[PATCH v2 1/7] arm64: mte: Fix/clarify the PG_mte_tagged semantics

Peter Collingbourne
2022-07-22T01:50:59Z

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

Currently the PG_mte_tagged page flag mostly means the page contains
valid tags and it should be set after the tags have been cleared or
restored. However, in mte_sync_tags() it is set before setting the tags
to avoid, in theory, a race with concurrent mprotect(PROT_MTE) for
shared pages. However, a concurrent mprotect(PROT_MTE) with a copy on
write in another thread can cause the new page to have stale tags.
Similarly, tag reading via ptrace() can read stale tags of the
PG_mte_tagged flag is set before actually clearing/restoring the tags.

Fix the PG_mte_tagged semantics so that it is only set after the tags
have been cleared or restored. This is safe for swap restoring into a
MAP_SHARED or CoW page since the core code takes the page lock. Add two
functions to test and set the PG_mte_tagged flag with acquire and
release semantics. The downside is that concurrent mprotect(PROT_MTE) on
a MAP_SHARED page may cause tag loss. This is already the case for KVM
guests if a VMM changes the page protection while the guest triggers a
user_mem_abort().

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Peter Collingbourne <pcc@google.com>
---
arch/arm64/include/asm/mte.h | 30 ++++++++++++++++++++++++++++++
arch/arm64/include/asm/pgtable.h | 2 +-
arch/arm64/kernel/cpufeature.c | 4 +++-
arch/arm64/kernel/elfcore.c | 2 +-
arch/arm64/kernel/hibernate.c | 2 +-
arch/arm64/kernel/mte.c | 12 +++++++-----
arch/arm64/kvm/guest.c | 4 ++--
arch/arm64/kvm/mmu.c | 4 ++--
arch/arm64/mm/copypage.c | 4 ++--
arch/arm64/mm/fault.c | 2 +-
arch/arm64/mm/mteswap.c | 2 +-
11 files changed, 51 insertions(+), 17 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index aa523591a44e..c69218c56980 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -37,6 +37,29 @@ void mte_free_tag_storage(char *storage);
/* track which pages have valid allocation tags */
#define PG_mte_tagged PG_arch_2

+static inline void set_page_mte_tagged(struct page *page)
+{
+ /*
+ * Ensure that the tags written prior to this function are visible
+ * before the page flags update.
+ */
+ smp_wmb();
+ set_bit(PG_mte_tagged, &page->flags);
+}
+
+static inline bool page_mte_tagged(struct page *page)
+{
+ bool ret = test_bit(PG_mte_tagged, &page->flags);
+
+ /*
+ * If the page is tagged, ensure ordering with a likely subsequent
+ * read of the tags.
+ */
+ if (ret)
+ smp_rmb();
+ return ret;
+}
+
void mte_zero_clear_page_tags(void *addr);
void mte_sync_tags(pte_t old_pte, pte_t pte);
void mte_copy_page_tags(void *kto, const void *kfrom);
@@ -54,6 +77,13 @@ size_t mte_probe_user_range(const char __user *uaddr, size_t size);
/* unused if !CONFIG_ARM64_MTE, silence the compiler */
#define PG_mte_tagged 0

+static inline set_page_mte_tagged(struct page *page)
+{
+}
+static inline bool page_mte_tagged(struct page *page)
+{
+ return false;
+}
static inline void mte_zero_clear_page_tags(void *addr)
{
}
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index b5df82aa99e6..82719fa42c0e 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1050,7 +1050,7 @@ static inline void arch_swap_invalidate_area(int type)
static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
{
if (system_supports_mte() && mte_restore_tags(entry, &folio->page))
- set_bit(PG_mte_tagged, &folio->flags);
+ set_page_mte_tagged(&folio->page);
}

#endif /* CONFIG_ARM64_MTE */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index fae4c7a785d8..c66f0ffaaf47 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2020,8 +2020,10 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
* Clear the tags in the zero page. This needs to be done via the
* linear map which has the Tagged attribute.
*/
- if (!test_and_set_bit(PG_mte_tagged, &ZERO_PAGE(0)->flags))
+ if (!page_mte_tagged(ZERO_PAGE(0))) {
mte_clear_page_tags(lm_alias(empty_zero_page));
+ set_page_mte_tagged(ZERO_PAGE(0));
+ }

