linux-kernel - Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG

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Date:   Thu, 8 Sep 2022 11:36:42 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG_INVALID

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@....com>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
>  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