[PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Vitaly Kuznetsov]

If KVM wasn't used at all before we crash the cleanup procedure fails with
 BUG: unable to handle page fault for address: ffffffffffffffc8
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 23215067 P4D 23215067 PUD 23217067 PMD 0
 Oops: 0000 [#8] SMP PTI
 CPU: 0 PID: 3542 Comm: bash Kdump: loaded Tainted: G      D           5.6.0-rc2+ #823
 RIP: 0010:crash_vmclear_local_loaded_vmcss.cold+0x19/0x51 [kvm_intel]

The root cause is that loaded_vmcss_on_cpu list is not yet initialized,
we initialize it in hardware_enable() but this only happens when we start
a VM.

Previously, we used to have a bitmap with enabled CPUs and that was
preventing [masking] the issue.

Initialized loaded_vmcss_on_cpu list earlier, right before we assign
crash_vmclear_loaded_vmcss pointer. blocked_vcpu_on_cpu list and
blocked_vcpu_on_cpu_lock are moved altogether for consistency.

Fixes: 31603d4fc2bb ("KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3aba51d782e2..39a5dde12b79 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2257,10 +2257,6 @@ static int hardware_enable(void)
 	    !hv_get_vp_assist_page(cpu))
 		return -EFAULT;
 
-	INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
-	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
-	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
-
 	r = kvm_cpu_vmxon(phys_addr);
 	if (r)
 		return r;
@@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
 
 static int __init vmx_init(void)
 {
-	int r;
+	int r, cpu;
 
 #if IS_ENABLED(CONFIG_HYPERV)
 	/*
@@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
 		return r;
 	}
 
+	for_each_possible_cpu(cpu) {
+		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
+		INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
+		spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
+	}
+
 #ifdef CONFIG_KEXEC_CORE
 	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
 			   crash_vmclear_local_loaded_vmcss);
-- 
2.25.1

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Sean Christopherson]

On Wed, Apr 01, 2020 at 10:13:48AM +0200, Vitaly Kuznetsov wrote:
> If KVM wasn't used at all before we crash the cleanup procedure fails with
>  BUG: unable to handle page fault for address: ffffffffffffffc8
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 23215067 P4D 23215067 PUD 23217067 PMD 0
>  Oops: 0000 [#8] SMP PTI
>  CPU: 0 PID: 3542 Comm: bash Kdump: loaded Tainted: G      D           5.6.0-rc2+ #823
>  RIP: 0010:crash_vmclear_local_loaded_vmcss.cold+0x19/0x51 [kvm_intel]
> 
> The root cause is that loaded_vmcss_on_cpu list is not yet initialized,
> we initialize it in hardware_enable() but this only happens when we start
> a VM.
> 
> Previously, we used to have a bitmap with enabled CPUs and that was
> preventing [masking] the issue.
> 
> Initialized loaded_vmcss_on_cpu list earlier, right before we assign
> crash_vmclear_loaded_vmcss pointer. blocked_vcpu_on_cpu list and
> blocked_vcpu_on_cpu_lock are moved altogether for consistency.
> 
> Fixes: 31603d4fc2bb ("KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3aba51d782e2..39a5dde12b79 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
>  	    !hv_get_vp_assist_page(cpu))
>  		return -EFAULT;
>  
> -	INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> -	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> -	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> -
>  	r = kvm_cpu_vmxon(phys_addr);
>  	if (r)
>  		return r;
> @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
>  
>  static int __init vmx_init(void)
>  {
> -	int r;
> +	int r, cpu;
>  
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	/*
> @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
>  		return r;
>  	}
>  
> +	for_each_possible_cpu(cpu) {
> +		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +		INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> +		spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));

Hmm, part of me thinks the posted interrupt per_cpu variables should
continue to be initialized during hardware_enable().  But it's a small
part of me :-)

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

> +	}
> +
>  #ifdef CONFIG_KEXEC_CORE
>  	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
>  			   crash_vmclear_local_loaded_vmcss);
> -- 
> 2.25.1
> 

