[PATCH v2 19/47] mm/gup: recheck the PUD under PTL in follow_pud_mask()

["David Hildenbrand (Arm)" <david@kernel.org>]

follow_pud_mask() reads the PUD without a lock. If it is a leaf, it takes
the lock and calls follow_huge_pud(), which reads the entry again, but
only tests whether it is present.

A leaf can be zapped concurrently and a page fault can fill the empty
entry with a page table.

This currently only applies when device-dax makes use of PUD entries,
and the fault after e.g., MADV_DONTNEED would find THPs to be disabled.

That requires a sequence like

1) Fault in the PUD
2) Disable THP
3) Have GUP race against concurrent MADV_DONTNEED + refault

Which is rather unlikely to happen in practice, which is why we never
received a report about such a problem.

Before commit 1b1676180246 ("mm/gup: handle huge pud for
follow_pud_mask()"), we performed that check in follow_devmap_pud() by
testing for pud_devmap().

Let's fix it by just structuring follow_pud_mask() similar to
follow_pmd_mask(), minus protnone stuff that does not apply and minus
FOLL_SPLIT_PMD handling. That makes it much easier to spot any such
inconsistencies.

Perform the pud_bad() check early in follow_pmd_mask() and just forward
the PUD value. We can drop the pud_present() check in follow_huge_pud(),
just like we have in follow_huge_pmd().

Even though we are performing more changes than strictly necessary, the
fix should be easy to backport.

Fixes: 1b1676180246 ("mm/gup: handle huge pud for follow_pud_mask()")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/gup.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 7d43ce717b24..f809361227e7 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -658,9 +658,6 @@ static struct page *follow_huge_pud(struct vm_area_struct *vma,
 
 	assert_spin_locked(pud_lockptr(mm, pudp));
 
-	if (!pud_present(pud))
-		return NULL;
-
 	if ((flags & FOLL_WRITE) &&
 	    !can_follow_write_pud(pud, pfn_to_page(pfn), vma, flags))
 		return NULL;
@@ -896,15 +893,17 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 }
 
 static struct page *follow_pmd_mask(struct vm_area_struct *vma,
-				    unsigned long address, pud_t *pudp,
-				    unsigned int flags,
-				    unsigned long *page_mask)
+		unsigned long address, pud_t *pudp, pud_t pud,
+		unsigned int flags, unsigned long *page_mask)
 {
 	pmd_t *pmd, pmdval;
 	spinlock_t *ptl;
 	struct page *page;
 	struct mm_struct *mm = vma->vm_mm;
 
+	if (unlikely(pud_bad(pud)))
+		return no_page_table(vma, flags, address);
+
 	pmd = pmd_offset(pudp, address);
 	pmdval = pmdp_get_lockless(pmd);
 	if (pmd_none(pmdval))
@@ -953,18 +952,23 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
 	pud = pudp_get(pudp);
 	if (!pud_present(pud))
 		return no_page_table(vma, flags, address);
-	if (pud_leaf(pud)) {
-		ptl = pud_lock(mm, pudp);
-		page = follow_huge_pud(vma, address, pudp, flags, page_mask);
+	if (!pud_leaf(pud))
+		return follow_pmd_mask(vma, address, pudp, pud, flags, page_mask);
+
+	ptl = pud_lock(mm, pudp);
+	pud = *pudp;
+	if (unlikely(!pud_present(pud))) {
 		spin_unlock(ptl);
-		if (page)
-			return page;
 		return no_page_table(vma, flags, address);
 	}
-	if (unlikely(pud_bad(pud)))
-		return no_page_table(vma, flags, address);
+	if (unlikely(!pud_leaf(pud))) {
+		spin_unlock(ptl);
+		return follow_pmd_mask(vma, address, pudp, pud, flags, page_mask);
+	}
 
-	return follow_pmd_mask(vma, address, pudp, flags, page_mask);
+	page = follow_huge_pud(vma, address, pudp, flags, page_mask);
+	spin_unlock(ptl);
+	return page;
 }
 
 static struct page *follow_p4d_mask(struct vm_area_struct *vma,

-- 
2.43.0