[PATCH 3/3] arm64: mte: Simplify swap tag restoration logic and fix uninitialized tag issue

Peter Collingbourne
2023-05-12T23:59:06Z

As a result of the previous two patches, there are no circumstances
in which a swapped-in page is installed in a page table without first
having arch_swap_restore() called on it. Therefore, we no longer need
the logic in set_pte_at() that restores the tags, so remove it.

Because we can now rely on the page being locked, we no longer need to
handle the case where a page is having its tags restored by multiple tasks
concurrently, so we can slightly simplify the logic in mte_restore_tags().

This patch also fixes an issue where a page can have PG_mte_tagged set
with uninitialized tags. The issue is that the mte_sync_page_tags()
function sets PG_mte_tagged if it initializes page tags. Then we
return to mte_sync_tags(), which sets PG_mte_tagged again. At best,
this is redundant. However, it is possible for mte_sync_page_tags()
to return without having initialized tags for the page, i.e. in the
case where check_swap is true (non-compound page), is_swap_pte(old_pte)
is false and pte_is_tagged is false. So at worst, we set PG_mte_tagged
on a page with uninitialized tags. This can happen if, for example,
page migration causes a PTE for an untagged page to be replaced. If the
userspace program subsequently uses mprotect() to enable PROT_MTE for
that page, the uninitialized tags will be exposed to userspace.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I8ad54476f3b2d0144ccd8ce0c1d7a2963e5ff6f3
Fixes: e059853d14ca ("arm64: mte: Fix/clarify the PG_mte_tagged semantics")
Cc: <stable@vger.kernel.org> # 6.1
---
The Fixes: tag (and the commit message in general) are written assuming
that this patch is landed in a maintainer tree instead of
"arm64: mte: Do not set PG_mte_tagged if tags were not initialized".

arch/arm64/include/asm/mte.h | 4 ++--
arch/arm64/include/asm/pgtable.h | 14 ++------------
arch/arm64/kernel/mte.c | 32 +++-----------------------------
arch/arm64/mm/mteswap.c | 7 +++----
4 files changed, 10 insertions(+), 47 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index 20dd06d70af5..dfea486a6a85 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -90,7 +90,7 @@ static inline bool try_page_mte_tagging(struct page *page)
}

void mte_zero_clear_page_tags(void *addr);
-void mte_sync_tags(pte_t old_pte, pte_t pte);
+void mte_sync_tags(pte_t pte);
void mte_copy_page_tags(void *kto, const void *kfrom);
void mte_thread_init_user(void);
void mte_thread_switch(struct task_struct *next);
@@ -122,7 +122,7 @@ static inline bool try_page_mte_tagging(struct page *page)
static inline void mte_zero_clear_page_tags(void *addr)
{
}
-static inline void mte_sync_tags(pte_t old_pte, pte_t pte)
+static inline void mte_sync_tags(pte_t pte)
{
}
static inline void mte_copy_page_tags(void *kto, const void *kfrom)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b6ba466e2e8a..efdf48392026 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -337,18 +337,8 @@ static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
* don't expose tags (instruction fetches don't check tags).
*/
if (system_supports_mte() && pte_access_permitted(pte, false) &&
- !pte_special(pte)) {
- pte_t old_pte = READ_ONCE(*ptep);
- /*
- * We only need to synchronise if the new PTE has tags enabled
- * or if swapping in (in which case another mapping may have
- * set tags in the past even if this PTE isn't tagged).
- * (!pte_none() && !pte_present()) is an open coded version of
- * is_swap_pte()
- */
- if (pte_tagged(pte) || (!pte_none(old_pte) && !pte_present(old_pte)))
- mte_sync_tags(old_pte, pte);
- }
+ !pte_special(pte) && pte_tagged(pte))
+ mte_sync_tags(pte);

__check_safe_pte_update(mm, ptep, pte);

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index f5bcb0dc6267..c40728046fed 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -35,41 +35,15 @@ DEFINE_STATIC_KEY_FALSE(mte_async_or_asymm_mode);
EXPORT_SYMBOL_GPL(mte_async_or_asymm_mode);
#endif

-static void mte_sync_page_tags(struct page *page, pte_t old_pte,
- bool check_swap, bool pte_is_tagged)
-{
- if (check_swap && is_swap_pte(old_pte)) {
- swp_entry_t entry = pte_to_swp_entry(old_pte);
-
- if (!non_swap_entry(entry))
- mte_restore_tags(entry, page);
- }
-
- if (!pte_is_tagged)
- return;
-
- if (try_page_mte_tagging(page)) {
- mte_clear_page_tags(page_address(page));
- set_page_mte_tagged(page);
- }
-}
-
-void mte_sync_tags(pte_t old_pte, pte_t pte)
+void mte_sync_tags(pte_t pte)
{
struct page *page = pte_page(pte);
long i, nr_pages = compound_nr(page);
- bool check_swap = nr_pages == 1;
- bool pte_is_tagged = pte_tagged(pte);
-
- /* Early out if there's nothing to do */
- if (!check_swap && !pte_is_tagged)
- return;

/* if PG_mte_tagged is set, tags have already been initialised */
for (i = 0; i < nr_pages; i++, page++) {
- if (!page_mte_tagged(page)) {
- mte_sync_page_tags(page, old_pte, check_swap,
- pte_is_tagged);
+ if (try_page_mte_tagging(page)) {
+ mte_clear_page_tags(page_address(page));
set_page_mte_tagged(page);
}
}
diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index cd508ba80ab1..3a78bf1b1364 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,10 +53,9 @@ void mte_restore_tags(swp_entry_t entry, struct page *page)
if (!tags)
return;

- if (try_page_mte_tagging(page)) {
- mte_restore_page_tags(page_address(page), tags);
- set_page_mte_tagged(page);
- }
+ WARN_ON_ONCE(!try_page_mte_tagging(page));
+ mte_restore_page_tags(page_address(page), tags);
+ set_page_mte_tagged(page);
}

void mte_invalidate_tags(int type, pgoff_t offset)
--
2.40.1.606.ga4b1b128d6-goog