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Paolo Bonzini]

On 01/04/20 10:13, Vitaly Kuznetsov wrote:
> If KVM wasn't used at all before we crash the cleanup procedure fails with
>  BUG: unable to handle page fault for address: ffffffffffffffc8
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 23215067 P4D 23215067 PUD 23217067 PMD 0
>  Oops: 0000 [#8] SMP PTI
>  CPU: 0 PID: 3542 Comm: bash Kdump: loaded Tainted: G      D           5.6.0-rc2+ #823
>  RIP: 0010:crash_vmclear_local_loaded_vmcss.cold+0x19/0x51 [kvm_intel]
> 
> The root cause is that loaded_vmcss_on_cpu list is not yet initialized,
> we initialize it in hardware_enable() but this only happens when we start
> a VM.
> 
> Previously, we used to have a bitmap with enabled CPUs and that was
> preventing [masking] the issue.
> 
> Initialized loaded_vmcss_on_cpu list earlier, right before we assign
> crash_vmclear_loaded_vmcss pointer. blocked_vcpu_on_cpu list and
> blocked_vcpu_on_cpu_lock are moved altogether for consistency.
> 
> Fixes: 31603d4fc2bb ("KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3aba51d782e2..39a5dde12b79 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
>  	    !hv_get_vp_assist_page(cpu))
>  		return -EFAULT;
>  
> -	INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> -	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> -	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> -
>  	r = kvm_cpu_vmxon(phys_addr);
>  	if (r)
>  		return r;
> @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
>  
>  static int __init vmx_init(void)
>  {
> -	int r;
> +	int r, cpu;
>  
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	/*
> @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
>  		return r;
>  	}
>  
> +	for_each_possible_cpu(cpu) {
> +		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +		INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> +		spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> +	}
> +
>  #ifdef CONFIG_KEXEC_CORE
>  	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
>  			   crash_vmclear_local_loaded_vmcss);
> 

Queued, thanks.

Paolo

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Baoquan He]

On 04/01/20 at 10:13am, Vitaly Kuznetsov wrote:
> If KVM wasn't used at all before we crash the cleanup procedure fails with
>  BUG: unable to handle page fault for address: ffffffffffffffc8
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 23215067 P4D 23215067 PUD 23217067 PMD 0
>  Oops: 0000 [#8] SMP PTI
>  CPU: 0 PID: 3542 Comm: bash Kdump: loaded Tainted: G      D           5.6.0-rc2+ #823
>  RIP: 0010:crash_vmclear_local_loaded_vmcss.cold+0x19/0x51 [kvm_intel]
> 
> The root cause is that loaded_vmcss_on_cpu list is not yet initialized,
> we initialize it in hardware_enable() but this only happens when we start
> a VM.
> 
> Previously, we used to have a bitmap with enabled CPUs and that was
> preventing [masking] the issue.
> 
> Initialized loaded_vmcss_on_cpu list earlier, right before we assign
> crash_vmclear_loaded_vmcss pointer. blocked_vcpu_on_cpu list and
> blocked_vcpu_on_cpu_lock are moved altogether for consistency.
> 
> Fixes: 31603d4fc2bb ("KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Kdump kernel hang can be reproduced on a bare metal machine of Intel always,
issue disappeared with this patch applied. Feel free to add:

Tested-by: Baoquan He <bhe@redhat.com>

> ---
>  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3aba51d782e2..39a5dde12b79 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
>  	    !hv_get_vp_assist_page(cpu))
>  		return -EFAULT;
>  
> -	INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> -	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> -	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> -
>  	r = kvm_cpu_vmxon(phys_addr);
>  	if (r)
>  		return r;
> @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
>  
>  static int __init vmx_init(void)
>  {
> -	int r;
> +	int r, cpu;
>  
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	/*
> @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
>  		return r;
>  	}
>  
> +	for_each_possible_cpu(cpu) {
> +		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +		INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> +		spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> +	}
> +
>  #ifdef CONFIG_KEXEC_CORE
>  	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
>  			   crash_vmclear_local_loaded_vmcss);
> -- 
> 2.25.1
> 

