[PATCH] mte: Initialize tag storage to KASAN_TAG_INVALID

[Vincenzo Frascino]

Hy Andrey,

On 6/9/22 19:34, Andrey Konovalov wrote:
> On Tue, Jun 7, 2022 at 1:32 PM Vincenzo Frascino
> <vincenzo.frascino at arm.com> 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.
> 

You are right, I will fix this in v2. Before re-posting I will wait and see if
there are more comments.

> 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 at arm.com>
>> Cc: Will Deacon <will at kernel.org>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
>> ---
>>  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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-- 
Regards,
Vincenzo