[PATCH v5] RISC-V: KVM: Validate SBI STA shmem alignment in kvm_sbi_ext_sta_set_reg()

[Jiakai Xu]

The RISC-V SBI Steal-Time Accounting (STA) extension requires the shared
memory physical address to be 64-byte aligned, and the shared memory size
to be at least 64 bytes.

KVM exposes the SBI STA shared memory configuration to userspace via
KVM_SET_ONE_REG. However, the current implementation of
kvm_sbi_ext_sta_set_reg() does not validate the alignment of the configured
shared memory address. As a result, userspace can install a misaligned
shared memory address that violates the SBI specification.

Such an invalid configuration may later reach runtime code paths that
assume a valid and properly aligned shared memory region. In particular,
KVM_RUN can trigger the following WARN_ON in
kvm_riscv_vcpu_record_steal_time():

  WARNING: arch/riscv/kvm/vcpu_sbi_sta.c:49 at
  kvm_riscv_vcpu_record_steal_time

WARN_ON paths are not expected to be reachable during normal runtime
execution, and may result in a kernel panic when panic_on_warn is enabled.

Fix this by validating the shared memory alignment at the
KVM_SET_ONE_REG boundary and rejecting misaligned configurations with
-EINVAL. The validation is performed on a temporary computed address and
only committed to vcpu->arch.sta.shmem once it is known to be valid, 
similar to the existing logic in kvm_sbi_sta_steal_time_set_shmem() and
kvm_sbi_ext_sta_handler().

With this change, invalid userspace state is rejected early and cannot
reach runtime code paths that rely on the SBI specification invariants.

A reproducer triggering the WARN_ON and the complete kernel log are
available at: https://github.com/j1akai/temp/tree/main/20260124

Fixes: f61ce890b1f074 ("RISC-V: KVM: Add support for SBI STA registers")
Signed-off-by: Jiakai Xu <xujiakai2025 at iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut at gmail.com>
---
V4 -> V5: Added parentheses to function name in subject.
V3 -> V4: Declared new_shmem at the top of kvm_sbi_ext_sta_set_reg().
          Initialized new_shmem to 0 instead of vcpu->arch.sta.shmem.
          Added blank lines per review feedback.
V2 -> V3: Added parentheses to function name in subject.
V1 -> V2: Added Fixes tag.

---
 arch/riscv/kvm/vcpu_sbi_sta.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kvm/vcpu_sbi_sta.c b/arch/riscv/kvm/vcpu_sbi_sta.c
index afa0545c3bcfc..bb13aa8eab7ee 100644
--- a/arch/riscv/kvm/vcpu_sbi_sta.c
+++ b/arch/riscv/kvm/vcpu_sbi_sta.c
@@ -181,6 +181,7 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
 				   unsigned long reg_size, const void *reg_val)
 {
 	unsigned long value;
+	gpa_t new_shmem = 0;
 
 	if (reg_size != sizeof(unsigned long))
 		return -EINVAL;
@@ -191,18 +192,18 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
 		if (IS_ENABLED(CONFIG_32BIT)) {
 			gpa_t hi = upper_32_bits(vcpu->arch.sta.shmem);
 
-			vcpu->arch.sta.shmem = value;
-			vcpu->arch.sta.shmem |= hi << 32;
+			new_shmem = value;
+			new_shmem |= hi << 32;
 		} else {
-			vcpu->arch.sta.shmem = value;
+			new_shmem = value;
 		}
 		break;
 	case KVM_REG_RISCV_SBI_STA_REG(shmem_hi):
 		if (IS_ENABLED(CONFIG_32BIT)) {
 			gpa_t lo = lower_32_bits(vcpu->arch.sta.shmem);
 
-			vcpu->arch.sta.shmem = ((gpa_t)value << 32);
-			vcpu->arch.sta.shmem |= lo;
+			new_shmem = ((gpa_t)value << 32);
+			new_shmem |= lo;
 		} else if (value != 0) {
 			return -EINVAL;
 		}
@@ -211,6 +212,11 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
 		return -ENOENT;
 	}
 
+	if (new_shmem && !IS_ALIGNED(new_shmem, 64))
+		return -EINVAL;
+
+	vcpu->arch.sta.shmem = new_shmem;
+
 	return 0;
 }
 
-- 
2.34.1