Messages in this thread | | | Subject | Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors | From | James Morse <> | Date | Fri, 21 Dec 2018 14:57:10 +0000 |
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Hi guys,
On 10/12/2018 14:59, Catalin Marinas wrote: > On Fri, Dec 07, 2018 at 08:38:11AM +0000, Julien Thierry wrote: >> On 12/06/2018 06:25 PM, Catalin Marinas wrote: >>> On Mon, Dec 03, 2018 at 01:55:18PM +0000, Julien Thierry wrote: >>>> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h >>>> index 07c3408..cabfcae 100644 >>>> --- a/arch/arm64/include/asm/uaccess.h >>>> +++ b/arch/arm64/include/asm/uaccess.h >>>> @@ -233,6 +233,23 @@ static inline void uaccess_enable_not_uao(void) >>>> +#define unsafe_user_region_active uaccess_region_active >>>> +static inline bool uaccess_region_active(void) >>>> +{ >>>> + if (system_uses_ttbr0_pan()) {
>>>> + } else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO)) { >>>> + return (read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN) ? >>>> + false : >>>> + !read_sysreg_s(SYS_PSTATE_PAN); >>>> + } >>> >>> ARM64_ALT_PAN_NOT_UAO implies ARM64_HAS_PAN which implies SCTLR_EL1.SPAN >>> is 0 at run-time. Is this to cope with the case of being called prior to >>> cpu_enable_pan()? >>> >> >> Yes, the issue I can into is that for cpufeatures, .cpu_enable() callbacks >> are called inside stop_machine() which obviously might_sleep and so attempts >> to check whether user_access is on. But for features that get enabled before >> PAN, the PAN bit will be set. > > OK, so the PSTATE.PAN bit only makes sense when SCTLR_EL1.SPAN is 0, IOW > the PAN hardware feature has been enabled. Maybe you could write it > (together with some comment): > > } else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO) && > !(read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN)) { > /* only if PAN is present and enabled */ > return !read_sysreg_s(SYS_PSTATE_PAN) > } > > On the cpufeature.c side of things, it seems that we enable the > static_branch before calling the cpu_enable. I wonder whether changing > the order here would help with avoid the SCTLR_EL1 read (not sure what > else it would break; cc'ing Suzuki).
Avoiding the system-register read would be good. Can we check alternatives_applied? It gets set later, and is obviously connected to the PAN alternatives being patched in to the uaccess routines.
Thanks,
James
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