Re: [PATCH v3 29/39] arm64: mte: Switch GCR_EL1 in kernel entry and exit

Vincenzo Frascino
2020-09-25T11:47:55Z

On 9/25/20 12:34 PM, Catalin Marinas wrote:
> On Fri, Sep 25, 2020 at 12:50:36AM +0200, Andrey Konovalov wrote:
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index ff34461524d4..c7cc1fdfbd1a 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -175,6 +175,49 @@ alternative_else_nop_endif
>> #endif
>> .endm
>>
>> + .macro mte_set_gcr, tmp, tmp2
>> +#ifdef CONFIG_ARM64_MTE
>> +alternative_if_not ARM64_MTE
>> + b 1f
>> +alternative_else_nop_endif
>
> You don't need the alternative here. The macro is only invoked in an
> alternative path already (I'd be surprised if it even works, we don't
> handle nested alternatives well).
>

Yes, you are right. I forgot to remove it.

>> + /*
>> + * Calculate and set the exclude mask preserving
>> + * the RRND (bit[16]) setting.
>> + */
>> + mrs_s \tmp2, SYS_GCR_EL1
>> + bfi \tmp2, \tmp, #0, #16
>> + msr_s SYS_GCR_EL1, \tmp2
>> + isb
>> +1:
>> +#endif
>> + .endm
>> +
>> + .macro mte_set_kernel_gcr, tsk, tmp, tmp2
>
> What's the point of a 'tsk' argument here?
>

It is unused. I kept the interface same in between kernel and user.
I can either add a comment or remove it. Which one do you prefer?

>> +#ifdef CONFIG_KASAN_HW_TAGS
>> +#ifdef CONFIG_ARM64_MTE
>
> Does KASAN_HW_TAGS depend on ARM64_MTE already? Just to avoid too may
> ifdefs. Otherwise, you can always write it as:
>
> #if defined(CONFIG_KASAN_HW_TAGS) && defined(CONFIG_ARM64_MTE)
>
> to save two lines (and its easier to read).
>

It is indeed. I forgot to remove CONFIG_ARM64_MTE.

>> +alternative_if_not ARM64_MTE
>> + b 1f
>> +alternative_else_nop_endif
>> + ldr_l \tmp, gcr_kernel_excl
>> +
>> + mte_set_gcr \tmp, \tmp2
>> +1:
>> +#endif
>> +#endif
>> + .endm
>> +
>> + .macro mte_set_user_gcr, tsk, tmp, tmp2
>> +#ifdef CONFIG_ARM64_MTE
>> +alternative_if_not ARM64_MTE
>> + b 1f
>> +alternative_else_nop_endif
>> + ldr \tmp, [\tsk, #THREAD_GCR_EL1_USER]
>> +
>> + mte_set_gcr \tmp, \tmp2
>> +1:
>> +#endif
>> + .endm
>> +
>> .macro kernel_entry, el, regsize = 64
>> .if \regsize == 32
>> mov w0, w0 // zero upper 32 bits of x0
>> @@ -214,6 +257,8 @@ alternative_else_nop_endif
>>
>> ptrauth_keys_install_kernel tsk, x20, x22, x23
>>
>> + mte_set_kernel_gcr tsk, x22, x23
>> +
>> scs_load tsk, x20
>> .else
>> add x21, sp, #S_FRAME_SIZE
>> @@ -332,6 +377,8 @@ alternative_else_nop_endif
>> /* No kernel C function calls after this as user keys are set. */
>> ptrauth_keys_install_user tsk, x0, x1, x2
>>
>> + mte_set_user_gcr tsk, x0, x1
>> +
>> apply_ssbd 0, x0, x1
>> .endif
>>
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index 393d0c794be4..c3b4f056fc54 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -22,6 +22,8 @@
>> #include <asm/ptrace.h>
>> #include <asm/sysreg.h>
>>
>> +u64 gcr_kernel_excl __ro_after_init;
>> +
>> static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>> {
>> pte_t old_pte = READ_ONCE(*ptep);
>> @@ -116,6 +118,13 @@ void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
>> return ptr;
>> }
>>
>> +void mte_init_tags(u64 max_tag)
>> +{
>> + u64 incl = GENMASK(max_tag & MTE_TAG_MAX, 0);
>> +
>> + gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
>> +}
>> +
>> static void update_sctlr_el1_tcf0(u64 tcf0)
>> {
>> /* ISB required for the kernel uaccess routines */
>> @@ -151,7 +160,11 @@ static void update_gcr_el1_excl(u64 excl)
>> static void set_gcr_el1_excl(u64 excl)
>> {
>> current->thread.gcr_user_excl = excl;
>> - update_gcr_el1_excl(excl);
>> +
>> + /*
>> + * SYS_GCR_EL1 will be set to current->thread.gcr_user_incl value
> ^^^^^^^^^^^^^
> That's gcr_user_excl now.
>
>> + * by mte_restore_gcr() in kernel_exit,
>
> I don't think mte_restore_gcr is still around in this patch.
>

This comment requires updating. I missed it.

--
Regards,
Vincenzo