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Message-ID: <20251108013601.902918-1-seanjc@google.com>
Date: Fri, 7 Nov 2025 17:36:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Chao Gao <chao.gao@...el.com>
Subject: [PATCH] KVM: x86: Allocate/free user_return_msrs at kvm.ko
(un)loading time
From: Chao Gao <chao.gao@...el.com>
Move user_return_msrs allocation/free from vendor modules (kvm-intel.ko and
kvm-amd.ko) (un)loading time to kvm.ko's to make it less risky to access
user_return_msrs in kvm.ko. Tying the lifetime of user_return_msrs to
vendor modules makes every access to user_return_msrs prone to
use-after-free issues as vendor modules may be unloaded at any time.
Opportunistically turn the per-CPU variable into full structs, as there's
no practical difference between statically allocating the memory and
allocating it unconditionally during module_init().
Zero out kvm_nr_uret_msrs on vendor module exit to further minimize the
chances of consuming stale data, and WARN on vendor module load if KVM
thinks there are existing user-return MSRs.
Note! The user-return MSRs also need to be "destroyed" if
ops->hardware_setup() fails, as both SVM and VMX expect common KVM to
clean up (because common code, not vendor code, is responsible for
kvm_nr_uret_msrs).
Signed-off-by: Chao Gao <chao.gao@...el.com>
Co-developed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/kvm/x86.c | 46 ++++++++++++++++------------------------------
1 file changed, 16 insertions(+), 30 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9c2e28028c2b..24dba35f3217 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -212,7 +212,7 @@ struct kvm_user_return_msrs {
u32 __read_mostly kvm_nr_uret_msrs;
EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_nr_uret_msrs);
static u32 __read_mostly kvm_uret_msrs_list[KVM_MAX_NR_USER_RETURN_MSRS];
-static struct kvm_user_return_msrs __percpu *user_return_msrs;
+static DEFINE_PER_CPU(struct kvm_user_return_msrs, user_return_msrs);
#define KVM_SUPPORTED_XCR0 (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
@@ -575,25 +575,14 @@ static inline void kvm_async_pf_hash_reset(struct kvm_vcpu *vcpu)
vcpu->arch.apf.gfns[i] = ~0;
}
-static int kvm_init_user_return_msrs(void)
+static void kvm_destroy_user_return_msrs(void)
{
- user_return_msrs = alloc_percpu(struct kvm_user_return_msrs);
- if (!user_return_msrs) {
- pr_err("failed to allocate percpu user_return_msrs\n");
- return -ENOMEM;
- }
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ WARN_ON_ONCE(per_cpu(user_return_msrs, cpu).registered);
+
kvm_nr_uret_msrs = 0;
- return 0;
-}
-
-static void kvm_free_user_return_msrs(void)
-{
- int cpu;
-
- for_each_possible_cpu(cpu)
- WARN_ON_ONCE(per_cpu_ptr(user_return_msrs, cpu)->registered);
-
- free_percpu(user_return_msrs);
}
static void kvm_on_user_return(struct user_return_notifier *urn)
@@ -656,7 +645,7 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_find_user_return_msr);
static void kvm_user_return_msr_cpu_online(void)
{
- struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
+ struct kvm_user_return_msrs *msrs = this_cpu_ptr(&user_return_msrs);
u64 value;
int i;
@@ -678,7 +667,7 @@ static void kvm_user_return_register_notifier(struct kvm_user_return_msrs *msrs)
int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
{
- struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
+ struct kvm_user_return_msrs *msrs = this_cpu_ptr(&user_return_msrs);
int err;
value = (value & mask) | (msrs->values[slot].host & ~mask);
@@ -696,13 +685,13 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_set_user_return_msr);
u64 kvm_get_user_return_msr(unsigned int slot)
{
- return this_cpu_ptr(user_return_msrs)->values[slot].curr;
+ return this_cpu_ptr(&user_return_msrs)->values[slot].curr;
}
EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_get_user_return_msr);
static void drop_user_return_notifiers(void)
{
- struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
+ struct kvm_user_return_msrs *msrs = this_cpu_ptr(&user_return_msrs);
if (msrs->registered)
kvm_on_user_return(&msrs->urn);
@@ -10077,13 +10066,9 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
return -ENOMEM;
}
- r = kvm_init_user_return_msrs();
- if (r)
- goto out_free_x86_emulator_cache;
-
r = kvm_mmu_vendor_module_init();
if (r)
- goto out_free_percpu;
+ goto out_free_x86_emulator_cache;
kvm_caps.supported_vm_types = BIT(KVM_X86_DEFAULT_VM);
kvm_caps.supported_mce_cap = MCG_CTL_P | MCG_SER_P;
@@ -10108,6 +10093,8 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
rdmsrq(MSR_IA32_ARCH_CAPABILITIES, kvm_host.arch_capabilities);
+ WARN_ON_ONCE(kvm_nr_uret_msrs);
+
r = ops->hardware_setup();
if (r != 0)
goto out_mmu_exit;
@@ -10180,9 +10167,8 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
kvm_x86_ops.enable_virtualization_cpu = NULL;
kvm_x86_call(hardware_unsetup)();
out_mmu_exit:
+ kvm_destroy_user_return_msrs();
kvm_mmu_vendor_module_exit();
-out_free_percpu:
- kvm_free_user_return_msrs();
out_free_x86_emulator_cache:
kmem_cache_destroy(x86_emulator_cache);
return r;
@@ -10210,8 +10196,8 @@ void kvm_x86_vendor_exit(void)
cancel_work_sync(&pvclock_gtod_work);
#endif
kvm_x86_call(hardware_unsetup)();
+ kvm_destroy_user_return_msrs();
kvm_mmu_vendor_module_exit();
- kvm_free_user_return_msrs();
kmem_cache_destroy(x86_emulator_cache);
#ifdef CONFIG_KVM_XEN
static_key_deferred_flush(&kvm_xen_enabled);
base-commit: 9052f4f6c539ea1fb7b282a34e6bb33154ce0b63
--
2.51.2.1041.gc1ab5b90ca-goog
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