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

[Andrey Konovalov]

On Tue, Jun 7, 2022 at 1:32 PM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> 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 (KHWASAN).

Hi Vincenzo,

I think we should move away from the KHWASAN name - it was used during
the early prototyping days for SW_TAGS KASAN. What you mean here is
HW_TAGS KASAN.

Thanks!


>
> 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(+)
>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 57b30bcf9f21..259a826363f1 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)
>  {
> @@ -107,6 +110,48 @@ 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;
> +
> +       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");
> +}
> +
>  static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
>  {
>         /* Enable MTE Sync Mode for EL1. */
> @@ -114,6 +159,8 @@ 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);
>  }
>
> --
> 2.36.1
>
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