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

On 2/10/21 6:03 PM, Catalin Marinas wrote:

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: stable@vger.kernel.org # 5.10.x Cc: Will Deacon will@kernel.org Reported-by: Luis Machado luis.machado@linaro.org

---

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));
  

Nit: I would live a white line before WARN_ON_ONCE() to improve readability and maybe transform it in WARN_ONCE() with a message (alternatively a comment on top) based on what you are explaining in the commit message.

Otherwise:

Reviewed-by: Vincenzo Frascino vincenzo.frascino@arm.com

/* limit access to the end of the page */
offset = offset_in_page(addr);