On 06/10/2022 17:33, Catalin Marinas wrote:
> 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@xxxxxxx>
> Reported-by: syzbot+c2c79c6d6eddc5262b77@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 69e3b846d8a7 ("arm64: mte: Sync tags for pages where PTE is untagged")
> Cc: <stable@xxxxxxxxxxxxxxx> # 5.14.x
> Cc: Steven Price <steven.price@xxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/0000000000004387dc05e5888ae5@xxxxxxxxxx/
Reviewed-by: Steven Price <steven.price@xxxxxxx>
> ---
>
> This seems to work for me but reproducing the issue is not entirely
> consistent. Once reviewed, we can merge it and then it will hit the
> various CI systems and syzbot.
As you say - it seems to work, hopefully the bots will agree!
Steve
>
> arch/arm64/kernel/mte.c | 9 +++++++--
> arch/arm64/mm/mteswap.c | 7 ++++++-
> 2 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index aca88470fb69..7467217c1eaf 100644
> --- 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 4334dec93bd4..bed803d8e158 100644
> --- 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;
> }
>
> base-commit: d2995249a2f72333a4ab4922ff3c42a76c023791
