Re: [PATCH v3 24/39] arm64: mte: Add in-kernel MTE helpers

Vincenzo Frascino
2020-09-25T11:25:58Z

On 9/25/20 11:15 AM, Catalin Marinas wrote:
> On Fri, Sep 25, 2020 at 12:50:31AM +0200, Andrey Konovalov wrote:
>> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
>> index 035003acfa87..bc0dc66a6a27 100644
>> --- a/arch/arm64/include/asm/esr.h
>> +++ b/arch/arm64/include/asm/esr.h
>> @@ -103,6 +103,7 @@
>> #define ESR_ELx_FSC (0x3F)
>> #define ESR_ELx_FSC_TYPE (0x3C)
>> #define ESR_ELx_FSC_EXTABT (0x10)
>> +#define ESR_ELx_FSC_MTE (0x11)
>> #define ESR_ELx_FSC_SERROR (0x11)
>> #define ESR_ELx_FSC_ACCESS (0x08)
>> #define ESR_ELx_FSC_FAULT (0x04)
>> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
>> new file mode 100644
>> index 000000000000..b0f27de8de33
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/mte-kasan.h
>> @@ -0,0 +1,60 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Copyright (C) 2020 ARM Ltd.
>> + */
>> +#ifndef __ASM_MTE_ASM_H
>> +#define __ASM_MTE_ASM_H
>> +
>> +#include <asm/compiler.h>
>> +
>> +#define __MTE_PREAMBLE ARM64_ASM_PREAMBLE ".arch_extension memtag\n"
>
> Can this not live in mte.h?
>

Yes, I can move it there in the next version.

>> +#define MTE_GRANULE_SIZE UL(16)
>> +#define MTE_GRANULE_MASK (~(MTE_GRANULE_SIZE - 1))
>> +#define MTE_TAG_SHIFT 56
>> +#define MTE_TAG_SIZE 4
>> +#define MTE_TAG_MASK GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
>> +#define MTE_TAG_MAX (MTE_TAG_MASK >> MTE_TAG_SHIFT)
>
> I'd still like these MTE_* macros in a separate mte-hwdef.h file. The
> only reason I see they were not in mte.h is because they need to be
> included in asm/cache.h. They are not KASAN specific.
>

Ok, fine I will reintroduce it in the next version.

>> +
>> +#ifndef __ASSEMBLY__
>> +
>> +#include <linux/types.h>
>> +
>> +#ifdef CONFIG_ARM64_MTE
>> +
>> +static inline u8 mte_get_ptr_tag(void *ptr)
>> +{
>> + u8 tag = (u8)(((u64)(ptr)) >> MTE_TAG_SHIFT);
>> +
>> + return tag;
>> +}
>
> So this returns the top 8 bits of the address (i.e. no masking with
> MTE_TAG_MASK). Fine by me.
>
>> +
>> +u8 mte_get_mem_tag(void *addr);
>> +u8 mte_get_random_tag(void);
>> +void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag);
>> +
>> +#else /* CONFIG_ARM64_MTE */
>> +
>> +static inline u8 mte_get_ptr_tag(void *ptr)
>> +{
>> + return 0xFF;
>> +}
>> +
>> +static inline u8 mte_get_mem_tag(void *addr)
>> +{
>> + return 0xFF;
>> +}
>> +static inline u8 mte_get_random_tag(void)
>> +{
>> + return 0xFF;
>> +}
>> +static inline void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
>> +{
>> + return addr;
>> +}
>
> Maybe these can stay in mte-kasan.h, although they are not a direct
> interface for KASAN AFAICT (the arch_* equivalent are defined in
> asm/memory.h. If there's no good reason, we could move them to mte.h.
>

This is here because it is not a direct interface as you noticed. I tried to
keep the separation (even if it I have something to fix based on your comment
below ;)).

The other kasan implementation define the arch_* indirection in asm/memory.h in
every architecture. I think maintaining the design is the best way to non create
confusion.

>> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
>> index 1c99fcadb58c..3a2bf3ccb26c 100644
>> --- a/arch/arm64/include/asm/mte.h
>> +++ b/arch/arm64/include/asm/mte.h
>> @@ -5,14 +5,13 @@
>> #ifndef __ASM_MTE_H
>> #define __ASM_MTE_H
>>
>> -#define MTE_GRANULE_SIZE UL(16)
>> -#define MTE_GRANULE_MASK (~(MTE_GRANULE_SIZE - 1))
>> -#define MTE_TAG_SHIFT 56
>> -#define MTE_TAG_SIZE 4
>> +#include <asm/mte-kasan.h>
>>
>> #ifndef __ASSEMBLY__
>>
>> +#include <linux/bitfield.h>
>> #include <linux/page-flags.h>
>> +#include <linux/types.h>
>>
>> #include <asm/pgtable-types.h>
>>
>> @@ -45,7 +44,9 @@ long get_mte_ctrl(struct task_struct *task);
>> int mte_ptrace_copy_tags(struct task_struct *child, long request,
>> unsigned long addr, unsigned long data);
>>
>> -#else
>> +void mte_assign_mem_tag_range(void *addr, size_t size);
>
> So mte_set_mem_tag_range() is KASAN specific but
> mte_assign_mem_tag_range() is not. Slightly confusing.
>

mte_assign_mem_tag_range() is the internal function implemented in assembler
which is not used directly by KASAN. Is it the name that you find confusing? Do
you have a better proposal?

