Re: [PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler

[Liran Alon]

On 22/11/17 10:45, Liran Alon wrote:
> On 22/11/17 09:56, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> >
> > Reported by syzkaller:
> >
> >     ------------[ cut here ]------------
> >     WARNING: CPU: 5 PID: 2939 at arch/x86/kvm/vmx.c:3844
> > free_loaded_vmcs+0x77/0x80 [kvm_intel]
> >     CPU: 5 PID: 2939 Comm: repro Not tainted 4.14.0+ #26
> >     RIP: 0010:free_loaded_vmcs+0x77/0x80 [kvm_intel]
> >     Call Trace:
> >      vmx_free_vcpu+0xda/0x130 [kvm_intel]
> >      kvm_arch_destroy_vm+0x192/0x290 [kvm]
> >      kvm_put_kvm+0x262/0x560 [kvm]
> >      kvm_vm_release+0x2c/0x30 [kvm]
> >      __fput+0x190/0x370
> >      task_work_run+0xa1/0xd0
> >      do_exit+0x4d2/0x13e0
> >      do_group_exit+0x89/0x140
> >      get_signal+0x318/0xb80
> >      do_signal+0x8c/0xb40
> >      exit_to_usermode_loop+0xe4/0x140
> >      syscall_return_slowpath+0x206/0x230
> >      entry_SYSCALL_64_fastpath+0x98/0x9a
> >
> > The syzkaller testcase will execute VMXON/VMLAUCH instructions, so the
> > vmx->nested stuff is populated, it will also issue KVM_SMI ioctl.
> > However,
> > the testcase is just a simple c program and not be lauched by something
> > like seabios which implements smi_handler. Commit 05cade71cf (KVM: nSVM:
> > fix SMI injection in guest mode) gets out of guest mode and set
> > nested.vmxon
> > to false for the duration of SMM according to SDM 34.14.1 "leave VMX
> > operation" upon entering SMM. We can't alloc/free the vmx->nested stuff
> > each time when entering/exiting SMM since it will induce more
> > overhead. So
> > the function vmx_pre_enter_smm() marks nested.vmxon false even if
> > vmx->nested
> > stuff is still populated. What it expected is em_rsm() can mark
> > nested.vmxon
> > to be true again. However, the smi_handler/rsm will not execute since
> > there
> > is no something like seabios in this scenario. The function free_nested()
> > fails to free the vmx->nested stuff since the vmx->nested.vmxon is false
> > which results in the above warning.
> >
> > This patch fixes it by also considering the no SMI handler case, luckily
> > vmx->nested.smm.vmxon is marked according to the value of
> > vmx->nested.vmxon
> > in vmx_pre_enter_smm(), we can take advantage of it and free vmx->nested
> > stuff when L1 goes down.
> >
> > Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Fixes: 05cade71cf (KVM: nSVM: fix SMI injection in guest mode)
> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index dccc0f7..ed22425 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -7372,7 +7372,7 @@ static inline void nested_release_vmcs12(struct
> > vcpu_vmx *vmx)
> >    */
> >   static void free_nested(struct vcpu_vmx *vmx)
> >   {
> > -    if (!vmx->nested.vmxon)
> > +    if (!vmx->nested.vmxon && !vmx->nested.smm.vmxon)
> >           return;
> >
> >       vmx->nested.vmxon = false;
> Funny bug. Great analysis.
> Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
Actually, I would add one more thing to patch:
I think we should also set "vmx->nested.smm.vmxon = false;" after 
"vmx->nested.vmxon = false;" to correctlyhandle the case VMXOFF is 
executed from SMI handler. Otherwise, when SMI handler executes RSM, we 
will reach vmx_pre_leave_smm() which will set again "vmx->nested.vmxon = 
true;" which I think shouldn't happen.

Regards,
-Liran