On Wed, 19 Mar 2025 00:26:14 +0000, Gavin Shan gshan@redhat.com wrote:
Hi Mark,
On 3/14/25 10:35 AM, Mark Brown wrote:
From: Mark Rutland mark.rutland@arm.com
[ Upstream commit 59419f10045bc955d2229819c7cf7a8b0b9c5b59 ]
In non-protected KVM modes, while the guest FPSIMD/SVE/SME state is live on the CPU, the host's active SVE VL may differ from the guest's maximum SVE VL:
For VHE hosts, when a VM uses NV, ZCR_EL2 contains a value constrained by the guest hypervisor, which may be less than or equal to that guest's maximum VL.
Note: in this case the value of ZCR_EL1 is immaterial due to E2H.
For nVHE/hVHE hosts, ZCR_EL1 contains a value written by the guest, which may be less than or greater than the guest's maximum VL.
Note: in this case hyp code traps host SVE usage and lazily restores ZCR_EL2 to the host's maximum VL, which may be greater than the guest's maximum VL.
This can be the case between exiting a guest and kvm_arch_vcpu_put_fp(). If a softirq is taken during this period and the softirq handler tries to use kernel-mode NEON, then the kernel will fail to save the guest's FPSIMD/SVE state, and will pend a SIGKILL for the current thread.
This happens because kvm_arch_vcpu_ctxsync_fp() binds the guest's live FPSIMD/SVE state with the guest's maximum SVE VL, and fpsimd_save_user_state() verifies that the live SVE VL is as expected before attempting to save the register state:
| if (WARN_ON(sve_get_vl() != vl)) { | force_signal_inject(SIGKILL, SI_KERNEL, 0, 0); | return; | }
Fix this and make this a bit easier to reason about by always eagerly switching ZCR_EL{1,2} at hyp during guest<->host transitions. With this happening, there's no need to trap host SVE usage, and the nVHE/nVHE __deactivate_cptr_traps() logic can be simplified to enable host access to all present FPSIMD/SVE/SME features.
In protected nVHE/hVHE modes, the host's state is always saved/restored by hyp, and the guest's state is saved prior to exit to the host, so from the host's PoV the guest never has live FPSIMD/SVE/SME state, and the host's ZCR_EL1 is never clobbered by hyp.
Fixes: 8c8010d69c132273 ("KVM: arm64: Save/restore SVE state for nVHE") Fixes: 2e3cf82063a00ea0 ("KVM: arm64: nv: Ensure correct VL is loaded before saving SVE state") Signed-off-by: Mark Rutland mark.rutland@arm.com Reviewed-by: Mark Brown broonie@kernel.org Tested-by: Mark Brown broonie@kernel.org Cc: Catalin Marinas catalin.marinas@arm.com Cc: Fuad Tabba tabba@google.com Cc: Marc Zyngier maz@kernel.org Cc: Oliver Upton oliver.upton@linux.dev Cc: Will Deacon will@kernel.org Reviewed-by: Oliver Upton oliver.upton@linux.dev Link: https://lore.kernel.org/r/20250210195226.1215254-9-mark.rutland@arm.com Signed-off-by: Marc Zyngier maz@kernel.org Signed-off-by: Mark Brown broonie@kernel.org
arch/arm64/kvm/fpsimd.c | 30 ----------------- arch/arm64/kvm/hyp/entry.S | 5 +++ arch/arm64/kvm/hyp/include/hyp/switch.h | 59 +++++++++++++++++++++++++++++++++ arch/arm64/kvm/hyp/nvhe/hyp-main.c | 13 ++++---- arch/arm64/kvm/hyp/nvhe/switch.c | 33 +++++++++++++++--- arch/arm64/kvm/hyp/vhe/switch.c | 4 +++ 6 files changed, 103 insertions(+), 41 deletions(-)
[...]
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c index 4e757a77322c9efc59cdff501745f7c80d452358..1c8e2ad32e8c396fc4b11d5fec2e86728f2829d9 100644 --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c @@ -5,6 +5,7 @@ */ #include <hyp/adjust_pc.h> +#include <hyp/switch.h> #include <asm/pgtable-types.h> #include <asm/kvm_asm.h> @@ -176,8 +177,12 @@ static void handle___kvm_vcpu_run(struct kvm_cpu_context *host_ctxt) sync_hyp_vcpu(hyp_vcpu); pkvm_put_hyp_vcpu(hyp_vcpu); } else {
struct kvm_vcpu *vcpu = kern_hyp_va(host_vcpu);- /* The host is fully trusted, run its vCPU directly. */
ret = __kvm_vcpu_run(host_vcpu);
fpsimd_lazy_switch_to_guest(vcpu);ret = __kvm_vcpu_run(vcpu);fpsimd_lazy_switch_to_host(vcpu);@host_vcpu should have been hypervisor's linear mapping address in v6.12. It looks incorrect to assume it's a kernel's linear mapping address and convert it (@host_vcpu) to the hypervisor's linear address agin, if I don't miss anything.
host_vcpu is passed as a parameter to the hypercall, and is definitely a kernel address.
However, at this stage, we have *already* converted it to a HYP VA:
https://web.git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/ar...
The result is that this change is turning a perfectly valid HYP VA into... something. Odds are that the masking/patching will not mess up the address, but this is completely buggy anyway. In general, kern_hyp_va() is not an idempotent operation.
Thanks for noticing that something was wrong.
Broonie, can you please look into this?
Greg, it may be more prudent to unstage this series from 6.12-stable until we know for sure this is the only problem.
M.