From 85f56708a443ec02a290878f00d79a1ff5110e41 Mon Sep 17 00:00:00 2001 From: Fuad Tabba Date: Mon, 6 Jul 2026 12:55:21 +0100 Subject: [PATCH] KVM: arm64: Fix sign-extension of MMIO loads A sign-extending load (LDRSB, LDRSH, LDRSW) from MMIO returns a zero-extended value to the guest. The architecture performs such a load as a memory read of the access size, then a sign-extension to the register width. For LDRSH (DDI 0487 M.b C6.2.225, with the Mem accessor at J1.2.3.111): data = Mem{16}(address, accdesc); X{regsize}(t) = SignExtend{regsize}(data); The byte order is handled inside the Mem accessor, keyed on the access size; the register width is separate, applied afterwards by SignExtend(). kvm_handle_mmio_return() runs these in the wrong order: it sign-extends the access-width data, then calls vcpu_data_host_to_guest(), which masks the value back to the access width (the size-keyed byte-order step). The mask drops the sign bits that sign-extension produced. Reorder so vcpu_data_host_to_guest() runs first, with the sign-extension to register width after it. trace_kvm_mmio() moves with it and now logs the access-width data before sign-extension. Fixes: b30070862edbd ("ARM64: KVM: MMIO support BE host running LE code") Reviewed-by: Oliver Upton Signed-off-by: Fuad Tabba Link: https://patch.msgid.link/20260706115522.954913-2-fuad.tabba@linux.dev Signed-off-by: Marc Zyngier --- arch/arm64/kvm/mmio.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c index e2285ed8c91d..d1c3a352d5a2 100644 --- a/arch/arm64/kvm/mmio.c +++ b/arch/arm64/kvm/mmio.c @@ -126,6 +126,10 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu) len = kvm_vcpu_dabt_get_as(vcpu); data = kvm_mmio_read_buf(run->mmio.data, len); + trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr, + &data); + data = vcpu_data_host_to_guest(vcpu, data, len); + if (kvm_vcpu_dabt_issext(vcpu) && len < sizeof(unsigned long)) { mask = 1U << ((len * 8) - 1); @@ -135,9 +139,6 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu) if (!kvm_vcpu_dabt_issf(vcpu)) data = data & 0xffffffff; - trace_kvm_mmio(KVM_TRACE_MMIO_READ, len, run->mmio.phys_addr, - &data); - data = vcpu_data_host_to_guest(vcpu, data, len); vcpu_set_reg(vcpu, kvm_vcpu_dabt_get_rd(vcpu), data); }