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SubjectRe: [PATCH v12 4/7] arm64: mte: Enable TCO in functions that can read beyond buffer limits
On Mon, Feb 08, 2021 at 04:56:14PM +0000, Vincenzo Frascino wrote:
> 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();
> +}

I would add a comment here along the lines of what's in the commit log:
these functions disable tag checking only if in MTE async mode since the
sync mode generates exceptions synchronously and the nofault or
load_unaligned_zeropad can handle them.

> +
> 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(); \
> \

Nitpick: for consistency with the __get_kernel_nofault() function,
please move the empty line above __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/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);
> }

Do we need to disable mte_async_mode in mte_enable_kernel_sync()? I
think currently that's only done at boot time but kasan may gain some
run-time features and change the mode dynamically.

--
Catalin

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