kasan_init_hw_tags_cpu();
}
diff --git a/arch/arm64/kernel/elfcore.c b/arch/arm64/kernel/elfcore.c
index 27ef7ad3ffd2..353009d7f307 100644
--- a/arch/arm64/kernel/elfcore.c
+++ b/arch/arm64/kernel/elfcore.c
@@ -47,7 +47,7 @@ static int mte_dump_tag_range(struct coredump_params *cprm,
* Pages mapped in user space as !pte_access_permitted() (e.g.
* PROT_EXEC only) may not have the PG_mte_tagged flag set.
*/
- if (!test_bit(PG_mte_tagged, &page->flags)) {
+ if (!page_mte_tagged(page)) {
put_page(page);
dump_skip(cprm, MTE_PAGE_TAG_STORAGE);
continue;
diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index af5df48ba915..788597a6b6a2 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c
@@ -271,7 +271,7 @@ static int swsusp_mte_save_tags(void)
if (!page)
continue;

- if (!test_bit(PG_mte_tagged, &page->flags))
+ if (!page_mte_tagged(page))
continue;

ret = save_tags(page, pfn);
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index b2b730233274..2287316639f3 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -41,14 +41,17 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
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 (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
+ set_page_mte_tagged(page);
return;
+ }
}

if (!pte_is_tagged)
return;

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

void mte_sync_tags(pte_t old_pte, pte_t pte)
@@ -64,7 +67,7 @@ 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 (!page_mte_tagged(page))
mte_sync_page_tags(page, old_pte, check_swap,
pte_is_tagged);
}
@@ -91,8 +94,7 @@ int memcmp_pages(struct page *page1, struct page *page2)
* pages is tagged, set_pte_at() may zero or change the tags of the
* other page via mte_sync_tags().
*/
- if (test_bit(PG_mte_tagged, &page1->flags) ||
- test_bit(PG_mte_tagged, &page2->flags))
+ if (page_mte_tagged(page1) || page_mte_tagged(page2))
return addr1 != addr2;

return ret;
@@ -398,7 +400,7 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
put_page(page);
break;
}
- WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));
+ WARN_ON_ONCE(!page_mte_tagged(page));

/* limit access to the end of the page */
offset = offset_in_page(addr);
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 8c607199cad1..3b04e69006b4 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -1058,7 +1058,7 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
maddr = page_address(page);

if (!write) {
- if (test_bit(PG_mte_tagged, &page->flags))
+ if (page_mte_tagged(page))
num_tags = mte_copy_tags_to_user(tags, maddr,
MTE_GRANULES_PER_PAGE);
else
@@ -1075,7 +1075,7 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
* completed fully
*/
if (num_tags == MTE_GRANULES_PER_PAGE)
- set_bit(PG_mte_tagged, &page->flags);
+ set_page_mte_tagged(page);

kvm_release_pfn_dirty(pfn);
}
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 87f1cd0df36e..c9012707f69c 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1075,9 +1075,9 @@ static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
return -EFAULT;

for (i = 0; i < nr_pages; i++, page++) {
- if (!test_bit(PG_mte_tagged, &page->flags)) {
+ if (!page_mte_tagged(page)) {
mte_clear_page_tags(page_address(page));
- set_bit(PG_mte_tagged, &page->flags);
+ set_page_mte_tagged(page);
}
}

diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
index 24913271e898..4223389b6180 100644
--- a/arch/arm64/mm/copypage.c
+++ b/arch/arm64/mm/copypage.c
@@ -21,9 +21,9 @@ void copy_highpage(struct page *to, struct page *from)

copy_page(kto, kfrom);

- if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
- set_bit(PG_mte_tagged, &to->flags);
+ if (system_supports_mte() && page_mte_tagged(from)) {
mte_copy_page_tags(kto, kfrom);
+ set_page_mte_tagged(to);
}
}
EXPORT_SYMBOL(copy_highpage);
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index c33f1fad2745..d095bfa16771 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -931,5 +931,5 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
void tag_clear_highpage(struct page *page)
{
mte_zero_clear_page_tags(page_address(page));
- set_bit(PG_mte_tagged, &page->flags);
+ set_page_mte_tagged(page);
}
diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
index 4334dec93bd4..a78c1db23c68 100644
--- a/arch/arm64/mm/mteswap.c
+++ b/arch/arm64/mm/mteswap.c
@@ -24,7 +24,7 @@ int mte_save_tags(struct page *page)
{
void *tag_storage, *ret;

- if (!test_bit(PG_mte_tagged, &page->flags))
+ if (!page_mte_tagged(page))
return 0;

tag_storage = mte_allocate_tag_storage();
--
2.37.1.359.gd136c6c3e2-goog