KVM: SVM: Don't BUG if userspace injects a soft interrupt with GIF=0

Don't BUG/WARN on interrupt injection due to GIF being cleared if the
injected event is a soft interrupt, which are not actually IRQs and thus
not subject to IRQ blocking conditions.  KVM doesn't currently use event
injection to handle incomplete soft interrupts, but it's trivial for
userspace to force the situation via KVM_SET_VCPU_EVENTS.

Opportunistically downgrade the BUG_ON() to WARN_ON(), there's no need to
bring down the whole host just because there might be some issue with
respect to guest GIF handling in KVM, or as evidenced here, an egregious
oversight with respect to KVM's uAPI.

  kernel BUG at arch/x86/kvm/svm/svm.c:3386!
  invalid opcode: 0000 [#1] SMP
  CPU: 15 PID: 926 Comm: smm_test Not tainted 5.17.0-rc3+ torvalds#264
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:svm_inject_irq+0xab/0xb0 [kvm_amd]
  Code: <0f> 0b 0f 1f 00 0f 1f 44 00 00 80 3d ac b3 01 00 00 55 48 89 f5 53
  RSP: 0018:ffffc90000b37d88 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff88810a234ac0 RCX: 0000000000000006
  RDX: 0000000000000000 RSI: ffffc90000b37df7 RDI: ffff88810a234ac0
  RBP: ffffc90000b37df7 R08: ffff88810a1fa410 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  R13: ffff888109571000 R14: ffff88810a234ac0 R15: 0000000000000000
  FS:  0000000001821380(0000) GS:ffff88846fdc0000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f74fc550008 CR3: 000000010a6fe000 CR4: 0000000000350ea0
  Call Trace:
   <TASK>
   inject_pending_event+0x2f7/0x4c0 [kvm]
   kvm_arch_vcpu_ioctl_run+0x791/0x17a0 [kvm]
   kvm_vcpu_ioctl+0x26d/0x650 [kvm]
   __x64_sys_ioctl+0x82/0xb0
   do_syscall_64+0x3b/0xc0
   entry_SYSCALL_64_after_hwframe+0x44/0xae
   </TASK>

Fixes: 219b65d ("KVM: SVM: Improve nested interrupt injection")
Cc: [email protected]
Signed-off-by: Maciej S. Szmigiero <[email protected]>
Co-Developed-by: Sean Christopherson <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>

arch/x86/kvm/svm/svm.c

@@ -3383,7 +3383,7 @@ static void svm_inject_irq(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	BUG_ON(!(gif_set(svm)));
+	WARN_ON(!vcpu->arch.interrupt.soft && !gif_set(svm));
 
 	trace_kvm_inj_virq(vcpu->arch.interrupt.nr);
 	++vcpu->stat.irq_injections;