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

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Message-ID: <zudnvpsirxbfjwh6plmj5utumgvj4tmzdddegneu3f22vfxdz3@dza2x6bur6pb>
Date: Thu, 5 Feb 2026 09:29:53 -0600
From: Andrew Jones <andrew.jones@....qualcomm.com>
To: Jiakai Xu <xujiakai2025@...as.ac.cn>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-kselftest@...r.kernel.org, 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 <jiakaiPeanut@...il.com>
Subject: Re: [PATCH v6 1/2] RISC-V: KVM: Validate SBI STA shmem alignment in
  kvm_sbi_ext_sta_set_reg()

On Thu, Feb 05, 2026 at 01:05:01AM +0000, Jiakai Xu wrote:
> 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;
>  }
>

Reviewed-by: Andrew Jones <andrew.jones@....qualcomm.com>