linux-kernel - [PATCH v1 2/2] mm/mprotect: drop pgprot_t parameter from change

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Date:   Fri, 23 Dec 2022 16:56:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH v1 2/2] mm/mprotect: drop pgprot_t parameter from change_protection()

Being able to provide a custom protection opens the door for
inconsistencies and BUGs: for example, accidentally allowing for more
permissions than desired by other mechanisms (e.g., softdirty tracking).
vma->vm_page_prot should be the single source of truth.

Only PROT_NUMA is special: there is no way we can erroneously allow
for more permissions when removing all permissions. Special-case using
the MM_CP_PROT_NUMA flag.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 include/linux/mm.h |  3 +--
 mm/mempolicy.c     |  3 +--
 mm/mprotect.c      | 14 +++++++++++---
 mm/userfaultfd.c   |  3 +--
 4 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f3f196e4d66d..b8be8c33ca20 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2148,8 +2148,7 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t pte);
 extern unsigned long change_protection(struct mmu_gather *tlb,
 			      struct vm_area_struct *vma, unsigned long start,
-			      unsigned long end, pgprot_t newprot,
-			      unsigned long cp_flags);
+			      unsigned long end, unsigned long cp_flags);
 extern int mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
 			  struct vm_area_struct **pprev, unsigned long start,
 			  unsigned long end, unsigned long newflags);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 61aa9aedb728..c3f02703a710 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -635,8 +635,7 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
 
 	tlb_gather_mmu(&tlb, vma->vm_mm);
 
-	nr_updated = change_protection(&tlb, vma, addr, end, PAGE_NONE,
-				       MM_CP_PROT_NUMA);
+	nr_updated = change_protection(&tlb, vma, addr, end, MM_CP_PROT_NUMA);
 	if (nr_updated)
 		count_vm_numa_events(NUMA_PTE_UPDATES, nr_updated);
 
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 908df12caa26..569cefa668a6 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -503,13 +503,21 @@ static unsigned long change_protection_range(struct mmu_gather *tlb,
 
 unsigned long change_protection(struct mmu_gather *tlb,
 		       struct vm_area_struct *vma, unsigned long start,
-		       unsigned long end, pgprot_t newprot,
-		       unsigned long cp_flags)
+		       unsigned long end, unsigned long cp_flags)
 {
+	pgprot_t newprot = vma->vm_page_prot;
 	unsigned long pages;
 
 	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
 
+	/*
+	 * Ordinary protection updates (mprotect, uffd-wp, softdirty tracking)
+	 * are expected to reflect their requirements via VMA flags such that
+	 * vma_set_page_prot() will adjust vma->vm_page_prot accordingly.
+	 */
+	if (cp_flags & MM_CP_PROT_NUMA)
+		newprot = PAGE_NONE;
+
 	if (is_vm_hugetlb_page(vma))
 		pages = hugetlb_change_protection(vma, start, end, newprot,
 						  cp_flags);
@@ -638,7 +646,7 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
 	vma_set_page_prot(vma);
 
-	change_protection(tlb, vma, start, end, vma->vm_page_prot, mm_cp_flags);
+	change_protection(tlb, vma, start, end, mm_cp_flags);
 
 	/*
 	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 351e8d6b398b..be7ee9d82e72 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -744,8 +744,7 @@ void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
 	if (vma_wants_manual_pte_write_upgrade(dst_vma))
 		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
 	tlb_gather_mmu(&tlb, dst_mm);
-	change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot,
-			  mm_cp_flags);
+	change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
 	tlb_finish_mmu(&tlb);
 }
 
-- 
2.38.1