>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index 52a0638ed967..833b63fdd5e2 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -13,8 +13,10 @@
>> #include <linux/swap.h>
>> #include <linux/swapops.h>
>> #include <linux/thread_info.h>
>> +#include <linux/types.h>
>> #include <linux/uio.h>
>>
>> +#include <asm/barrier.h>
>> #include <asm/cpufeature.h>
>> #include <asm/mte.h>
>> #include <asm/ptrace.h>
>> @@ -72,6 +74,48 @@ int memcmp_pages(struct page *page1, struct page *page2)
>> return ret;
>> }
>>
>> +u8 mte_get_mem_tag(void *addr)
>> +{
>> + if (!system_supports_mte())
>> + return 0xFF;
>> +
>> + asm volatile(__MTE_PREAMBLE "ldg %0, [%0]"
>> + : "+r" (addr));
>
> Nitpick: do we need volatile or plain asm would do?
>

No we clearly don't anymore :) I will remove it in the next iteration.

> I wonder whether we'd need the "memory" clobber. I don't see how this
> would fail though, maybe later on with stack tagging if the compiler
> writes tags behind our back.
>

As you said, I do not see how this can fail either. We can be overcautious
though here and add a comment that the clobber has been added in prevision of
stack tagging.

>> +
>> + return 0xF0 | mte_get_ptr_tag(addr);
>
> Since mte_get_ptr_tag() returns the top byte of the address, we don't
> need the additional 0xF0 or'ing. LDG only sets bits 59:56.
>

Yes, this can clearly go away.

>> +}
>> +
>> +u8 mte_get_random_tag(void)
>> +{
>> + void *addr;
>> +
>> + if (!system_supports_mte())
>> + return 0xFF;
>> +
>> + asm volatile(__MTE_PREAMBLE "irg %0, %0"
>> + : "+r" (addr));
>> +
>> + return 0xF0 | mte_get_ptr_tag(addr);
>
> Same here.
>

Agreed.

>> +}
>> +
>> +void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
>> +{
>> + void *ptr = addr;
>> +
>> + if ((!system_supports_mte()) || (size == 0))
>> + return addr;
>> +
>> + /* Make sure that size is aligned. */
>> + WARN_ON(size & (MTE_GRANULE_SIZE - 1));
>
> Doesn't the address need to be aligned as well?
>

Yes, we need an extra WARN_ON here. I will add it in the next version.

>> +
>> + tag = 0xF0 | tag;
>> + ptr = (void *)__tag_set(ptr, tag);
>> +
>> + mte_assign_mem_tag_range(ptr, size);
>> +
>> + return ptr;
>> +}
>> +
>> static void update_sctlr_el1_tcf0(u64 tcf0)
>> {
>> /* ISB required for the kernel uaccess routines */
>> diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
>> index 03ca6d8b8670..aa0ab01252fe 100644
>> --- a/arch/arm64/lib/mte.S
>> +++ b/arch/arm64/lib/mte.S
>> @@ -149,3 +149,22 @@ SYM_FUNC_START(mte_restore_page_tags)
>>
>> ret
>> SYM_FUNC_END(mte_restore_page_tags)
>> +
>> +/*
>> + * Assign allocation tags for a region of memory based on the pointer tag
>> + * x0 - source pointer
>> + * x1 - size
>> + *
>> + * Note: size must be non-zero and MTE_GRANULE_SIZE aligned
>
> Doesn't the address need to be aligned as well?
>

The comment can be extended.

>> + */
>> +SYM_FUNC_START(mte_assign_mem_tag_range)
>> + /* if (src == NULL) return; */
>> + cbz x0, 2f
>> + /* if (size == 0) return; */
>> + cbz x1, 2f
>
> I find these checks unnecessary, as I said a couple of times before,
> just document the function pre-conditions. They are also incomplete
> (i.e. you check for NULL but not alignment).
>

I thought we agreed to harden the code further, based on [1]. Maybe I
misunderstood. I am going to remove them and extend the comment in the next version.

[1]
https://lore.kernel.org/linux-arm-kernel/921c4ed0-b5b5-bc01-5418-c52d80f1af59@arm.com/

>> +1: stg x0, [x0]
>> + add x0, x0, #MTE_GRANULE_SIZE
>> + subs x1, x1, #MTE_GRANULE_SIZE
>> + b.gt 1b
>> +2: ret
>> +SYM_FUNC_END(mte_assign_mem_tag_range)
>> --
>> 2.28.0.681.g6f77f65b4e-goog
>

--
Regards,
Vincenzo