[PATCH v3] xpti: fix bug in double fault handling

[PATCH v3] xpti: fix bug in double fault handling

[Juergen Gross]

When entering the hypervisor via the double fault handler resetting
xen_cr3 was missing. This led to switching to pv_cr3 when returning
from the next following exception, so repair this in order to allow
exception handling to work even after a double fault.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 xen/arch/x86/x86_64/entry.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 45d9842d09..1cd7d93892 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -767,12 +767,14 @@ ENTRY(double_fault)
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
-        test  %rbx, %rbx
+        neg   %rbx
         jz    .Ldblf_cr3_okay
         jns   .Ldblf_cr3_load
+        mov   %rbx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         neg   %rbx
 .Ldblf_cr3_load:
         mov   %rbx, %cr3
+        movq $0, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
 .Ldblf_cr3_okay:
 
         movq  %rsp,%rdi
-- 
2.13.6


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Re: [PATCH v3] xpti: fix bug in double fault handling

[Andrew Cooper]

On 23/04/18 14:59, Juergen Gross wrote:
> When entering the hypervisor via the double fault handler resetting
> xen_cr3 was missing. This led to switching to pv_cr3 when returning
> from the next following exception, so repair this in order to allow
> exception handling to work even after a double fault.
>
> Signed-off-by: Juergen Gross <jgross@suse.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>

> ---
>  xen/arch/x86/x86_64/entry.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
> index 45d9842d09..1cd7d93892 100644
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -767,12 +767,14 @@ ENTRY(double_fault)
>          /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
>  
>          mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
> -        test  %rbx, %rbx
> +        neg   %rbx
>          jz    .Ldblf_cr3_okay
>          jns   .Ldblf_cr3_load
> +        mov   %rbx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
>          neg   %rbx
>  .Ldblf_cr3_load:
>          mov   %rbx, %cr3
> +        movq $0, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
>  .Ldblf_cr3_okay:
>  
>          movq  %rsp,%rdi


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Re: [PATCH v3] xpti: fix bug in double fault handling

[Jan Beulich]

>>> On 23.04.18 at 15:59, <jgross@suse.com> wrote:
> When entering the hypervisor via the double fault handler resetting
> xen_cr3 was missing. This led to switching to pv_cr3 when returning
> from the next following exception, so repair this in order to allow
> exception handling to work even after a double fault.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>
(and I'm glad Andrew had noticed the issue before me putting this in)

Jan



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