[PATCH RFC 03/14] mm: compare pfns only if the entry is present when inserting pfns/pages

[David Hildenbrand <david@redhat.com>]

Doing a pte_pfn() etc. of something that is not a present page table
entry is wrong. Let's check in all relevant cases where we want to
upgrade write permissions when inserting pfns/pages whether the entry
is actually present.

It's not expected to have caused real harm in practice, so this is more a
cleanup than a fix for something that would likely trigger in some
weird circumstances.

At some point, we should likely unify the two pte handling paths,
similar to how we did it for pmds/puds.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c | 4 ++--
 mm/memory.c      | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8e0e3cfd9f223..e52360df87d15 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1392,7 +1392,7 @@ static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
 		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
 					  fop.pfn;
 
-		if (write) {
+		if (write && pmd_present(*pmd)) {
 			if (pmd_pfn(*pmd) != pfn) {
 				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
 				return -EEXIST;
@@ -1541,7 +1541,7 @@ static void insert_pud(struct vm_area_struct *vma, unsigned long addr,
 		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
 					  fop.pfn;
 
-		if (write) {
+		if (write && pud_present(*pud)) {
 			if (WARN_ON_ONCE(pud_pfn(*pud) != pfn))
 				return;
 			entry = pud_mkyoung(*pud);
diff --git a/mm/memory.c b/mm/memory.c
index a1b5575db52ac..9a1acd057ce59 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2137,7 +2137,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
 	pte_t pteval = ptep_get(pte);
 
 	if (!pte_none(pteval)) {
-		if (!mkwrite)
+		if (!mkwrite || !pte_present(pteval))
 			return -EBUSY;
 
 		/* see insert_pfn(). */
@@ -2434,7 +2434,7 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
 		return VM_FAULT_OOM;
 	entry = ptep_get(pte);
 	if (!pte_none(entry)) {
-		if (mkwrite) {
+		if (mkwrite && pte_present(entry)) {
 			/*
 			 * For read faults on private mappings the PFN passed
 			 * in may not match the PFN we have mapped if the
-- 
2.49.0