On Thu, Dec 03, 2020 at 07:06:01PM +1100, Stephen Rothwell wrote:
> diff --cc arch/arm64/include/asm/uaccess.h
> index d841a560fae7,abb31aa1f8ca..000000000000
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@@ -186,64 -159,20 +159,43 @@@ static inline void __uaccess_enable_hw_
> CONFIG_ARM64_PAN));
> }
>
> - #define __uaccess_disable(alt) \
> - do { \
> - if (!uaccess_ttbr0_disable()) \
> - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), alt, \
> - CONFIG_ARM64_PAN)); \
> - } while (0)
> -
> - #define __uaccess_enable(alt) \
> - do { \
> - if (!uaccess_ttbr0_enable()) \
> - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt, \
> - CONFIG_ARM64_PAN)); \
> - } while (0)
> -
> +/*
> + * The Tag Check Flag (TCF) mode for MTE is per EL, hence TCF0
> + * affects EL0 and TCF affects EL1 irrespective of which TTBR is
> + * used.
> + * The kernel accesses TTBR0 usually with LDTR/STTR instructions
> + * when UAO is available, so these would act as EL0 accesses using
> + * TCF0.
> + * However futex.h code uses exclusives which would be executed as
> + * EL1, this can potentially cause a tag check fault even if the
> + * user disables TCF0.
> + *
> + * To address the problem we set the PSTATE.TCO bit in uaccess_enable()
> + * and reset it in uaccess_disable().
> + *
> + * The Tag check override (TCO) bit disables temporarily the tag checking
> + * preventing the issue.
> + */
> - static inline void uaccess_disable(void)
> + static inline void uaccess_disable_privileged(void)
> {
> + asm volatile(ALTERNATIVE("nop", SET_PSTATE_TCO(0),
> + ARM64_MTE, CONFIG_KASAN_HW_TAGS));
> +
> - __uaccess_disable(ARM64_HAS_PAN);
> + if (uaccess_ttbr0_disable())
> + return;
> +
> + __uaccess_enable_hw_pan();
> }
>
> - static inline void uaccess_enable(void)
> + static inline void uaccess_enable_privileged(void)
> {
> + asm volatile(ALTERNATIVE("nop", SET_PSTATE_TCO(1),
> + ARM64_MTE, CONFIG_KASAN_HW_TAGS));
> +
> - __uaccess_enable(ARM64_HAS_PAN);
> - }
> -
> - /*
> - * These functions are no-ops when UAO is present.
> - */
> - static inline void uaccess_disable_not_uao(void)
> - {
> - __uaccess_disable(ARM64_ALT_PAN_NOT_UAO);
> - }
> + if (uaccess_ttbr0_enable())
> + return;
>
> - static inline void uaccess_enable_not_uao(void)
> - {
> - __uaccess_enable(ARM64_ALT_PAN_NOT_UAO);
> + __uaccess_disable_hw_pan();
> }
>
> /*
Thanks Stephen, it looks fine. I may clean it up a bit with dedicated
functions for the asm statements but after -rc1.
--
Catalin
