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

Catalin Marinas
2020-09-25T11:34:41Z

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).

> + /*
> + * 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?

> +#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).

> +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.

--
Catalin