2018-02-01 12:16:59

by Jürgen Groß

Subject: [PATCH] x86/xen: init %gs very early to avoid page faults with stack protector

When running as Xen pv guest %gs is initialized some time after
C code is started. Depending on stack protector usage this might be
too late, resulting in page faults.

So setup %gs and MSR_GS_BASE in assembly code already.

Cc: [email protected]
Signed-off-by: Juergen Gross <[email protected]>
---
arch/x86/xen/xen-head.S | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 497cc55a0c16..b47d87076efb 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -9,7 +9,9 @@

#include <asm/boot.h>
#include <asm/asm.h>
+#include <asm/msr.h>
#include <asm/page_types.h>
+#include <asm/percpu.h>
#include <asm/unwind_hints.h>

#include <xen/interface/elfnote.h>
@@ -35,6 +37,18 @@ ENTRY(startup_xen)
mov %_ASM_SI, xen_start_info
mov $init_thread_union+THREAD_SIZE, %_ASM_SP

+ /* Set up %gs.
+ *
+ * The base of %gs always points to the bottom of the irqstack
+ * union. If the stack protector canary is enabled, it is
+ * located at %gs:40. Note that, on SMP, the boot cpu uses
+ * init data section till per cpu areas are set up.
+ */
+ movl $MSR_GS_BASE,%ecx
+ movq $INIT_PER_CPU_VAR(irq_stack_union),%rax
+ cdq
+ wrmsr
+
jmp xen_start_kernel
END(startup_xen)
__FINIT
--
2.13.6

2018-02-01 12:23:03

by Andrew Cooper

Subject: Re: [PATCH] x86/xen: init %gs very early to avoid page faults with stack protector

On 01/02/18 12:16, Juergen Gross wrote:
> When running as Xen pv guest %gs is initialized some time after
> C code is started. Depending on stack protector usage this might be
> too late, resulting in page faults.
>
> So setup %gs and MSR_GS_BASE in assembly code already.
>
> Cc: [email protected]
> Signed-off-by: Juergen Gross <[email protected]>
> ---
> arch/x86/xen/xen-head.S | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 497cc55a0c16..b47d87076efb 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -9,7 +9,9 @@
>
> #include <asm/boot.h>
> #include <asm/asm.h>
> +#include <asm/msr.h>
> #include <asm/page_types.h>
> +#include <asm/percpu.h>
> #include <asm/unwind_hints.h>
>
> #include <xen/interface/elfnote.h>
> @@ -35,6 +37,18 @@ ENTRY(startup_xen)
> mov %_ASM_SI, xen_start_info
> mov $init_thread_union+THREAD_SIZE, %_ASM_SP
>
> + /* Set up %gs.
> + *
> + * The base of %gs always points to the bottom of the irqstack
> + * union. If the stack protector canary is enabled, it is
> + * located at %gs:40. Note that, on SMP, the boot cpu uses
> + * init data section till per cpu areas are set up.
> + */
> + movl $MSR_GS_BASE,%ecx
> + movq $INIT_PER_CPU_VAR(irq_stack_union),%rax
> + cdq
> + wrmsr

You surely want a #ifdef __x86_64__ ?  This path is common to the 32bit
entry as well?

~Andrew

> +
> jmp xen_start_kernel
> END(startup_xen)
> __FINIT

2018-02-01 12:25:57

by Jürgen Groß

Subject: Re: [PATCH] x86/xen: init %gs very early to avoid page faults with stack protector

On 01/02/18 13:22, Andrew Cooper wrote:
> On 01/02/18 12:16, Juergen Gross wrote:
>> When running as Xen pv guest %gs is initialized some time after
>> C code is started. Depending on stack protector usage this might be
>> too late, resulting in page faults.
>>
>> So setup %gs and MSR_GS_BASE in assembly code already.
>>
>> Cc: [email protected]
>> Signed-off-by: Juergen Gross <[email protected]>
>> ---
>> arch/x86/xen/xen-head.S | 14 ++++++++++++++
>> 1 file changed, 14 insertions(+)
>>
>> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
>> index 497cc55a0c16..b47d87076efb 100644
>> --- a/arch/x86/xen/xen-head.S
>> +++ b/arch/x86/xen/xen-head.S
>> @@ -9,7 +9,9 @@
>>
>> #include <asm/boot.h>
>> #include <asm/asm.h>
>> +#include <asm/msr.h>
>> #include <asm/page_types.h>
>> +#include <asm/percpu.h>
>> #include <asm/unwind_hints.h>
>>
>> #include <xen/interface/elfnote.h>
>> @@ -35,6 +37,18 @@ ENTRY(startup_xen)
>> mov %_ASM_SI, xen_start_info
>> mov $init_thread_union+THREAD_SIZE, %_ASM_SP
>>
>> + /* Set up %gs.
>> + *
>> + * The base of %gs always points to the bottom of the irqstack
>> + * union. If the stack protector canary is enabled, it is
>> + * located at %gs:40. Note that, on SMP, the boot cpu uses
>> + * init data section till per cpu areas are set up.
>> + */
>> + movl $MSR_GS_BASE,%ecx
>> + movq $INIT_PER_CPU_VAR(irq_stack_union),%rax
>> + cdq
>> + wrmsr
>
> You surely want a #ifdef __x86_64__ ?  This path is common to the 32bit
> entry as well?

Oh, indeed! Thanks for noticing.

V2 coming soon...


Juergen