arm64: mte: Do not set PG_mte_tagged if tags were not initialized

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.

Fix it by removing the redundant call to set_page_mte_tagged().

Fixes: e059853d14ca ("arm64: mte: Fix/clarify the PG_mte_tagged semantics")
Signed-off-by: Peter Collingbourne <pcc@google.com>
Cc: <stable@vger.kernel.org> # 6.1
Link: https://linux-review.googlesource.com/id/Ib02d004d435b2ed87603b858ef7480f7b1463052
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20230420214327.2357985-1-pcc@google.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index f5bcb0dc6267385e3401ed2735429b45af56e8e7..7e89968bd28244ced303586a35bb2e81dc1ffad6 100644 (file)
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -66,13 +66,10 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
                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)) {
+       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);
-                       set_page_mte_tagged(page);
-               }
-       }
 
        /* ensure the tags are visible before the PTE is set */
        smp_wmb();