KVM: x86: Fix KVM_CAP_SYNC_REGS's sync_regs() TOCTOU issues

In a spirit of using a sledgehammer to crack a nut, make sync_regs() feed
__set_sregs() and kvm_vcpu_ioctl_x86_set_vcpu_events() with kernel's own
copy of data.

Both __set_sregs() and kvm_vcpu_ioctl_x86_set_vcpu_events() assume they
have exclusive rights to structs they operate on. While this is true when
coming from an ioctl handler (caller makes a local copy of user's data),
sync_regs() breaks this contract; a pointer to a user-modifiable memory
(vcpu->run->s.regs) is provided. This can lead to a situation when incoming
data is checked and/or sanitized only to be re-set by a user thread running
in parallel.
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Fixes: 01643c51 ("KVM: x86: KVM_CAP_SYNC_REGS")
Link: https://lore.kernel.org/r/20230728001606.2275586-2-mhal@rbox.coSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -11777,15 +11777,22 @@ static int sync_regs(struct kvm_vcpu *vcpu)
 		__set_regs(vcpu, &vcpu->run->s.regs.regs);
 		vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_REGS;
 	}
+
 	if (vcpu->run->kvm_dirty_regs & KVM_SYNC_X86_SREGS) {
-		if (__set_sregs(vcpu, &vcpu->run->s.regs.sregs))
+		struct kvm_sregs sregs = vcpu->run->s.regs.sregs;
+
+		if (__set_sregs(vcpu, &sregs))
 			return -EINVAL;
+
 		vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_SREGS;
 	}
+
 	if (vcpu->run->kvm_dirty_regs & KVM_SYNC_X86_EVENTS) {
-		if (kvm_vcpu_ioctl_x86_set_vcpu_events(
-				vcpu, &vcpu->run->s.regs.events))
+		struct kvm_vcpu_events events = vcpu->run->s.regs.events;
+
+		if (kvm_vcpu_ioctl_x86_set_vcpu_events(vcpu, &events))
 			return -EINVAL;
+
 		vcpu->run->kvm_dirty_regs &= ~KVM_SYNC_X86_EVENTS;
 	}