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* [PATCH] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page
@ 2021-02-10 18:03 Catalin Marinas
  2021-02-10 18:52 ` Luis Machado
                   ` (2 more replies)
  0 siblings, 3 replies; 6+ messages in thread
From: Catalin Marinas @ 2021-02-10 18:03 UTC (permalink / raw)
  To: linux-arm-kernel
  Cc: Luis Machado, Kevin Brodsky, Vincenzo Frascino, Steven Price,
	stable, Will Deacon

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

^ permalink raw reply related	[flat|nested] 6+ messages in thread
* [PATCH] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page
@ 2021-02-16 18:56 vivek
  0 siblings, 0 replies; 6+ messages in thread
From: vivek @ 2021-02-16 18:56 UTC (permalink / raw)
  To: bitu.kv; +Cc: Catalin Marinas, # 5 . 10 . x, Will Deacon

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

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
---
 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 e99edde..3e6331b 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 dc9ada6..80b62fe 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);
-- 
2.7.4


^ permalink raw reply related	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2021-02-16 18:57 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-02-10 18:03 [PATCH] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page Catalin Marinas
2021-02-10 18:52 ` Luis Machado
2021-02-11 10:35   ` Catalin Marinas
2021-02-11 10:56 ` Vincenzo Frascino
2021-02-12 16:45 ` Catalin Marinas
2021-02-16 18:56 vivek

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