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Subject[PATCH v12 4/7] arm64: mte: Enable TCO in functions that can read beyond buffer limits
Date
load_unaligned_zeropad() and __get/put_kernel_nofault() functions can
read passed some buffer limits which may include some MTE granule with a
different tag.

When MTE async mode is enable, the load operation crosses the boundaries
and the next granule has a different tag the PE sets the TFSR_EL1.TF1 bit
as if an asynchronous tag fault is happened.

Enable Tag Check Override (TCO) in these functions before the load and
disable it afterwards to prevent this to happen.

Note: The same condition can be hit in MTE sync mode but we deal with it
through the exception handling.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reported-by: Branislav Rankov <Branislav.Rankov@arm.com>
Tested-by: Branislav Rankov <Branislav.Rankov@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
arch/arm64/include/asm/uaccess.h | 19 +++++++++++++++++++
arch/arm64/include/asm/word-at-a-time.h | 4 ++++
arch/arm64/kernel/mte.c | 10 ++++++++++
3 files changed, 33 insertions(+)

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 0deb88467111..f43d78aee593 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -188,6 +188,21 @@ static inline void __uaccess_enable_tco(void)
ARM64_MTE, CONFIG_KASAN_HW_TAGS));
}

+/* Whether the MTE asynchronous mode is enabled. */
+DECLARE_STATIC_KEY_FALSE(mte_async_mode);
+
+static inline void __uaccess_disable_tco_async(void)
+{
+ if (static_branch_unlikely(&mte_async_mode))
+ __uaccess_disable_tco();
+}
+
+static inline void __uaccess_enable_tco_async(void)
+{
+ if (static_branch_unlikely(&mte_async_mode))
+ __uaccess_enable_tco();
+}
+
static inline void uaccess_disable_privileged(void)
{
__uaccess_disable_tco();
@@ -307,8 +322,10 @@ do { \
do { \
int __gkn_err = 0; \
\
+ __uaccess_enable_tco_async(); \
__raw_get_mem("ldr", *((type *)(dst)), \
(__force type *)(src), __gkn_err); \
+ __uaccess_disable_tco_async(); \
if (unlikely(__gkn_err)) \
goto err_label; \
} while (0)
@@ -379,9 +396,11 @@ do { \
#define __put_kernel_nofault(dst, src, type, err_label) \
do { \
int __pkn_err = 0; \
+ __uaccess_enable_tco_async(); \
\
__raw_put_mem("str", *((type *)(src)), \
(__force type *)(dst), __pkn_err); \
+ __uaccess_disable_tco_async(); \
if (unlikely(__pkn_err)) \
goto err_label; \
} while(0)
diff --git a/arch/arm64/include/asm/word-at-a-time.h b/arch/arm64/include/asm/word-at-a-time.h
index 3333950b5909..c62d9fa791aa 100644
--- a/arch/arm64/include/asm/word-at-a-time.h
+++ b/arch/arm64/include/asm/word-at-a-time.h
@@ -55,6 +55,8 @@ static inline unsigned long load_unaligned_zeropad(const void *addr)
{
unsigned long ret, offset;

+ __uaccess_enable_tco_async();
+
/* Load word from unaligned pointer addr */
asm(
"1: ldr %0, %3\n"
@@ -76,6 +78,8 @@ static inline unsigned long load_unaligned_zeropad(const void *addr)
: "=&r" (ret), "=&r" (offset)
: "r" (addr), "Q" (*(unsigned long *)addr));

+ __uaccess_disable_tco_async();
+
return ret;
}

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 92078e1eb627..60531afc706e 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -27,6 +27,10 @@ u64 gcr_kernel_excl __ro_after_init;

static bool report_fault_once = true;

+/* Whether the MTE asynchronous mode is enabled. */
+DEFINE_STATIC_KEY_FALSE(mte_async_mode);
+EXPORT_SYMBOL_GPL(mte_async_mode);
+
static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
{
pte_t old_pte = READ_ONCE(*ptep);
@@ -170,6 +174,12 @@ void mte_enable_kernel_sync(void)
void mte_enable_kernel_async(void)
{
__mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
+
+ /*
+ * This function is called on each active smp core, we do not
+ * to take cpu_hotplug_lock again.
+ */
+ static_branch_enable_cpuslocked(&mte_async_mode);
}

void mte_set_report_once(bool state)
--
2.30.0
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