Re: [PATCH] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page

The ptrace(PTRACE_PEEKMTETAGS) implementation checks whether the user
page has valid tags (mapped with PROT_MTE) by testing the PG_mte_tagged
page flag. If this bit is cleared, ptrace(PTRACE_PEEKMTETAGS) returns
-EIO.

A newly created (PROT_MTE) mapping points to the zero page which had its
tags zeroed during cpu_enable_mte(). If there were no prior writes to
this mapping, ptrace(PTRACE_PEEKMTETAGS) fails with -EIO since the zero
page does not have the PG_mte_tagged flag set.

Set PG_mte_tagged on the zero page when its tags are cleared during
boot. In addition, to avoid ptrace(PTRACE_PEEKMTETAGS) succeeding on
!PROT_MTE mappings pointing to the zero page, change the
__access_remote_tags() check to (vm_flags & VM_MTE) instead of
PG_mte_tagged.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Cc: <redacted> # 5.10.x
Cc: Will Deacon <will@kernel.org>
Reported-by: Luis Machado <redacted>
---

The fix is actually checking VM_MTE instead of PG_mte_tagged in
__access_remote_tags() but I added the WARN_ON(!PG_mte_tagged) and
setting the flag on the zero page in case we break this assumption in
the future.

  arch/arm64/kernel/cpufeature.c | 6 +-----
  arch/arm64/kernel/mte.c        | 3 ++-
  2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index e99eddec0a46..3e6331b64932 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -1701,16 +1701,12 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused)
  #ifdef CONFIG_ARM64_MTE
  static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
  {
-	static bool cleared_zero_page = false;
-
  	/*
  	 * Clear the tags in the zero page. This needs to be done via the
  	 * linear map which has the Tagged attribute.
  	 */
-	if (!cleared_zero_page) {
-		cleared_zero_page = true;
+	if (!test_and_set_bit(PG_mte_tagged, &ZERO_PAGE(0)->flags))
  		mte_clear_page_tags(lm_alias(empty_zero_page));
-	}
  
  	kasan_init_hw_tags_cpu();
  }

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index dc9ada64feed..80b62fe49dcf 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c

@@ -329,11 +329,12 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
  		 * would cause the existing tags to be cleared if the page
  		 * was never mapped with PROT_MTE.
  		 */
-		if (!test_bit(PG_mte_tagged, &page->flags)) {
+		if (!(vma->vm_flags & VM_MTE)) {
  			ret = -EOPNOTSUPP;
  			put_page(page);
  			break;
  		}
+		WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));
  
  		/* limit access to the end of the page */
  		offset = offset_in_page(addr);