Re: [PATCH 04/11] KVM: arm64: Move PC rollback on SError to HYP

On 2020-10-27 14:56, James Morse wrote:

Hi Marc,

On 26/10/2020 13:34, Marc Zyngier wrote:

quoted

Instead of handling the "PC rollback on SError during HVC" at EL1 
(which
requires disclosing PC to a potentially untrusted kernel), let's move
this fixup to ... fixup_guest_exit(), which is where we do all fixups.

quoted

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h 

b/arch/arm64/kvm/hyp/include/hyp/switch.h
index d687e574cde5..668f02c7b0b3 100644

--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h

@@ -411,6 +411,21 @@ static inline bool fixup_guest_exit(struct 

kvm_vcpu *vcpu, u64 *exit_code)
 	if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
 		vcpu->arch.fault.esr_el2 = read_sysreg_el2(SYS_ESR);

+	if (ARM_SERROR_PENDING(*exit_code)) {
+		u8 esr_ec = kvm_vcpu_trap_get_class(vcpu);
+
+		/*
+		 * HVC already have an adjusted PC, which we need to
+		 * correct in order to return to after having injected
+		 * the SError.
+		 *
+		 * SMC, on the other hand, is *trapped*, meaning its
+		 * preferred return address is the SMC itself.
+		 */
+		if (esr_ec == ESR_ELx_EC_HVC32 || esr_ec == ESR_ELx_EC_HVC64)
+			*vcpu_pc(vcpu) -= 4;

Isn't *vcpu_pc(vcpu) the PC of the previous entry for this vcpu?....
its not the PC of the
exit until __sysreg_save_el2_return_state() saves it, which happens 
just after
fixup_guest_exit().

Hmmm. Good point. The move was obviously done in haste, thank you for 
pointing
this blatant bug.

Mess with ELR_EL2 directly?

Yes, that's the best course of action. We never run this code anyway.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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