[PATCH 5.10 086/104] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page

Greg Kroah-Hartman
2021-02-15T15:42:24Z

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

commit 68d54ceeec0e5fee4fb8048e6a04c193f32525ca upstream.

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>
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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/kernel/cpufeature.c | 6 +-----
arch/arm64/kernel/mte.c | 3 ++-
2 files changed, 3 insertions(+), 6 deletions(-)

--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1696,16 +1696,12 @@ static void bti_enable(const struct arm6
#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));
- }
}
#endif /* CONFIG_ARM64_MTE */

--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -239,11 +239,12 @@ static int __access_remote_tags(struct m
* 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);