diff mbox series

[1/2] x86/hyperv: fix invalid writes to MSRs during root partition kexec

Message ID 20221026134715.1438789-2-anrayabh@linux.microsoft.com
State New
Headers
Series Fix MSR access errors during kexec in root partition |

Commit Message

Anirudh Rayabharam Oct. 26, 2022, 1:47 p.m. UTC 
  hv_cleanup resets the hypercall page by setting the MSR to 0. However,
the root partition is not allowed to write to the GPA bits of the MSR.
Instead, it uses the hypercall page provided by the MSR. Similar is the
case with the reference TSC MSR.

Clear only the enable bit instead of zeroing the entire MSR to make
the code valid for root partition too.

Signed-off-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
---
 arch/x86/hyperv/hv_init.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Comments

Michael Kelley (LINUX) Oct. 26, 2022, 2:58 p.m. UTC | #1 
From: Anirudh Rayabharam <anrayabh@linux.microsoft.com> Sent: Wednesday, October 26, 2022 6:47 AM
> 
> hv_cleanup resets the hypercall page by setting the MSR to 0. However,
> the root partition is not allowed to write to the GPA bits of the MSR.
> Instead, it uses the hypercall page provided by the MSR. Similar is the
> case with the reference TSC MSR.
> 
> Clear only the enable bit instead of zeroing the entire MSR to make
> the code valid for root partition too.

When the enable bit is cleared (but not the PFN) in the MSR, do we know
for sure that Hyper-V removes the overlay page for the PFN?  Making sure
that the overlay page is removed is the main reason for clearing the entire
MSR.   If we're going to leave the PFN in place and just clear the enable bit,
we need to confirm with the Hyper-V guys that the overlay page will be
removed.

Michael

> 
> Signed-off-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 29774126e931..76ff63d69461 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -537,6 +537,7 @@ void __init hyperv_init(void)
>  void hyperv_cleanup(void)
>  {
>  	union hv_x64_msr_hypercall_contents hypercall_msr;
> +	u64 tsc_msr;
> 
>  	unregister_syscore_ops(&hv_syscore_ops);
> 
> @@ -552,12 +553,14 @@ void hyperv_cleanup(void)
>  	hv_hypercall_pg = NULL;
> 
>  	/* Reset the hypercall page */
> -	hypercall_msr.as_uint64 = 0;
> +	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +	hypercall_msr.enable = 0;
>  	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> 
>  	/* Reset the TSC page */
> -	hypercall_msr.as_uint64 = 0;
> -	wrmsrl(HV_X64_MSR_REFERENCE_TSC, hypercall_msr.as_uint64);
> +	rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
> +	tsc_msr &= ~BIT_ULL(0);
> +	wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
>  }
> 
>  void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die)
> --
> 2.34.1
Anirudh Rayabharam Oct. 27, 2022, 10:01 a.m. UTC | #2 
On Wed, Oct 26, 2022 at 02:58:16PM +0000, Michael Kelley (LINUX) wrote:
> From: Anirudh Rayabharam <anrayabh@linux.microsoft.com> Sent: Wednesday, October 26, 2022 6:47 AM
> > 
> > hv_cleanup resets the hypercall page by setting the MSR to 0. However,
> > the root partition is not allowed to write to the GPA bits of the MSR.
> > Instead, it uses the hypercall page provided by the MSR. Similar is the
> > case with the reference TSC MSR.
> > 
> > Clear only the enable bit instead of zeroing the entire MSR to make
> > the code valid for root partition too.
> 
> When the enable bit is cleared (but not the PFN) in the MSR, do we know
> for sure that Hyper-V removes the overlay page for the PFN?  Making sure
> that the overlay page is removed is the main reason for clearing the entire
> MSR.   If we're going to leave the PFN in place and just clear the enable bit,
> we need to confirm with the Hyper-V guys that the overlay page will be
> removed.

I checked the hypervisor code. Just clearing the enable bit does cause
the overlay page to be unmapped by the hypervisor.

Thanks,
Anirudh.

> 
> Michael
> 
> > 
> > Signed-off-by: Anirudh Rayabharam <anrayabh@linux.microsoft.com>
> > ---
> >  arch/x86/hyperv/hv_init.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 29774126e931..76ff63d69461 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -537,6 +537,7 @@ void __init hyperv_init(void)
> >  void hyperv_cleanup(void)
> >  {
> >  	union hv_x64_msr_hypercall_contents hypercall_msr;
> > +	u64 tsc_msr;
> > 
> >  	unregister_syscore_ops(&hv_syscore_ops);
> > 
> > @@ -552,12 +553,14 @@ void hyperv_cleanup(void)
> >  	hv_hypercall_pg = NULL;
> > 
> >  	/* Reset the hypercall page */
> > -	hypercall_msr.as_uint64 = 0;
> > +	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > +	hypercall_msr.enable = 0;
> >  	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > 
> >  	/* Reset the TSC page */
> > -	hypercall_msr.as_uint64 = 0;
> > -	wrmsrl(HV_X64_MSR_REFERENCE_TSC, hypercall_msr.as_uint64);
> > +	rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
> > +	tsc_msr &= ~BIT_ULL(0);
> > +	wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
> >  }
> > 
> >  void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die)
> > --
> > 2.34.1
diff mbox series

Patch

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 29774126e931..76ff63d69461 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -537,6 +537,7 @@  void __init hyperv_init(void)
 void hyperv_cleanup(void)
 {
 	union hv_x64_msr_hypercall_contents hypercall_msr;
+	u64 tsc_msr;
 
 	unregister_syscore_ops(&hv_syscore_ops);
 
@@ -552,12 +553,14 @@  void hyperv_cleanup(void)
 	hv_hypercall_pg = NULL;
 
 	/* Reset the hypercall page */
-	hypercall_msr.as_uint64 = 0;
+	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
+	hypercall_msr.enable = 0;
 	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
 
 	/* Reset the TSC page */
-	hypercall_msr.as_uint64 = 0;
-	wrmsrl(HV_X64_MSR_REFERENCE_TSC, hypercall_msr.as_uint64);
+	rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
+	tsc_msr &= ~BIT_ULL(0);
+	wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
 }
 
 void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die)