KVM: riscv: Fix Spectre-v1 in floating-point register access

User-controlled indices are used to index into floating-point registers.
Sanitize them with array_index_nospec() to prevent speculative
out-of-bounds access.

Reviewed-by: Radim Krčmář <radim.krcmar@oss.qualcomm.com>
Signed-off-by: Lukas Gerlach <lukas.gerlach@cispa.de>
Link: https://lore.kernel.org/r/20260303-kvm-riscv-spectre-v1-v2-3-192caab8e0dc@cispa.de
Signed-off-by: Anup Patel <anup@brainfault.org>

Author: Lukas Gerlach

arch/riscv/kvm/vcpu_fp.c

@@ -10,6 +10,7 @@
 #include <linux/errno.h>
 #include <linux/err.h>
 #include <linux/kvm_host.h>
+#include <linux/nospec.h>
 #include <linux/uaccess.h>
 #include <asm/cpufeature.h>
 
@@ -93,9 +94,11 @@ int kvm_riscv_vcpu_get_reg_fp(struct kvm_vcpu *vcpu,
 		if (reg_num == KVM_REG_RISCV_FP_F_REG(fcsr))
 			reg_val = &cntx->fp.f.fcsr;
 		else if ((KVM_REG_RISCV_FP_F_REG(f[0]) <= reg_num) &&
-			  reg_num <= KVM_REG_RISCV_FP_F_REG(f[31]))
+			  reg_num <= KVM_REG_RISCV_FP_F_REG(f[31])) {
+			reg_num = array_index_nospec(reg_num,
+					ARRAY_SIZE(cntx->fp.f.f));
 			reg_val = &cntx->fp.f.f[reg_num];
-		else
+		} else
 			return -ENOENT;
 	} else if ((rtype == KVM_REG_RISCV_FP_D) &&
 		   riscv_isa_extension_available(vcpu->arch.isa, d)) {
@@ -107,6 +110,8 @@ int kvm_riscv_vcpu_get_reg_fp(struct kvm_vcpu *vcpu,
 			   reg_num <= KVM_REG_RISCV_FP_D_REG(f[31])) {
 			if (KVM_REG_SIZE(reg->id) != sizeof(u64))
 				return -EINVAL;
+			reg_num = array_index_nospec(reg_num,
+					ARRAY_SIZE(cntx->fp.d.f));
 			reg_val = &cntx->fp.d.f[reg_num];
 		} else
 			return -ENOENT;
@@ -138,9 +143,11 @@ int kvm_riscv_vcpu_set_reg_fp(struct kvm_vcpu *vcpu,
 		if (reg_num == KVM_REG_RISCV_FP_F_REG(fcsr))
 			reg_val = &cntx->fp.f.fcsr;
 		else if ((KVM_REG_RISCV_FP_F_REG(f[0]) <= reg_num) &&
-			  reg_num <= KVM_REG_RISCV_FP_F_REG(f[31]))
+			  reg_num <= KVM_REG_RISCV_FP_F_REG(f[31])) {
+			reg_num = array_index_nospec(reg_num,
+					ARRAY_SIZE(cntx->fp.f.f));
 			reg_val = &cntx->fp.f.f[reg_num];
-		else
+		} else
 			return -ENOENT;
 	} else if ((rtype == KVM_REG_RISCV_FP_D) &&
 		   riscv_isa_extension_available(vcpu->arch.isa, d)) {
@@ -152,6 +159,8 @@ int kvm_riscv_vcpu_set_reg_fp(struct kvm_vcpu *vcpu,
 			   reg_num <= KVM_REG_RISCV_FP_D_REG(f[31])) {
 			if (KVM_REG_SIZE(reg->id) != sizeof(u64))
 				return -EINVAL;
+			reg_num = array_index_nospec(reg_num,
+					ARRAY_SIZE(cntx->fp.d.f));
 			reg_val = &cntx->fp.d.f[reg_num];
 		} else
 			return -ENOENT;