[PATCH v8 28/43] arm64: mte: Reset the page tag in page->flags

Andrey Konovalov
2020-11-04T23:20:18Z

From: Vincenzo Frascino <vincenzo.frascino@arm.com>

The hardware tag-based KASAN for compatibility with the other modes
stores the tag associated to a page in page->flags.
Due to this the kernel faults on access when it allocates a page with an
initial tag and the user changes the tags.

Reset the tag associated by the kernel to a page in all the meaningful
places to prevent kernel faults on access.

Note: An alternative to this approach could be to modify page_to_virt().
This though could end up being racy, in fact if a CPU checks the
PG_mte_tagged bit and decides that the page is not tagged but another
CPU maps the same with PROT_MTE and becomes tagged the subsequent kernel
access would fail.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
Change-Id: I8451d438bb63364de2a3e68041e3a27866921d4e
---
arch/arm64/kernel/mte.c | 1 +
arch/arm64/mm/copypage.c | 1 +
arch/arm64/mm/mteswap.c | 1 +
3 files changed, 3 insertions(+)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 8f99c65837fd..06ba6c923ab7 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -34,6 +34,7 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
return;
}

+ page_kasan_tag_reset(page);
mte_clear_page_tags(page_address(page));
}

diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 70a71f38b6a9..348f4627da08 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -22,6 +22,7 @@ void copy_highpage(struct page *to, struct page *from)
copy_page(kto, kfrom);

if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
+ page_kasan_tag_reset(to);
set_bit(PG_mte_tagged, &to->flags);
mte_copy_page_tags(kto, kfrom);
}
diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index c52c1847079c..0e7eccbe598a 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,6 +53,7 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
if (!tags)
return false;

+ page_kasan_tag_reset(page);
mte_restore_page_tags(page_address(page), tags);

return true;
--
2.29.1.341.ge80a0c044ae-goog