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: Andrey Konovalov <andreyknvl@google.com>
Reported-by: Peter Collingbourne <pcc@google.com>
Tested-by: Peter 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: Andrew Morton <akpm@linux-foundation.org>

mm/page_alloc.c

@@ -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)