linux-kernel - [PATCH v6 1/2] RISC-V: KVM: Validate SBI STA shmem alignment in kvm_sbi_ext_sta_set

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Message-Id: <20260205010502.2554381-2-xujiakai2025@iscas.ac.cn>
Date: Thu,  5 Feb 2026 01:05:01 +0000
From: Jiakai Xu <xujiakai2025@...as.ac.cn>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	kvm-riscv@...ts.infradead.org,
	linux-riscv@...ts.infradead.org,
	linux-kselftest@...r.kernel.org
Cc: Anup Patel <anup@...infault.org>,
	Atish Patra <atish.patra@...ux.dev>,
	Paul Walmsley <pjw@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>,
	Andrew Jones <ajones@...tanamicro.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	Jiakai Xu <xujiakai2025@...as.ac.cn>,
	Jiakai Xu <jiakaiPeanut@...il.com>
Subject: [PATCH v6 1/2] RISC-V: KVM: Validate SBI STA shmem alignment in  kvm_sbi_ext_sta_set_reg()

The RISC-V SBI Steal-Time Accounting (STA) extension requires the shared
memory physical address to be 64-byte aligned, or set to all-ones to
explicitly disable steal-time accounting.

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 computed shared memory GPA at the
KVM_SET_ONE_REG boundary. A temporary GPA is constructed and checked
before committing it to vcpu->arch.sta.shmem. The validation allows
either a 64-byte aligned GPA or INVALID_GPA (all-ones), which disables
STA as defined by the SBI specification.

This prevents invalid userspace state from reaching runtime code paths
that assume SBI STA invariants and avoids unexpected WARN_ON behavior.

Fixes: f61ce890b1f074 ("RISC-V: KVM: Add support for SBI STA registers")
Signed-off-by: Jiakai Xu <xujiakai2025@...as.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@...il.com>
---
V5 -> V6: Initialized new_shmem to INVALID_GPA as suggested.
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..3b834709b429f 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 = INVALID_GPA;

 	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 != INVALID_GPA && !IS_ALIGNED(new_shmem, 64))
+		return -EINVAL;
+
+	vcpu->arch.sta.shmem = new_shmem;
+
 	return 0;
 }

-- 
2.34.1