Re: [PATCH v8 30/43] arm64: kasan: Allow enabling in-kernel MTE

Vincenzo Frascino
2020-11-05T11:13:18Z

Hi Andrey,

On 11/4/20 11:18 PM, Andrey Konovalov wrote:
> Hardware tag-based KASAN relies on Memory Tagging Extension (MTE)
> feature and requires it to be enabled. MTE supports
>
> This patch adds a new mte_init_tags() helper, that enables MTE in
> Synchronous mode in EL1 and is intended to be called from KASAN runtime
> during initialization.
>
> The Tag Checking operation causes a synchronous data abort as
> a consequence of a tag check fault when MTE is configured in
> synchronous mode.
>
> As part of this change enable match-all tag for EL1 to allow the
> kernel to access user pages without faulting. This is required because
> the kernel does not have knowledge of the tags set by the user in a
> page.
>
> Note: For MTE, the TCF bit field in SCTLR_EL1 affects only EL1 in a
> similar way as TCF0 affects EL0.
>
> MTE that is built on top of the Top Byte Ignore (TBI) feature hence we
> enable it as part of this patch as well.
>

seems that in this patch you dropped me as author. Would you mind to clarify the
reason?

> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Co-developed-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> Change-Id: I4d67497268bb7f0c2fc5dcacefa1e273df4af71d
> ---
> arch/arm64/include/asm/mte-kasan.h | 6 ++++++
> arch/arm64/kernel/mte.c | 7 +++++++
> arch/arm64/mm/proc.S | 23 ++++++++++++++++++++---
> 3 files changed, 33 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
> index 3a70fb1807fd..ae75feaea2d4 100644
> --- a/arch/arm64/include/asm/mte-kasan.h
> +++ b/arch/arm64/include/asm/mte-kasan.h
> @@ -29,6 +29,8 @@ 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);
>
> +void __init mte_init_tags(u64 max_tag);
> +
> #else /* CONFIG_ARM64_MTE */
>
> static inline u8 mte_get_ptr_tag(void *ptr)
> @@ -49,6 +51,10 @@ static inline void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
> return addr;
> }
>
> +static inline void mte_init_tags(u64 max_tag)
> +{
> +}
> +
> #endif /* CONFIG_ARM64_MTE */
>
> #endif /* __ASSEMBLY__ */
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 06ba6c923ab7..fcfbefcc3174 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -121,6 +121,13 @@ void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
> return ptr;
> }
>
> +void __init mte_init_tags(u64 max_tag)
> +{
> + /* Enable MTE Sync Mode for EL1. */
> + sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> + isb();

I am fine with the approach of letting cpu_enable_mte() call directly
kasan_init_tags(), but how does it work of the other 2 implementation of KASAN?
Is it still called in arch_setup()?

I would prefer to keep the code that initializes the sync mode in
cpu_enable_mte() (calling kasan_init_tags() before then that) or in a separate
function since setting the mode has nothing to do with initializing the tags.
The second approach probably would come handy when we introduce async mode.

> +}
> +
> static void update_sctlr_el1_tcf0(u64 tcf0)
> {
> /* ISB required for the kernel uaccess routines */
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index 23c326a06b2d..7c3304fb15d9 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -40,9 +40,15 @@
> #define TCR_CACHE_FLAGS TCR_IRGN_WBWA | TCR_ORGN_WBWA
>
> #ifdef CONFIG_KASAN_SW_TAGS
> -#define TCR_KASAN_FLAGS TCR_TBI1
> +#define TCR_KASAN_SW_FLAGS TCR_TBI1
> #else
> -#define TCR_KASAN_FLAGS 0
> +#define TCR_KASAN_SW_FLAGS 0
> +#endif
> +
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define TCR_KASAN_HW_FLAGS SYS_TCR_EL1_TCMA1 | TCR_TBI1
> +#else
> +#define TCR_KASAN_HW_FLAGS 0
> #endif
>
> /*
> @@ -427,6 +433,10 @@ SYM_FUNC_START(__cpu_setup)
> */
> mov_q x5, MAIR_EL1_SET
> #ifdef CONFIG_ARM64_MTE
> + mte_tcr .req x20
> +
> + mov mte_tcr, #0
> +
> /*
> * Update MAIR_EL1, GCR_EL1 and TFSR*_EL1 if MTE is supported
> * (ID_AA64PFR1_EL1[11:8] > 1).
> @@ -447,6 +457,9 @@ SYM_FUNC_START(__cpu_setup)
> /* clear any pending tag check faults in TFSR*_EL1 */
> msr_s SYS_TFSR_EL1, xzr
> msr_s SYS_TFSRE0_EL1, xzr
> +
> + /* set the TCR_EL1 bits */
> + mov_q mte_tcr, TCR_KASAN_HW_FLAGS
> 1:
> #endif
> msr mair_el1, x5
> @@ -456,7 +469,11 @@ SYM_FUNC_START(__cpu_setup)
> */
> mov_q x10, TCR_TxSZ(VA_BITS) | TCR_CACHE_FLAGS | TCR_SMP_FLAGS | \
> TCR_TG_FLAGS | TCR_KASLR_FLAGS | TCR_ASID16 | \
> - TCR_TBI0 | TCR_A1 | TCR_KASAN_FLAGS
> + TCR_TBI0 | TCR_A1 | TCR_KASAN_SW_FLAGS
> +#ifdef CONFIG_ARM64_MTE
> + orr x10, x10, mte_tcr
> + .unreq mte_tcr
> +#endif
> tcr_clear_errata_bits x10, x9, x5
>
> #ifdef CONFIG_ARM64_VA_BITS_52
>

--
Regards,
Vincenzo