arm64: mte: Avoid setting PG_mte_tagged if no tags cleared or restored

Prior to commit 69e3b846d8a7 ("arm64: mte: Sync tags for pages where PTE
is untagged"), mte_sync_tags() was only called for pte_tagged() entries
(those mapped with PROT_MTE). Therefore mte_sync_tags() could safely use
test_and_set_bit(PG_mte_tagged, &page->flags) without inadvertently
setting PG_mte_tagged on an untagged page.

The above commit was required as guests may enable MTE without any
control at the stage 2 mapping, nor a PROT_MTE mapping in the VMM.
However, the side-effect was that any page with a PTE that looked like
swap (or migration) was getting PG_mte_tagged set automatically. A
subsequent page copy (e.g. migration) copied the tags to the destination
page even if the tags were owned by KASAN.

This issue was masked by the page_kasan_tag_reset() call introduced in
commit e5b8d9218951 ("arm64: mte: reset the page tag in page->flags").
When this commit was reverted (20794545c146), KASAN started reporting
access faults because the overriding tags in a page did not match the
original page->flags (with CONFIG_KASAN_HW_TAGS=y):

  BUG: KASAN: invalid-access in copy_page+0x10/0xd0 arch/arm64/lib/copy_page.S:26
  Read at addr f5ff000017f2e000 by task syz-executor.1/2218
  Pointer tag: [f5], memory tag: [f2]

Move the PG_mte_tagged bit setting from mte_sync_tags() to the actual
place where tags are cleared (mte_sync_page_tags()) or restored
(mte_restore_tags()).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: syzbot+c2c79c6d6eddc5262b77@syzkaller.appspotmail.com
Fixes: 69e3b846d8a7 ("arm64: mte: Sync tags for pages where PTE is untagged")
Cc: <stable@vger.kernel.org> # 5.14.x
Cc: Steven Price <steven.price@arm.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/0000000000004387dc05e5888ae5@google.com/
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20221006163354.3194102-1-catalin.marinas@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index aca8847..7467217 100644 (file)
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -48,7 +48,12 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
        if (!pte_is_tagged)
                return;
 
-       mte_clear_page_tags(page_address(page));
+       /*
+        * Test PG_mte_tagged again in case it was racing with another
+        * set_pte_at().
+        */
+       if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+               mte_clear_page_tags(page_address(page));
 }
 
 void mte_sync_tags(pte_t old_pte, pte_t pte)
@@ -64,7 +69,7 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
 
        /* if PG_mte_tagged is set, tags have already been initialised */
        for (i = 0; i < nr_pages; i++, page++) {
-               if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+               if (!test_bit(PG_mte_tagged, &page->flags))
                        mte_sync_page_tags(page, old_pte, check_swap,
                                           pte_is_tagged);
        }

diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index 4334dec..bed803d 100644 (file)
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,7 +53,12 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
        if (!tags)
                return false;
 
-       mte_restore_page_tags(page_address(page), tags);
+       /*
+        * Test PG_mte_tagged again in case it was racing with another
+        * set_pte_at().
+        */
+       if (!test_and_set_bit(PG_mte_tagged, &page->flags))
+               mte_restore_page_tags(page_address(page), tags);
 
        return true;
 }