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Jim Mattson]

On Wed, Apr 1, 2020 at 1:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> If KVM wasn't used at all before we crash the cleanup procedure fails with
>  BUG: unable to handle page fault for address: ffffffffffffffc8
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 23215067 P4D 23215067 PUD 23217067 PMD 0
>  Oops: 0000 [#8] SMP PTI
>  CPU: 0 PID: 3542 Comm: bash Kdump: loaded Tainted: G      D           5.6.0-rc2+ #823
>  RIP: 0010:crash_vmclear_local_loaded_vmcss.cold+0x19/0x51 [kvm_intel]
>
> The root cause is that loaded_vmcss_on_cpu list is not yet initialized,
> we initialize it in hardware_enable() but this only happens when we start
> a VM.
>
> Previously, we used to have a bitmap with enabled CPUs and that was
> preventing [masking] the issue.
>
> Initialized loaded_vmcss_on_cpu list earlier, right before we assign
> crash_vmclear_loaded_vmcss pointer. blocked_vcpu_on_cpu list and
> blocked_vcpu_on_cpu_lock are moved altogether for consistency.
>
> Fixes: 31603d4fc2bb ("KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 3aba51d782e2..39a5dde12b79 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
>             !hv_get_vp_assist_page(cpu))
>                 return -EFAULT;
>
> -       INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> -       INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> -       spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> -
>         r = kvm_cpu_vmxon(phys_addr);
>         if (r)
>                 return r;
> @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
>
>  static int __init vmx_init(void)
>  {
> -       int r;
> +       int r, cpu;
>
>  #if IS_ENABLED(CONFIG_HYPERV)
>         /*
> @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
>                 return r;
>         }
>
> +       for_each_possible_cpu(cpu) {
> +               INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +               INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> +               spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> +       }

Just above this chunk, we have:

r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
if (r) {
        vmx_exit();
...

If we take that early exit, because vmx_setup_l1d_flush() fails, we
won't initialize loaded_vmcss_on_cpu. However, vmx_exit() calls
kvm_exit(), which calls on_each_cpu(hardware_disable_nolock, NULL, 1).
Hardware_disable_nolock() then calls kvm_arch_hardware_disable(),
which calls kvm_x86_ops.hardware_disable() [the vmx.c
hardware_disable()], which calls vmclear_local_loaded_vmcss().

I believe that vmclear_local_loaded_vmcss() will then try to
dereference a NULL pointer, since per_cpu(loaded_vmcss_on_cpu, cpu) is
uninitialzed.

>  #ifdef CONFIG_KEXEC_CORE
>         rcu_assign_pointer(crash_vmclear_loaded_vmcss,
>                            crash_vmclear_local_loaded_vmcss);
> --
> 2.25.1
>

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Sean Christopherson]

On Thu, Aug 20, 2020 at 01:08:22PM -0700, Jim Mattson wrote:
> On Wed, Apr 1, 2020 at 1:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 3aba51d782e2..39a5dde12b79 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
> >             !hv_get_vp_assist_page(cpu))
> >                 return -EFAULT;
> >
> > -       INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > -       INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > -       spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > -
> >         r = kvm_cpu_vmxon(phys_addr);
> >         if (r)
> >                 return r;
> > @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
> >
> >  static int __init vmx_init(void)
> >  {
> > -       int r;
> > +       int r, cpu;
> >
> >  #if IS_ENABLED(CONFIG_HYPERV)
> >         /*
> > @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
> >                 return r;
> >         }
> >
> > +       for_each_possible_cpu(cpu) {
> > +               INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > +               INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > +               spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > +       }
> 
> Just above this chunk, we have:
> 
> r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
> if (r) {
>         vmx_exit();
> ...
> 
> If we take that early exit, because vmx_setup_l1d_flush() fails, we
> won't initialize loaded_vmcss_on_cpu. However, vmx_exit() calls
> kvm_exit(), which calls on_each_cpu(hardware_disable_nolock, NULL, 1).
> Hardware_disable_nolock() then calls kvm_arch_hardware_disable(),
> which calls kvm_x86_ops.hardware_disable() [the vmx.c
> hardware_disable()], which calls vmclear_local_loaded_vmcss().
> 
> I believe that vmclear_local_loaded_vmcss() will then try to
> dereference a NULL pointer, since per_cpu(loaded_vmcss_on_cpu, cpu) is
> uninitialzed.

