[PATCH v3 02/18] x86/reboot: Harden virtualization hooks for emergency reboot

Sean Christopherson
2023-05-12T23:50:45Z

Provide dedicated helpers to (un)register virt hooks used during an
emergency crash/reboot, and WARN if there is an attempt to overwrite
the registered callback, or an attempt to do an unpaired unregister.

Opportunsitically use rcu_assign_pointer() instead of RCU_INIT_POINTER(),
mainly so that the set/unset paths are more symmetrical, but also because
any performance gains from using RCU_INIT_POINTER() are meaningless for
this code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/reboot.h | 5 +++--
arch/x86/kernel/reboot.c | 30 ++++++++++++++++++++++++------
arch/x86/kvm/vmx/vmx.c | 6 ++----
3 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
index 2551baec927d..d9a38d379d18 100644
--- a/arch/x86/include/asm/reboot.h
+++ b/arch/x86/include/asm/reboot.h
@@ -25,8 +25,9 @@ void __noreturn machine_real_restart(unsigned int type);
#define MRR_BIOS 0
#define MRR_APM 1

-typedef void crash_vmclear_fn(void);
-extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
+typedef void (cpu_emergency_virt_cb)(void);
+void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
+void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
void cpu_emergency_disable_virtualization(void);

typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 299b970e5f82..739e09527dbb 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -794,17 +794,35 @@ void machine_crash_shutdown(struct pt_regs *regs)
*
* protected by rcu.
*/
-crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
-EXPORT_SYMBOL_GPL(crash_vmclear_loaded_vmcss);
+static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
+
+void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback)
+{
+ if (WARN_ON_ONCE(rcu_access_pointer(cpu_emergency_virt_callback)))
+ return;
+
+ rcu_assign_pointer(cpu_emergency_virt_callback, callback);
+}
+EXPORT_SYMBOL_GPL(cpu_emergency_register_virt_callback);
+
+void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback)
+{
+ if (WARN_ON_ONCE(rcu_access_pointer(cpu_emergency_virt_callback) != callback))
+ return;
+
+ rcu_assign_pointer(cpu_emergency_virt_callback, NULL);
+ synchronize_rcu();
+}
+EXPORT_SYMBOL_GPL(cpu_emergency_unregister_virt_callback);

static inline void cpu_crash_vmclear_loaded_vmcss(void)
{
- crash_vmclear_fn *do_vmclear_operation = NULL;
+ cpu_emergency_virt_cb *callback;

rcu_read_lock();
- do_vmclear_operation = rcu_dereference(crash_vmclear_loaded_vmcss);
- if (do_vmclear_operation)
- do_vmclear_operation();
+ callback = rcu_dereference(cpu_emergency_virt_callback);
+ if (callback)
+ callback();
rcu_read_unlock();
}

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 317f72baf0c3..fc9cdb4114cc 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8547,8 +8547,7 @@ static void __vmx_exit(void)
{
allow_smaller_maxphyaddr = false;

- RCU_INIT_POINTER(crash_vmclear_loaded_vmcss, NULL);
- synchronize_rcu();
+ cpu_emergency_unregister_virt_callback(crash_vmclear_local_loaded_vmcss);

vmx_cleanup_l1d_flush();
}
@@ -8598,8 +8597,7 @@ static int __init vmx_init(void)
pi_init_cpu(cpu);
}

- rcu_assign_pointer(crash_vmclear_loaded_vmcss,
- crash_vmclear_local_loaded_vmcss);
+ cpu_emergency_register_virt_callback(crash_vmclear_local_loaded_vmcss);

vmx_check_vmcs12_offsets();

--
2.40.1.606.ga4b1b128d6-goog