[PATCH v2] KVM: arm64: Preserve GPRs for AArch32 CP64 reads generating an UNDEF

[PATCH v2] KVM: arm64: Preserve GPRs for AArch32 CP64 reads generating an UNDEF

[Karl Mehltretter]

kvm_handle_cp_64() only seeds params.regval for writes. If a CP64 read is
decoded but UNDEFs, emulate_cp() still returns handled and the caller
writes params.regval back to Rt/Rt2.

This can happen for PMU counter read accesses generating an UNDEF. KVM
injects the exception into the guest, so the MRRC GPRs must remain
unchanged.

Instead, the uninitialised regval is copied into the guest GPRs. With stack
auto-initialisation this is a deterministic zero or pattern value. With
CONFIG_INIT_STACK_NONE it may be stale host stack data.

Match kvm_handle_cp_32() and kvm_handle_sys_reg() by seeding regval from
the GPRs before emulation.

Fixes: 62a89c44954f0 ("arm64: KVM: 32bit handling of coprocessor traps")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Runtime tested on a Raspberry Pi 400 with a minimal KVM harness running
an AArch32 guest. On the unpatched 6.1.21-v8+ vendor kernel, the PMCCNTR
MRRC test took UNDEF with r0=0x00000001/r1=0x00000000 instead of the
guest's sentinel values. With this patch on v7.2-rc3-278-g38436106b2f5,
the same test preserved r0=0x12345678/r1=0x9abcdef0.

Changes in v2:
- Rework the commit message to use correct terminology, omit the
  unreachable GIC example, fix the Fixes tag and drop the added
  comment in the source code (Marc Zyngier).

Link to v1: https://lore.kernel.org/r/20260801153616.71960-1-kmehltretter@gmail.com

--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -4860,11 +4860,9 @@
 	/*
 	 * Make a 64-bit value out of Rt and Rt2. As we use the same trap
 	 * backends between AArch32 and AArch64, we get away with it.
 	 */
-	if (params.is_write) {
-		params.regval = vcpu_get_reg(vcpu, Rt) & 0xffffffff;
-		params.regval |= vcpu_get_reg(vcpu, Rt2) << 32;
-	}
+	params.regval = vcpu_get_reg(vcpu, Rt) & 0xffffffff;
+	params.regval |= vcpu_get_reg(vcpu, Rt2) << 32;
 
 	/*
 	 * If the table contains a handler, handle the
-- 
2.51.0

Re: [PATCH v2] KVM: arm64: Preserve GPRs for AArch32 CP64 reads generating an UNDEF

[Marc Zyngier]

On Sun, 02 Aug 2026 19:22:22 +0100,
Karl Mehltretter <kmehltretter@gmail.com> wrote:
> 
> kvm_handle_cp_64() only seeds params.regval for writes. If a CP64 read is
> decoded but UNDEFs, emulate_cp() still returns handled and the caller
> writes params.regval back to Rt/Rt2.
> 
> This can happen for PMU counter read accesses generating an UNDEF. KVM
> injects the exception into the guest, so the MRRC GPRs must remain
> unchanged.
> 
> Instead, the uninitialised regval is copied into the guest GPRs. With stack
> auto-initialisation this is a deterministic zero or pattern value. With
> CONFIG_INIT_STACK_NONE it may be stale host stack data.
> 
> Match kvm_handle_cp_32() and kvm_handle_sys_reg() by seeding regval from
> the GPRs before emulation.
> 
> Fixes: 62a89c44954f0 ("arm64: KVM: 32bit handling of coprocessor traps")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.