I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
finished.

> >  #ifdef CONFIG_KEXEC_CORE
> >         rcu_assign_pointer(crash_vmclear_loaded_vmcss,
> >                            crash_vmclear_local_loaded_vmcss);
> > --
> > 2.25.1
> >

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Jim Mattson]

On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Thu, Aug 20, 2020 at 01:08:22PM -0700, Jim Mattson wrote:
> > On Wed, Apr 1, 2020 at 1:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
> > >  1 file changed, 7 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 3aba51d782e2..39a5dde12b79 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
> > >             !hv_get_vp_assist_page(cpu))
> > >                 return -EFAULT;
> > >
> > > -       INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > > -       INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > > -       spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > > -
> > >         r = kvm_cpu_vmxon(phys_addr);
> > >         if (r)
> > >                 return r;
> > > @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
> > >
> > >  static int __init vmx_init(void)
> > >  {
> > > -       int r;
> > > +       int r, cpu;
> > >
> > >  #if IS_ENABLED(CONFIG_HYPERV)
> > >         /*
> > > @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
> > >                 return r;
> > >         }
> > >
> > > +       for_each_possible_cpu(cpu) {
> > > +               INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > > +               INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > > +               spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > > +       }
> >
> > Just above this chunk, we have:
> >
> > r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
> > if (r) {
> >         vmx_exit();
> > ...
> >
> > If we take that early exit, because vmx_setup_l1d_flush() fails, we
> > won't initialize loaded_vmcss_on_cpu. However, vmx_exit() calls
> > kvm_exit(), which calls on_each_cpu(hardware_disable_nolock, NULL, 1).
> > Hardware_disable_nolock() then calls kvm_arch_hardware_disable(),
> > which calls kvm_x86_ops.hardware_disable() [the vmx.c
> > hardware_disable()], which calls vmclear_local_loaded_vmcss().
> >
> > I believe that vmclear_local_loaded_vmcss() will then try to
> > dereference a NULL pointer, since per_cpu(loaded_vmcss_on_cpu, cpu) is
> > uninitialzed.
>
> I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
> pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
> impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
> finished.

Unless I'm missing something, there's no check for a non-zero
kvm_usage_count on this path. There is such a check in
hardware_disable_all_nolock(), but not in hardware_disable_nolock().

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Jim Mattson]

On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@google.com> wrote:
>
> On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > On Thu, Aug 20, 2020 at 01:08:22PM -0700, Jim Mattson wrote:
> > > On Wed, Apr 1, 2020 at 1:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > > > ---
> > > >  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
> > > >  1 file changed, 7 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > > index 3aba51d782e2..39a5dde12b79 100644
> > > > --- a/arch/x86/kvm/vmx/vmx.c
> > > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > > @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
> > > >             !hv_get_vp_assist_page(cpu))
> > > >                 return -EFAULT;
> > > >
> > > > -       INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > > > -       INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > > > -       spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > > > -
> > > >         r = kvm_cpu_vmxon(phys_addr);
> > > >         if (r)
> > > >                 return r;
> > > > @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
> > > >
> > > >  static int __init vmx_init(void)
> > > >  {
> > > > -       int r;
> > > > +       int r, cpu;
> > > >
> > > >  #if IS_ENABLED(CONFIG_HYPERV)
> > > >         /*
> > > > @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
> > > >                 return r;
> > > >         }
> > > >
> > > > +       for_each_possible_cpu(cpu) {
> > > > +               INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > > > +               INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > > > +               spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > > > +       }
> > >
> > > Just above this chunk, we have:
> > >
> > > r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
> > > if (r) {
> > >         vmx_exit();
> > > ...
> > >
> > > If we take that early exit, because vmx_setup_l1d_flush() fails, we
> > > won't initialize loaded_vmcss_on_cpu. However, vmx_exit() calls
> > > kvm_exit(), which calls on_each_cpu(hardware_disable_nolock, NULL, 1).
> > > Hardware_disable_nolock() then calls kvm_arch_hardware_disable(),
> > > which calls kvm_x86_ops.hardware_disable() [the vmx.c
> > > hardware_disable()], which calls vmclear_local_loaded_vmcss().
> > >
> > > I believe that vmclear_local_loaded_vmcss() will then try to
> > > dereference a NULL pointer, since per_cpu(loaded_vmcss_on_cpu, cpu) is
> > > uninitialzed.
> >
> > I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
> > pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
> > impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
> > finished.
>
> Unless I'm missing something, there's no check for a non-zero
> kvm_usage_count on this path. There is such a check in
> hardware_disable_all_nolock(), but not in hardware_disable_nolock().

