Messages in this thread | | | Subject | Re: [PATCH v6 3/6] arm64/kvm: context-switch ptrauth registers | From | Amit Daniel Kachhap <> | Date | Fri, 1 Mar 2019 11:47:27 +0530 |
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On 2/21/19 9:21 PM, Dave Martin wrote: > On Thu, Feb 21, 2019 at 12:29:42PM +0000, Mark Rutland wrote: >> On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote: >>> From: Mark Rutland <mark.rutland@arm.com> >>> >>> When pointer authentication is supported, a guest may wish to use it. >>> This patch adds the necessary KVM infrastructure for this to work, with >>> a semi-lazy context switch of the pointer auth state. >>> >>> Pointer authentication feature is only enabled when VHE is built >>> in the kernel and present into CPU implementation so only VHE code >>> paths are modified. >> >> Nit: s/into/in the/ >> >>> >>> When we schedule a vcpu, we disable guest usage of pointer >>> authentication instructions and accesses to the keys. While these are >>> disabled, we avoid context-switching the keys. When we trap the guest >>> trying to use pointer authentication functionality, we change to eagerly >>> context-switching the keys, and enable the feature. The next time the >>> vcpu is scheduled out/in, we start again. However the host key registers >>> are saved in vcpu load stage as they remain constant for each vcpu >>> schedule. >>> >>> Pointer authentication consists of address authentication and generic >>> authentication, and CPUs in a system might have varied support for >>> either. Where support for either feature is not uniform, it is hidden >>> from guests via ID register emulation, as a result of the cpufeature >>> framework in the host. >>> >>> Unfortunately, address authentication and generic authentication cannot >>> be trapped separately, as the architecture provides a single EL2 trap >>> covering both. If we wish to expose one without the other, we cannot >>> prevent a (badly-written) guest from intermittently using a feature >>> which is not uniformly supported (when scheduled on a physical CPU which >>> supports the relevant feature). Hence, this patch expects both type of >>> authentication to be present in a cpu. >>> >>> Signed-off-by: Mark Rutland <mark.rutland@arm.com> >>> [Only VHE, key switch from from assembly, kvm_supports_ptrauth >>> checks, save host key in vcpu_load] >> >> Hmm, why do we need to do the key switch in assembly, given it's not >> used in-kernel right now? >> >> Is that in preparation for in-kernel pointer auth usage? If so, please >> call that out in the commit message. > > [...] > >> Huh, so we're actually doing the switch in C code... >> >>> # KVM code is run at a different exception code with a different map, so >>> # compiler instrumentation that inserts callbacks or checks into the code may >>> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S >>> index 675fdc1..b78cc15 100644 >>> --- a/arch/arm64/kvm/hyp/entry.S >>> +++ b/arch/arm64/kvm/hyp/entry.S >>> @@ -64,6 +64,12 @@ ENTRY(__guest_enter) >>> >>> add x18, x0, #VCPU_CONTEXT >>> >>> +#ifdef CONFIG_ARM64_PTR_AUTH >>> + // Prepare parameter for __ptrauth_switch_to_guest(vcpu, host, guest). >>> + mov x2, x18 >>> + bl __ptrauth_switch_to_guest >>> +#endif >> >> ... and conditionally *calling* that switch code from assembly ... >> >>> + >>> // Restore guest regs x0-x17 >>> ldp x0, x1, [x18, #CPU_XREG_OFFSET(0)] >>> ldp x2, x3, [x18, #CPU_XREG_OFFSET(2)] >>> @@ -118,6 +124,17 @@ ENTRY(__guest_exit) >>> >>> get_host_ctxt x2, x3 >>> >>> +#ifdef CONFIG_ARM64_PTR_AUTH >>> + // Prepare parameter for __ptrauth_switch_to_host(vcpu, guest, host). >>> + // Save x0, x2 which are used later in callee saved registers. >>> + mov x19, x0 >>> + mov x20, x2 >>> + sub x0, x1, #VCPU_CONTEXT >>> + ldr x29, [x2, #CPU_XREG_OFFSET(29)] >>> + bl __ptrauth_switch_to_host >>> + mov x0, x19 >>> + mov x2, x20 >>> +#endif >> >> ... which adds a load of boilerplate for no immediate gain. >> >> Do we really need to do this in assembly today? > > If we will need to move this to assembly when we add in-kernel ptrauth > support, it may be best to have it in assembly from the start, to reduce > unnecessary churn. > > But having a mix of C and assembly is likely to make things more > complicated: we should go with one or the other IMHO. ok, I will check on this.
Thanks, Amit D > > Cheers > ---Dave >
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