mm: page table check

Check user page table entries at the time they are added and removed.

Allows to synchronously catch memory corruption issues related to double
mapping.

When a pte for an anonymous page is added into page table, we verify
that this pte does not already point to a file backed page, and vice
versa if this is a file backed page that is being added we verify that
this page does not have an anonymous mapping

We also enforce that read-only sharing for anonymous pages is allowed
(i.e.  cow after fork).  All other sharing must be for file pages.

Page table check allows to protect and debug cases where "struct page"
metadata became corrupted for some reason.  For example, when refcnt or
mapcount become invalid.

Link: https://lkml.kernel.org/r/20211221154650.1047963-4-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Xu <weixugc@google.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 270 insertions, 0 deletions

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
new file mode 100644
index 000000000000..7504e7caa2a1
--- /dev/null
+++ b/mm/page_table_check.c
@@ -0,0 +1,270 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (c) 2021, Google LLC.
+ * Pasha Tatashin <pasha.tatashin@soleen.com>
+ */
+#include <linux/mm.h>
+#include <linux/page_table_check.h>
+
+#undef pr_fmt
+#define pr_fmt(fmt)	"page_table_check: " fmt
+
+struct page_table_check {
+	atomic_t anon_map_count;
+	atomic_t file_map_count;
+};
+
+static bool __page_table_check_enabled __initdata =
+				IS_ENABLED(CONFIG_PAGE_TABLE_CHECK_ENFORCED);
+
+DEFINE_STATIC_KEY_TRUE(page_table_check_disabled);
+EXPORT_SYMBOL(page_table_check_disabled);
+
+static int __init early_page_table_check_param(char *buf)
+{
+	if (!buf)
+		return -EINVAL;
+
+	if (strcmp(buf, "on") == 0)
+		__page_table_check_enabled = true;
+	else if (strcmp(buf, "off") == 0)
+		__page_table_check_enabled = false;
+
+	return 0;
+}
+
+early_param("page_table_check", early_page_table_check_param);
+
+static bool __init need_page_table_check(void)
+{
+	return __page_table_check_enabled;
+}
+
+static void __init init_page_table_check(void)
+{
+	if (!__page_table_check_enabled)
+		return;
+	static_branch_disable(&page_table_check_disabled);
+}
+
+struct page_ext_operations page_table_check_ops = {
+	.size = sizeof(struct page_table_check),
+	.need = need_page_table_check,
+	.init = init_page_table_check,
+};
+
+static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
+{
+	BUG_ON(!page_ext);
+	return (void *)(page_ext) + page_table_check_ops.offset;
+}
+
+static inline bool pte_user_accessible_page(pte_t pte)
+{
+	return (pte_val(pte) & _PAGE_PRESENT) && (pte_val(pte) & _PAGE_USER);
+}
+
+static inline bool pmd_user_accessible_page(pmd_t pmd)
+{
+	return pmd_leaf(pmd) && (pmd_val(pmd) & _PAGE_PRESENT) &&
+		(pmd_val(pmd) & _PAGE_USER);
+}
+
+static inline bool pud_user_accessible_page(pud_t pud)
+{
+	return pud_leaf(pud) && (pud_val(pud) & _PAGE_PRESENT) &&
+		(pud_val(pud) & _PAGE_USER);
+}
+
+/*
+ * An enty is removed from the page table, decrement the counters for that page
+ * verify that it is of correct type and counters do not become negative.
+ */
+static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
+				   unsigned long pfn, unsigned long pgcnt)
+{
+	struct page_ext *page_ext;
+	struct page *page;
+	bool anon;
+	int i;
+
+	if (!pfn_valid(pfn))
+		return;
+
+	page = pfn_to_page(pfn);
+	page_ext = lookup_page_ext(page);
+	anon = PageAnon(page);
+
+	for (i = 0; i < pgcnt; i++) {
+		struct page_table_check *ptc = get_page_table_check(page_ext);
+
+		if (anon) {
+			BUG_ON(atomic_read(&ptc->file_map_count));
+			BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
+		} else {
+			BUG_ON(atomic_read(&ptc->anon_map_count));
+			BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
+		}
+		page_ext = page_ext_next(page_ext);
+	}
+}
+
+/*
+ * A new enty is added to the page table, increment the counters for that page
