RISC-V: KVM: Synchronize hrtimer callback during teardown

Overview

In the Linux kernel, the following vulnerability has been resolved:

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.

Affected

• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < 9bdbae64ae0dab93041ebc57ec4e93def793fc2d
• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < 2af03b606aadaaeb386658ef798c79bdcdef87c1
• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < e66aced74f0122380603c9910a71d77618b28f0e
• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < ea91f05a55b807ead3166546a2c75f4185ca645f
• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < 3cc6184bd5920c9685d87f4e260568feba08650d
• Linux >= 3a9f66cb25e18a3eeca36c08d9f823a35b3ddc22 < aaad136d56d91252517272b68cd533e5714698d5
• Linux 5.16

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.