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Message-ID: <649ce069-050e-83c8-24d6-6aab6bb3f471@arm.com>
Date:   Thu, 8 Sep 2022 14:49:25 +0100
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Subject: Re: [PATCH v2] mte: Initialize tag storage to KASAN_TAG_INVALID
Content-Language: en-US
To:     Vladimir Murzin <vladimir.murzin@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>
References: <20220907110015.11489-1-vincenzo.frascino@arm.com>
 <198b1486-d402-9061-a6f0-e522a548f040@arm.com>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
In-Reply-To: <198b1486-d402-9061-a6f0-e522a548f040@arm.com>
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Hi Vladimir,

On 9/8/22 11:36, Vladimir Murzin wrote:
> 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@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>> ---
>>  arch/arm64/kernel/mte.c | 47 +++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 47 insertions(+)
> 
> Nothing in Documentation/ ?
> 

I can have a separate patch that adds documentation of the kernel parameter.

>>
>> 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()
> 

Good point, I was not aware of this API. Thanks!

>> +	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
> 

Do you have any preference? :D

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

Yes I agree, I can print "Tag Storage Initialized to 0x.."

>> +}
>> +
>>  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

-- 
Regards,
Vincenzo