diff mbox series

[v2,1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON

Message ID	20230722231508.1030269-2-pasha.tatashin@soleen.com
State	New
Headers	Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; From: Pasha Tatashin <pasha.tatashin@soleen.com> To: pasha.tatashin@soleen.com, akpm@linux-foundation.org, corbet@lwn.net, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, rick.p.edgecombe@intel.com Subject: [PATCH v2 1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON Date: Sat, 22 Jul 2023 23:15:06 +0000 Message-ID: <20230722231508.1030269-2-pasha.tatashin@soleen.com> In-Reply-To: <20230722231508.1030269-1-pasha.tatashin@soleen.com> References: <20230722231508.1030269-1-pasha.tatashin@soleen.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk
Series	page table check warn instead of panic \| [v2,0/3] page table check warn instead of panic [v2,1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON [v2,2/3] doc/vm: add information about page_table_check warn_on behavior [v2,3/3] mm/page_table_check: Check writable zero page in page table check

Commit Message

Pasha Tatashin July 22, 2023, 11:15 p.m. UTC

  Currently, page_table_check when detects errors panics the kernel. Instead,
print a warning as it is more useful compared to unconditionally crashing
the machine.

However, once a warning is detected, the counting of page_table_check
becomes unbalanced, therefore,  disable its activity until the next boot.

In case of where machine hardening requires a more secure environment, it
is still possible to crash machine on page_table_check errors via
panic_on_warn sysctl option.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 mm/page_table_check.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

Comments

Matthew Wilcox July 23, 2023, 1:56 a.m. UTC | #1

On Sat, Jul 22, 2023 at 11:15:06PM +0000, Pasha Tatashin wrote:
>  static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
>  {
> -	BUG_ON(!page_ext);
> +	PAGE_TABLE_CHECK_WARN(!page_ext);
> +
>  	return (void *)(page_ext) + page_table_check_ops.offset;
>  }

[...]

> @@ -137,15 +144,15 @@ void __page_table_check_zero(struct page *page, unsigned int order)
>  	struct page_ext *page_ext;
>  	unsigned long i;
>  
> -	BUG_ON(PageSlab(page));
> +	PAGE_TABLE_CHECK_WARN(PageSlab(page));
>  
>  	page_ext = page_ext_get(page);
> -	BUG_ON(!page_ext);
> +	PAGE_TABLE_CHECK_WARN(!page_ext);
>  	for (i = 0; i < (1ul << order); i++) {
>  		struct page_table_check *ptc = get_page_table_check(page_ext);

Seems like we're going to warn about !page_ext twice?  Or more than
twice -- once per tail page?

But then we'll crash because page_ext was NULL and offset was small?

Pasha Tatashin July 23, 2023, 3:35 a.m. UTC | #2

On Sat, Jul 22, 2023 at 9:56 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Sat, Jul 22, 2023 at 11:15:06PM +0000, Pasha Tatashin wrote:
> >  static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
> >  {
> > -     BUG_ON(!page_ext);
> > +     PAGE_TABLE_CHECK_WARN(!page_ext);
> > +
> >       return (void *)(page_ext) + page_table_check_ops.offset;
> >  }
>
> [...]
>
> > @@ -137,15 +144,15 @@ void __page_table_check_zero(struct page *page, unsigned int order)
> >       struct page_ext *page_ext;
> >       unsigned long i;
> >
> > -     BUG_ON(PageSlab(page));
> > +     PAGE_TABLE_CHECK_WARN(PageSlab(page));
> >
> >       page_ext = page_ext_get(page);
> > -     BUG_ON(!page_ext);
> > +     PAGE_TABLE_CHECK_WARN(!page_ext);
> >       for (i = 0; i < (1ul << order); i++) {
> >               struct page_table_check *ptc = get_page_table_check(page_ext);
>
> Seems like we're going to warn about !page_ext twice?  Or more than
> twice -- once per tail page?
>
> But then we'll crash because page_ext was NULL and offset was small?

Good catch, page_ext should not be NULL, yet I do not want to add
BUG_ON, let me fix this by warning and gracefully returning if
page_ext is NULL

Pasha

diff mbox series

Patch

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 93ec7690a0d8..ad4447e999f8 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -22,6 +22,12 @@  static bool __page_table_check_enabled __initdata =
 DEFINE_STATIC_KEY_TRUE(page_table_check_disabled);
 EXPORT_SYMBOL(page_table_check_disabled);
 
+#define PAGE_TABLE_CHECK_WARN(v)						\
+	do {									\
+		if (WARN_ON_ONCE(v))						\
+			static_branch_enable(&page_table_check_disabled);	\
+	} while (false)
+
 static int __init early_page_table_check_param(char *buf)
 {
 	return kstrtobool(buf, &__page_table_check_enabled);
@@ -50,7 +56,8 @@  struct page_ext_operations page_table_check_ops = {
 
 static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
 {
-	BUG_ON(!page_ext);
+	PAGE_TABLE_CHECK_WARN(!page_ext);
+
 	return (void *)(page_ext) + page_table_check_ops.offset;
 }
 
@@ -72,18 +79,18 @@  static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(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);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
+			PAGE_TABLE_CHECK_WARN(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_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_dec_return(&ptc->file_map_count) < 0);
 		}
 		page_ext = page_ext_next(page_ext);
 	}
@@ -110,18 +117,18 @@  static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(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);
+			PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
+			PAGE_TABLE_CHECK_WARN(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_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+			PAGE_TABLE_CHECK_WARN(atomic_inc_return(&ptc->file_map_count) < 0);
 		}
 		page_ext = page_ext_next(page_ext);
 	}
@@ -137,15 +144,15 @@  void __page_table_check_zero(struct page *page, unsigned int order)
 	struct page_ext *page_ext;
 	unsigned long i;
 
-	BUG_ON(PageSlab(page));
+	PAGE_TABLE_CHECK_WARN(PageSlab(page));
 
 	page_ext = page_ext_get(page);
-	BUG_ON(!page_ext);
+	PAGE_TABLE_CHECK_WARN(!page_ext);
 	for (i = 0; i < (1ul << 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_TABLE_CHECK_WARN(atomic_read(&ptc->anon_map_count));
+		PAGE_TABLE_CHECK_WARN(atomic_read(&ptc->file_map_count));
 		page_ext = page_ext_next(page_ext);
 	}
 	page_ext_put(page_ext);