Re: [PATCH v5 08/40] arm64: mte: Switch GCR_EL1 in kernel entry and exit

From: Dmitry Vyukov
Date: Wed Oct 28 2020 - 19:07:06 EST

On Mon, Oct 12, 2020 at 10:45 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
>
> From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
>
> When MTE is present, the GCR_EL1 register contains the tags mask that
> allows to exclude tags from the random generation via the IRG instruction.
>
> With the introduction of the new Tag-Based KASAN API that provides a
> mechanism to reserve tags for special reasons, the MTE implementation
> has to make sure that the GCR_EL1 setting for the kernel does not affect
> the userspace processes and viceversa.
>
> Save and restore the kernel/user mask in GCR_EL1 in kernel entry and exit.
>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> ---
> Change-Id: I0081cba5ace27a9111bebb239075c9a466af4c84
> ---
> arch/arm64/include/asm/mte-def.h | 1 -
> arch/arm64/include/asm/mte-kasan.h | 6 +++++
> arch/arm64/include/asm/mte.h | 2 ++
> arch/arm64/kernel/asm-offsets.c | 3 +++
> arch/arm64/kernel/cpufeature.c | 3 +++
> arch/arm64/kernel/entry.S | 41 ++++++++++++++++++++++++++++++
> arch/arm64/kernel/mte.c | 22 +++++++++++++---
> 7 files changed, 74 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/mte-def.h b/arch/arm64/include/asm/mte-def.h
> index 8401ac5840c7..2d73a1612f09 100644
> --- a/arch/arm64/include/asm/mte-def.h
> +++ b/arch/arm64/include/asm/mte-def.h
> @@ -10,6 +10,5 @@
> #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)
>
> #endif /* __ASM_MTE_DEF_H */
> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
> index 3a70fb1807fd..a4c61b926d4a 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 mte_init_tags(u64 max_tag);

This should be marked as __init?


> #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/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index cf1cd181dcb2..d02aff9f493d 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -18,6 +18,8 @@
>
> #include <asm/pgtable-types.h>
>
> +extern u64 gcr_kernel_excl;
> +
> void mte_clear_page_tags(void *addr);
> unsigned long mte_copy_tags_from_user(void *to, const void __user *from,
> unsigned long n);
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 7d32fc959b1a..dfe6ed8446ac 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -47,6 +47,9 @@ int main(void)
> #ifdef CONFIG_ARM64_PTR_AUTH
> DEFINE(THREAD_KEYS_USER, offsetof(struct task_struct, thread.keys_user));
> DEFINE(THREAD_KEYS_KERNEL, offsetof(struct task_struct, thread.keys_kernel));
> +#endif
> +#ifdef CONFIG_ARM64_MTE
> + DEFINE(THREAD_GCR_EL1_USER, offsetof(struct task_struct, thread.gcr_user_excl));
> #endif
> BLANK();
> DEFINE(S_X0, offsetof(struct pt_regs, regs[0]));
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index eca06b8c74db..e76634ad5bc7 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1721,6 +1721,9 @@ static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
>
> /* Enable in-kernel MTE only if KASAN_HW_TAGS is enabled */
> if (IS_ENABLED(CONFIG_KASAN_HW_TAGS)) {
> + /* Enable the kernel exclude mask for random tags generation */
> + write_sysreg_s(SYS_GCR_EL1_RRND | gcr_kernel_excl, SYS_GCR_EL1);
> +
> /* Enable MTE Sync Mode for EL1 */
> sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> isb();
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ff34461524d4..eeaac91021bf 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -175,6 +175,43 @@ alternative_else_nop_endif
> #endif
> .endm
>
> + .macro mte_set_gcr, tmp, tmp2
> +#ifdef CONFIG_ARM64_MTE
> + /*
> + * 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
> +#endif
> + .endm
> +
> + .macro mte_set_kernel_gcr, tmp, tmp2
> +#ifdef CONFIG_KASAN_HW_TAGS
> +alternative_if_not ARM64_MTE
> + b 1f
> +alternative_else_nop_endif
> + ldr_l \tmp, gcr_kernel_excl
> +
> + mte_set_gcr \tmp, \tmp2
> +1:
> +#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 +251,8 @@ alternative_else_nop_endif
>
> ptrauth_keys_install_kernel tsk, x20, x22, x23
>
> + mte_set_kernel_gcr x22, x23
> +
> scs_load tsk, x20
> .else
> add x21, sp, #S_FRAME_SIZE
> @@ -332,6 +371,8 @@ 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 a9f03be75cef..ca8206b7f9a6 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -23,6 +23,8 @@
> #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);
> @@ -121,6 +123,17 @@ void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
> return ptr;
> }
>
> +void mte_init_tags(u64 max_tag)
> +{
> + /*
> + * The format of the tags in KASAN is 0xFF and in MTE is 0xF.
> + * This conversion is required to extract the MTE tag from a KASAN one.
> + */
> + u64 incl = GENMASK(FIELD_GET(MTE_TAG_MASK >> MTE_TAG_SHIFT, max_tag), 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 */
> @@ -156,7 +169,11 @@ 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_excl value
> + * by mte_set_user_gcr() in kernel_exit,
> + */
> }
>
> void flush_mte_state(void)
> @@ -182,7 +199,6 @@ void mte_thread_switch(struct task_struct *next)
> /* avoid expensive SCTLR_EL1 accesses if no change */
> if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
> update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
> - update_gcr_el1_excl(next->thread.gcr_user_excl);
> }
>
> void mte_suspend_exit(void)
> @@ -190,7 +206,7 @@ void mte_suspend_exit(void)
> if (!system_supports_mte())
> return;
>
> - update_gcr_el1_excl(current->thread.gcr_user_excl);
> + update_gcr_el1_excl(gcr_kernel_excl);
> }
>
> long set_mte_ctrl(struct task_struct *task, unsigned long arg)
> --
> 2.28.0.1011.ga647a8990f-goog
>