RISC-V: KVM: Synchronize hrtimer callback during teardown

The non-Sstc hrtimer callback clears next_set before its final uses of
the enclosing vCPU.  If teardown observes next_set as false while the
callback is still running, kvm_riscv_vcpu_timer_cancel() skips
hrtimer_cancel() and kvm_destroy_vcpus() can free the vCPU before the
callback enters kvm_riscv_vcpu_set_interrupt().

A guest can arm the timer with SBI TIME and request shutdown with SBI
legacy shutdown or SRST.  A VMM that honors KVM_EXIT_SYSTEM_EVENT and
destroys the VM supplies the teardown side of the race; no post-launch
host ioctl is needed to arm or request teardown.

On upstream master 62cc90241548, generic KASAN reported:

  BUG: KASAN: slab-use-after-free in do_raw_spin_lock
  Write of size 4 at addr ff60000005e58898

  kvm_riscv_vcpu_set_interrupt
  kvm_riscv_vcpu_hrtimer_expired
  __hrtimer_run_queues
  hrtimer_interrupt

The object was allocated by KVM_CREATE_VCPU and freed concurrently by:

  kvm_destroy_vcpus
  kvm_arch_destroy_vm
  kvm_destroy_vm
  __fput

For deterministic validation, I added mdelay(1000) immediately after
the existing next_set = false assignment.  This only widens the
existing post-clear callback window.  A no-delay trace build naturally
reached the callback-after-teardown-start/before-deinit ordering in 12
of 200 runs, but 1,500 stock-kernel stress iterations did not produce a
KASAN report, so natural reproduction is timing-sensitive.

Always invoke hrtimer_cancel() for an initialized timer.  Preserve the
existing -EINVAL result when the timer is no longer set, but only after
synchronizing with a running callback.

With this patch, hrtimer_cancel() blocked for the full widened callback
window before vCPU destruction.  KASAN reported no error in 100
fixed-and-widened runs or 200 fix-only timing-sweep runs.

Fixes: 3a9f66cb25e1 ("RISC-V: KVM: Add timer functionality")
Cc: stable@vger.kernel.org
Assisted-by: OpenAI:GPT-5.6
Signed-off-by: Myeonghun Pak <mhun512@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260731163550.46991-1-mhun512@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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