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 <[email protected]>
Reported-by: [email protected]
Fixes: 69e3b846d8a7 ("arm64: mte: Sync tags for pages where PTE is untagged")
Cc: <[email protected]> # 5.14.x
Cc: Steven Price <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Reviewed-by: Steven Price <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

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