Commit 420ef683 authored by Andrey Konovalov's avatar Andrey Konovalov Committed by Andrew Morton
Browse files

kasan: reset page tags properly with sampling 

The implementation of page_alloc poisoning sampling assumed that
tag_clear_highpage resets page tags for __GFP_ZEROTAGS allocations. 
However, this is no longer the case since commit 70c248ac ("mm: kasan:
Skip unpoisoning of user pages").

This leads to kernel crashes when MTE-enabled userspace mappings are used
with Hardware Tag-Based KASAN enabled.

Reset page tags for __GFP_ZEROTAGS allocations in post_alloc_hook().

Also clarify and fix related comments.

[andreyknvl@google.com: update comment]
 Link: https://lkml.kernel.org/r/5dbd866714b4839069e2d8469ac45b60953db290.1674592780.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/24ea20c1b19c2b4b56cf9f5b354915f8dbccfc77.1674592496.git.andreyknvl@google.com


Fixes: 44383cef ("kasan: allow sampling page_alloc allocations for HW_TAGS")
Signed-off-by: default avatarAndrey Konovalov <andreyknvl@google.com>
Reported-by: default avatarPeter Collingbourne <pcc@google.com>
Tested-by: default avatarPeter Collingbourne <pcc@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 2e126aa2

mm/page_alloc.c

+6 −5
Original line number Diff line number Diff line
@@ -2476,7 +2476,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order, 
	bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) &&

			!should_skip_init(gfp_flags);

	bool zero_tags = init && (gfp_flags & __GFP_ZEROTAGS);

	bool reset_tags = !zero_tags;

	bool reset_tags = true;

	int i;



	set_page_private(page, 0);
@@ -2503,7 +2503,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order, 
	 * (which happens only when memory should be initialized as well).

	 */

	if (zero_tags) {

		/* Initialize both memory and tags. */

		/* Initialize both memory and memory tags. */

		for (i = 0; i != 1 << order; ++i)

			tag_clear_highpage(page + i);
@@ -2521,14 +2521,15 @@ inline void post_alloc_hook(struct page *page, unsigned int order, 
		} else {

			/*

			 * KASAN decided to exclude this allocation from being

			 * poisoned due to sampling. Skip poisoning as well.

			 * (un)poisoned due to sampling. Make KASAN skip

			 * poisoning when the allocation is freed.

			 */

			SetPageSkipKASanPoison(page);

		}

	}

	/*

	 * If memory tags have not been set, reset the page tags to ensure

	 * page_address() dereferencing does not fault.

	 * If memory tags have not been set by KASAN, reset the page tags to

	 * ensure page_address() dereferencing does not fault.

	 */

	if (reset_tags) {

		for (i = 0; i != 1 << order; ++i)