arm64: mte: reset the page tag in page->flags

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.

Link: https://lkml.kernel.org/r/9073d4e973747a6f78d5bdd7ebe17f290d087096.1606161801.git.andreyknvl@google.com
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Marco Elver <elver@google.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index 4200377..9c9f47e 100644 (file)
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c
@@ -371,6 +371,11 @@ static void swsusp_mte_restore_tags(void)
                unsigned long pfn = xa_state.xa_index;
                struct page *page = pfn_to_online_page(pfn);
 
+               /*
+                * It is not required to invoke page_kasan_tag_reset(page)
+                * at this point since the tags stored in page->flags are
+                * already restored.
+                */
                mte_restore_page_tags(page_address(page), tags);
 
                mte_free_tag_storage(tags);

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 78f81c4..3a6216b 100644 (file)
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -34,6 +34,15 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
                        return;
        }
 
+       page_kasan_tag_reset(page);
+       /*
+        * We need smp_wmb() in between setting the flags and clearing the
+        * tags because if another thread reads page->flags and builds a
+        * tagged address out of it, there is an actual dependency to the
+        * memory access, but on the current thread we do not guarantee that
+        * the new page->flags are visible before the tags were updated.
+        */
+       smp_wmb();
        mte_clear_page_tags(page_address(page));
 }
 

diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 70a71f3..b5447e5 100644 (file)
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -23,6 +23,15 @@ void copy_highpage(struct page *to, struct page *from)
 
        if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
                set_bit(PG_mte_tagged, &to->flags);
+               page_kasan_tag_reset(to);
+               /*
+                * We need smp_wmb() in between setting the flags and clearing the
+                * tags because if another thread reads page->flags and builds a
+                * tagged address out of it, there is an actual dependency to the
+                * memory access, but on the current thread we do not guarantee that
+                * the new page->flags are visible before the tags were updated.
+                */
+               smp_wmb();
                mte_copy_page_tags(kto, kfrom);
        }
 }

diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index c52c184..7c4ef56 100644 (file)
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -53,6 +53,15 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
        if (!tags)
                return false;
 
+       page_kasan_tag_reset(page);
+       /*
+        * We need smp_wmb() in between setting the flags and clearing the
+        * tags because if another thread reads page->flags and builds a
+        * tagged address out of it, there is an actual dependency to the
+        * memory access, but on the current thread we do not guarantee that
+        * the new page->flags are visible before the tags were updated.
+        */
+       smp_wmb();
        mte_restore_page_tags(page_address(page), tags);
 
        return true;