[PATCH 5.15 001/120] arm64: mte: Avoid setting PG_mte_tagged if no tags cleared or restored

Greg Kroah-Hartman
2022-12-05T19:36:38Z

From: Catalin Marinas <catalin.marinas@arm.com>

[ Upstream commit a8e5e5146ad08d794c58252bab00b261045ef16d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 dacca0684ea3..a3898bac5ae6 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -53,7 +53,12 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
* the new page->flags are visible before the tags were updated.
*/
smp_wmb();
- 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)
@@ -69,7 +74,7 @@ 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 7c4ef56265ee..fd6cabc6d033 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -62,7 +62,12 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
* the new page->flags are visible before the tags were updated.
*/
smp_wmb();
- 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;
}
--
2.35.1