Commit 0225fd5e authored Apr 29, 2020 by Marc Zyngier

KVM: arm64: Fix 32bit PC wrap-around



In the unlikely event that a 32bit vcpu traps into the hypervisor
on an instruction that is located right at the end of the 32bit
range, the emulation of that instruction is going to increment
PC past the 32bit range. This isn't great, as userspace can then
observe this value and get a bit confused.

Conversly, userspace can do things like (in the context of a 64bit
guest that is capable of 32bit EL0) setting PSTATE to AArch64-EL0,
set PC to a 64bit value, change PSTATE to AArch32-USR, and observe
that PC hasn't been truncated. More confusion.

Fix both by:
- truncating PC increments for 32bit guests
- sanitizing all 32bit regs every time a core reg is changed by
  userspace, and that PSTATE indicates a 32bit mode.

Cc: stable@vger.kernel.org
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>

parent 958e8e14

arch/arm64/kvm/guest.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -200,6 +200,13 @@ static int set_core_reg(struct kvm_vcpu vcpu, const struct kvm_one_reg reg)
		}

		memcpy((u32 *)regs + off, valp, KVM_REG_SIZE(reg->id));

		if (*vcpu_cpsr(vcpu) & PSR_MODE32_BIT) {
		int i;

		for (i = 0; i < 16; i++)
		vcpu_reg32(vcpu, i) = (u32)vcpu_reg32(vcpu, i);
		}
		out:
		return err;
		}

virt/kvm/arm/hyp/aarch32.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -125,12 +125,16 @@ static void __hyp_text kvm_adjust_itstate(struct kvm_vcpu *vcpu)
		*/
		void __hyp_text kvm_skip_instr32(struct kvm_vcpu *vcpu, bool is_wide_instr)
		{
		u32 pc = *vcpu_pc(vcpu);
		bool is_thumb;

		is_thumb = !!(*vcpu_cpsr(vcpu) & PSR_AA32_T_BIT);
		if (is_thumb && !is_wide_instr)
		*vcpu_pc(vcpu) += 2;
		pc += 2;
		else
		*vcpu_pc(vcpu) += 4;
		pc += 4;

		*vcpu_pc(vcpu) = pc;

		kvm_adjust_itstate(vcpu);
		}

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