arch/riscv/kvm/vcpu_sbi_sta.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-)
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
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
---
arch/riscv/kvm/vcpu_sbi_sta.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/arch/riscv/kvm/vcpu_sbi_sta.c b/arch/riscv/kvm/vcpu_sbi_sta.c
index afa0545c3bcfc..7dfe671c42eaa 100644
--- a/arch/riscv/kvm/vcpu_sbi_sta.c
+++ b/arch/riscv/kvm/vcpu_sbi_sta.c
@@ -186,23 +186,25 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
return -EINVAL;
value = *(const unsigned long *)reg_val;
+ gpa_t new_shmem = vcpu->arch.sta.shmem;
+
switch (reg_num) {
case KVM_REG_RISCV_SBI_STA_REG(shmem_lo):
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;
}
@@ -210,7 +212,10 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
default:
return -ENOENT;
}
+ if (new_shmem && !IS_ALIGNED(new_shmem, 64))
+ return -EINVAL;
+ vcpu->arch.sta.shmem = new_shmem;
return 0;
}
--
2.34.1On Sat, Jan 24, 2026 at 02:20:42AM +0000, Jiakai Xu wrote:
...
> diff --git a/arch/riscv/kvm/vcpu_sbi_sta.c b/arch/riscv/kvm/vcpu_sbi_sta.c
> index afa0545c3bcfc..7dfe671c42eaa 100644
> --- a/arch/riscv/kvm/vcpu_sbi_sta.c
> +++ b/arch/riscv/kvm/vcpu_sbi_sta.c
> @@ -186,23 +186,25 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
> return -EINVAL;
> value = *(const unsigned long *)reg_val;
>
> + gpa_t new_shmem = vcpu->arch.sta.shmem;
Please declare new_shmem at the top of the function and there's no
need to initialize it to vcpu->arch.sta.shmem. Actually it appears you
meant to initialize it to NULL, based on the 'if (new_shmem ...)' check
below.
> +
> switch (reg_num) {
> case KVM_REG_RISCV_SBI_STA_REG(shmem_lo):
> 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;
> }
> @@ -210,7 +212,10 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
> default:
> return -ENOENT;
> }
Please add a blank line here.
> + if (new_shmem && !IS_ALIGNED(new_shmem, 64))
> + return -EINVAL;
>
> + vcpu->arch.sta.shmem = new_shmem;
And another blank line here.
> return 0;
> }
>
> --
> 2.34.1
>
Thanks,
drew
Thanks for the review.
I’ll send a v2 shortly with these changes.
Thanks,
Jiakai
On Sat, Jan 31, 2026 at 4:48 AM Andrew Jones
<andrew.jones@oss.qualcomm.com> wrote:
>
> On Sat, Jan 24, 2026 at 02:20:42AM +0000, Jiakai Xu wrote:
> ...
> > diff --git a/arch/riscv/kvm/vcpu_sbi_sta.c b/arch/riscv/kvm/vcpu_sbi_sta.c
> > index afa0545c3bcfc..7dfe671c42eaa 100644
> > --- a/arch/riscv/kvm/vcpu_sbi_sta.c
> > +++ b/arch/riscv/kvm/vcpu_sbi_sta.c
> > @@ -186,23 +186,25 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
> > return -EINVAL;
> > value = *(const unsigned long *)reg_val;
> >
> > + gpa_t new_shmem = vcpu->arch.sta.shmem;
>
> Please declare new_shmem at the top of the function and there's no
> need to initialize it to vcpu->arch.sta.shmem. Actually it appears you
> meant to initialize it to NULL, based on the 'if (new_shmem ...)' check
> below.
>
> > +
> > switch (reg_num) {
> > case KVM_REG_RISCV_SBI_STA_REG(shmem_lo):
> > 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;
> > }
> > @@ -210,7 +212,10 @@ static int kvm_sbi_ext_sta_set_reg(struct kvm_vcpu *vcpu, unsigned long reg_num,
> > default:
> > return -ENOENT;
> > }
>
> Please add a blank line here.
>
> > + if (new_shmem && !IS_ALIGNED(new_shmem, 64))
> > + return -EINVAL;
> >
> > + vcpu->arch.sta.shmem = new_shmem;
>
> And another blank line here.
>
> > return 0;
> > }
> >
> > --
> > 2.34.1
> >
>
> Thanks,
> drew
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