+ * verify that it is of correct type and is not being mapped with a different
+ * type to a different process.
+ */
+static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
+				 unsigned long pfn, unsigned long pgcnt,
+				 bool rw)
+{
+	struct page_ext *page_ext;
+	struct page *page;
+	bool anon;
+	int i;
+
+	if (!pfn_valid(pfn))
+		return;
+
+	page = pfn_to_page(pfn);
+	page_ext = lookup_page_ext(page);
+	anon = PageAnon(page);
+
+	for (i = 0; i < pgcnt; i++) {
+		struct page_table_check *ptc = get_page_table_check(page_ext);
+
+		if (anon) {
+			BUG_ON(atomic_read(&ptc->file_map_count));
+			BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
+		} else {
+			BUG_ON(atomic_read(&ptc->anon_map_count));
+			BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
+		}
+		page_ext = page_ext_next(page_ext);
+	}
+}
+
+/*
+ * page is on free list, or is being allocated, verify that counters are zeroes
+ * crash if they are not.
+ */
+void __page_table_check_zero(struct page *page, unsigned int order)
+{
+	struct page_ext *page_ext = lookup_page_ext(page);
+	int i;
+
+	BUG_ON(!page_ext);
+	for (i = 0; i < (1 << order); i++) {
+		struct page_table_check *ptc = get_page_table_check(page_ext);
+
+		BUG_ON(atomic_read(&ptc->anon_map_count));
+		BUG_ON(atomic_read(&ptc->file_map_count));
+		page_ext = page_ext_next(page_ext);
+	}
+}
+
+void __page_table_check_pte_clear(struct mm_struct *mm, unsigned long addr,
+				  pte_t pte)
+{
+	if (&init_mm == mm)
+		return;
+
+	if (pte_user_accessible_page(pte)) {
+		page_table_check_clear(mm, addr, pte_pfn(pte),
+				       PAGE_SIZE >> PAGE_SHIFT);
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pte_clear);
+
+void __page_table_check_pmd_clear(struct mm_struct *mm, unsigned long addr,
+				  pmd_t pmd)
+{
+	if (&init_mm == mm)
+		return;
+
+	if (pmd_user_accessible_page(pmd)) {
+		page_table_check_clear(mm, addr, pmd_pfn(pmd),
+				       PMD_PAGE_SIZE >> PAGE_SHIFT);
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pmd_clear);
+
+void __page_table_check_pud_clear(struct mm_struct *mm, unsigned long addr,
+				  pud_t pud)
+{
+	if (&init_mm == mm)
+		return;
+
+	if (pud_user_accessible_page(pud)) {
+		page_table_check_clear(mm, addr, pud_pfn(pud),
+				       PUD_PAGE_SIZE >> PAGE_SHIFT);
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pud_clear);
+
+void __page_table_check_pte_set(struct mm_struct *mm, unsigned long addr,
+				pte_t *ptep, pte_t pte)
+{
+	pte_t old_pte;
+
+	if (&init_mm == mm)
+		return;
+
+	old_pte = *ptep;
+	if (pte_user_accessible_page(old_pte)) {
+		page_table_check_clear(mm, addr, pte_pfn(old_pte),
+				       PAGE_SIZE >> PAGE_SHIFT);
+	}
+
+	if (pte_user_accessible_page(pte)) {
+		page_table_check_set(mm, addr, pte_pfn(pte),
+				     PAGE_SIZE >> PAGE_SHIFT,
+				     pte_write(pte));
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pte_set);
+
+void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
+				pmd_t *pmdp, pmd_t pmd)
+{
+	pmd_t old_pmd;
+
+	if (&init_mm == mm)
+		return;
+
+	old_pmd = *pmdp;
+	if (pmd_user_accessible_page(old_pmd)) {
+		page_table_check_clear(mm, addr, pmd_pfn(old_pmd),
+				       PMD_PAGE_SIZE >> PAGE_SHIFT);
+	}
+
+	if (pmd_user_accessible_page(pmd)) {
+		page_table_check_set(mm, addr, pmd_pfn(pmd),
+				     PMD_PAGE_SIZE >> PAGE_SHIFT,
+				     pmd_write(pmd));
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pmd_set);
+
+void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
+				pud_t *pudp, pud_t pud)
+{
+	pud_t old_pud;
+
+	if (&init_mm == mm)
+		return;
+
+	old_pud = *pudp;
+	if (pud_user_accessible_page(old_pud)) {
+		page_table_check_clear(mm, addr, pud_pfn(old_pud),
+				       PUD_PAGE_SIZE >> PAGE_SHIFT);
+	}
+
+	if (pud_user_accessible_page(pud)) {
+		page_table_check_set(mm, addr, pud_pfn(pud),
+				     PUD_PAGE_SIZE >> PAGE_SHIFT,
+				     pud_write(pud));
+	}
+}
+EXPORT_SYMBOL(__page_table_check_pud_set);