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 70c248aca9e7 ("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: 44383cef54c0 ("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>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 717f12e83b85a65248cef4cf935b37b667bfb039..5ebce58026f126345c7f45d264dc21ed8206b534 100644 (file)
--- a/mm/page_alloc.c
+++ b/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)