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

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Oct 28, 2020 at 11:07 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Oct 12, 2020 at 10:45 PM Andrey Konovalov <andreyknvl@google.com> wrote:
> >
> > From: Vincenzo Frascino <vincenzo.frascino@arm.com>
> >
> > 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@arm.com>
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> > 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?

Makes sense. I'll add this change into the other series together with
the patch that marks kasan_init_tags() 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
> >

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