Commit 68d54cee authored Feb 10, 2021 by Catalin Marinas

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: 34bfeea4 ("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>
Tested-by: Luis Machado <luis.machado@linaro.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210210180316.23654-1-catalin.marinas@arm.com

parent 22cd5edb

arch/arm64/kernel/cpufeature.c

+1 −5

Original line number	Diff line number	Diff line
		@@ -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();
		}

arch/arm64/kernel/mte.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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);