RISC-V: KVM: Validate SBI STA shmem alignment in kvm_sbi_ext_sta_set_reg()

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@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260303010859.1763177-2-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>