Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG_INVALID

From: Vladimir Murzin
Date: Thu Sep 08 2022 - 06:36:59 EST


Hi Vincenzo,

On 9/7/22 12:00, Vincenzo Frascino wrote:
> When the kernel is entered on aarch64, the MTE allocation tags are in an
> UNKNOWN state.
>
> With MTE enabled, the tags are initialized:
> - When a page is allocated and the user maps it with PROT_MTE.
> - On allocation, with in-kernel MTE enabled (HW_TAGS KASAN).
>
> If the tag pool is zeroed by the hardware at reset, it makes it
> difficult to track potential places where the initialization of the
> tags was missed.
>
> This can be observed under QEMU for aarch64, which initializes the MTE
> allocation tags to zero.
>
> Initialize to tag storage to KASAN_TAG_INVALID to catch potential
> places where the initialization of the tags was missed.
>
> This is done introducing a new kernel command line parameter
> "mte.tags_init" that enables the debug option.
>
> Note: The proposed solution should be considered a debug option because
> it might have performance impact on large machines at boot.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
> arch/arm64/kernel/mte.c | 47 +++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 47 insertions(+)

Nothing in Documentation/ ?

>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index b2b730233274..af9a8eba9be4 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -6,6 +6,7 @@
> #include <linux/bitops.h>
> #include <linux/cpu.h>
> #include <linux/kernel.h>
> +#include <linux/memblock.h>
> #include <linux/mm.h>
> #include <linux/prctl.h>
> #include <linux/sched.h>
> @@ -35,6 +36,8 @@ DEFINE_STATIC_KEY_FALSE(mte_async_or_asymm_mode);
> EXPORT_SYMBOL_GPL(mte_async_or_asymm_mode);
> #endif
>
> +static bool mte_tags_init __ro_after_init;
> +
> static void mte_sync_page_tags(struct page *page, pte_t old_pte,
> bool check_swap, bool pte_is_tagged)
> {
> @@ -98,6 +101,48 @@ int memcmp_pages(struct page *page1, struct page *page2)
> return ret;
> }
>
> +/* mte.tags_init=off/on */
> +static int __init early_mte_tags_init(char *arg)
> +{
> + if (!arg)
> + return -EINVAL;
> +
> + if (!strcmp(arg, "off"))
> + mte_tags_init = false;
> + else if (!strcmp(arg, "on"))
> + mte_tags_init = true;
> + else
> + return -EINVAL;
> +

You might want to offload parsing to kstrtobool()

> + return 0;
> +}
> +early_param("mte.tags_init", early_mte_tags_init);
> +
> +static inline void __mte_tag_storage_init(void)
> +{
> + static bool mte_tags_uninitialized = true;
> + phys_addr_t pa_start, pa_end;
> + u64 index;
> +
> + if (mte_tags_init && !mte_tags_uninitialized)
> + return;
> +
> + for_each_mem_range(index, &pa_start, &pa_end) {
> + void *va_start = (void *)__phys_to_virt(pa_start);
> + void *va_end = (void *)__phys_to_virt(pa_end);
> + size_t va_size = (u64)va_end - (u64)va_start;
> +
> + if (va_start >= va_end)
> + break;
> +
> + mte_set_mem_tag_range(va_start, va_size, KASAN_TAG_INVALID, false);
> + }
> +
> + /* Tags are now initialized to KASAN_TAG_INVALID */
> + mte_tags_uninitialized = false;
> + pr_info("MTE: Tag Storage Initialized\n");

Why All Words Start With Capital Letter? :D

Anyway, you might want to advertise tag value used for initialization.

> +}
> +
> static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
> {
> /* Enable MTE Sync Mode for EL1. */
> @@ -105,6 +150,8 @@ static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
> SYS_FIELD_PREP(SCTLR_EL1, TCF, tcf));
> isb();
>
> + __mte_tag_storage_init();
> +
> pr_info_once("MTE: enabled in %s mode at EL1\n", mode);
> }
>

Cheers
Vladimir