However, cpus_hardware_enabled shouldn't have any bits set, so
everything's fine. Nothing to see here, after all.

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Sean Christopherson]

On Mon, Aug 24, 2020 at 03:45:26PM -0700, Jim Mattson wrote:
> On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@google.com> wrote:
> >
> > On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > > I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
> > > pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
> > > impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
> > > finished.
> >
> > Unless I'm missing something, there's no check for a non-zero
> > kvm_usage_count on this path. There is such a check in
> > hardware_disable_all_nolock(), but not in hardware_disable_nolock().
> 
> However, cpus_hardware_enabled shouldn't have any bits set, so
> everything's fine. Nothing to see here, after all.

Ugh, I forgot that hardware_disable_all_nolock() does a BUG_ON() instead of
bailing on !kvm_usage_count.

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Mon, Aug 24, 2020 at 03:45:26PM -0700, Jim Mattson wrote:
>> On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@google.com> wrote:
>> >
>> > On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
>> > <sean.j.christopherson@intel.com> wrote:
>> > > I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
>> > > pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
>> > > impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
>> > > finished.
>> >
>> > Unless I'm missing something, there's no check for a non-zero
>> > kvm_usage_count on this path. There is such a check in
>> > hardware_disable_all_nolock(), but not in hardware_disable_nolock().
>> 
>> However, cpus_hardware_enabled shouldn't have any bits set, so
>> everything's fine. Nothing to see here, after all.
>
> Ugh, I forgot that hardware_disable_all_nolock() does a BUG_ON() instead of
> bailing on !kvm_usage_count.

But we can't hit this BUG_ON(), right? I'm failing to see how
hardware_disable_all_nolock() can be reached with kvm_usage_count==0.

-- 
Vitaly

Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

[Sean Christopherson]

On Tue, Sep 01, 2020 at 12:36:40PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
> 
> > On Mon, Aug 24, 2020 at 03:45:26PM -0700, Jim Mattson wrote:
> >> On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@google.com> wrote:
> >> >
> >> > On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
> >> > <sean.j.christopherson@intel.com> wrote:
> >> > > I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
> >> > > pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
> >> > > impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
> >> > > finished.
> >> >
> >> > Unless I'm missing something, there's no check for a non-zero
> >> > kvm_usage_count on this path. There is such a check in
> >> > hardware_disable_all_nolock(), but not in hardware_disable_nolock().
> >> 
> >> However, cpus_hardware_enabled shouldn't have any bits set, so
> >> everything's fine. Nothing to see here, after all.
> >
> > Ugh, I forgot that hardware_disable_all_nolock() does a BUG_ON() instead of
> > bailing on !kvm_usage_count.
> 
> But we can't hit this BUG_ON(), right? I'm failing to see how
> hardware_disable_all_nolock() can be reached with kvm_usage_count==0.

Correct, I was mostly talking to myself.