */

		if (pmd_none(pmdval))

			return no_page_table(vma, flags);

		goto retry;

	}

	if (pmd_devmap(*pmd)) {

	if (pmd_devmap(pmdval)) {

		ptl = pmd_lock(mm, pmd);

		page = follow_devmap_pmd(vma, address, pmd, flags);

		spin_unlock(ptl);

		if (page)

			return page;

	}

	if (likely(!pmd_trans_huge(*pmd)))

	if (likely(!pmd_trans_huge(pmdval)))

		return follow_page_pte(vma, address, pmd, flags);



	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))

	if ((flags & FOLL_NUMA) && pmd_protnone(pmdval))

		return no_page_table(vma, flags);



retry_locked:

	ptl = pmd_lock(mm, pmd);

	if (unlikely(pmd_none(*pmd))) {

		spin_unlock(ptl);

		return no_page_table(vma, flags);

	}

	if (unlikely(!pmd_present(*pmd))) {

		spin_unlock(ptl);

		if (likely(!(flags & FOLL_MIGRATION)))