[PATCH v2] xen/arm32: Distinguish guest SError from Xen data aborts

[PATCH v2] xen/arm32: Distinguish guest SError from Xen data aborts

[Wei Chen]

ARM32 doesn't have an exception similar to hyp_sync of ARM64 to catch
the synchronous data abort (For example, a NULL pointer has been referenced).
Hence the SError and sync data abort will be caught by the same data abort
exception.

Since commit "3f16c8cb" we treat all data aborts caught by this excetpion
as SError. This means, we will forward Xen synchronous data abort to guest,
if the serror_op=FORWARD. This is obviously incorrect. But we don't have
any method to distinguish SError from Xen data aborts.

But we can distinguish guest generated SError from Xen data aborts. So we
want to change the policy to handle data aborts for ARM32:
1. If this data abort is guest generated SError, we will handle this data
   abort follow the SError handle option setting.
2. If this data abort is synchronous data abort or Xen generate SError, we
   will PANIC the whole system.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
---
v1->v2:
Add an in-code comment to describe this change.

---
 xen/arch/arm/arm32/traps.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/arm32/traps.c b/xen/arch/arm/arm32/traps.c
index 5bc5f64..48baa64 100644
--- a/xen/arch/arm/arm32/traps.c
+++ b/xen/arch/arm/arm32/traps.c
@@ -62,7 +62,18 @@ asmlinkage void do_trap_prefetch_abort(struct cpu_user_regs *regs)
 
 asmlinkage void do_trap_data_abort(struct cpu_user_regs *regs)
 {
-    do_trap_hyp_serror(regs);
+    /*
+     * We cannot distinguish Xen SErrors from synchronous data aborts. We
+     * want to avoid treating any Xen synchronous aborts as SErrors and
+     * forwarding them to the guest. Instead, crash the system in all
+     * cases when the abort comes from Xen. Even if they are Xen SErrors
+     * it would be a reasonable thing to do, and the default behavior with
+     * serror_op == DIVERSE.
+     */
+    if ( VABORT_GEN_BY_GUEST(regs) )
+        do_trap_guest_serror(regs);
+    else
+        do_unexpected_trap("Data Abort", regs);
 }
 
 /*
-- 
2.7.4


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Re: [PATCH v2] xen/arm32: Distinguish guest SError from Xen data aborts

[Stefano Stabellini]

On Thu, 4 May 2017, Wei Chen wrote:
> ARM32 doesn't have an exception similar to hyp_sync of ARM64 to catch
> the synchronous data abort (For example, a NULL pointer has been referenced).
> Hence the SError and sync data abort will be caught by the same data abort
> exception.
> 
> Since commit "3f16c8cb" we treat all data aborts caught by this excetpion
> as SError. This means, we will forward Xen synchronous data abort to guest,
> if the serror_op=FORWARD. This is obviously incorrect. But we don't have
> any method to distinguish SError from Xen data aborts.
> 
> But we can distinguish guest generated SError from Xen data aborts. So we
> want to change the policy to handle data aborts for ARM32:
> 1. If this data abort is guest generated SError, we will handle this data
>    abort follow the SError handle option setting.
> 2. If this data abort is synchronous data abort or Xen generate SError, we
>    will PANIC the whole system.
> 
> Signed-off-by: Wei Chen <Wei.Chen@arm.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> v1->v2:
> Add an in-code comment to describe this change.
> 
> ---
>  xen/arch/arm/arm32/traps.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/arm32/traps.c b/xen/arch/arm/arm32/traps.c
> index 5bc5f64..48baa64 100644
> --- a/xen/arch/arm/arm32/traps.c
> +++ b/xen/arch/arm/arm32/traps.c
> @@ -62,7 +62,18 @@ asmlinkage void do_trap_prefetch_abort(struct cpu_user_regs *regs)
>  
>  asmlinkage void do_trap_data_abort(struct cpu_user_regs *regs)
>  {
> -    do_trap_hyp_serror(regs);
> +    /*
> +     * We cannot distinguish Xen SErrors from synchronous data aborts. We
> +     * want to avoid treating any Xen synchronous aborts as SErrors and
> +     * forwarding them to the guest. Instead, crash the system in all
> +     * cases when the abort comes from Xen. Even if they are Xen SErrors
> +     * it would be a reasonable thing to do, and the default behavior with
> +     * serror_op == DIVERSE.
> +     */
> +    if ( VABORT_GEN_BY_GUEST(regs) )
> +        do_trap_guest_serror(regs);
> +    else
> +        do_unexpected_trap("Data Abort", regs);
>  }
>  
>  /*
> -- 
> 2.7.4
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> https://lists.xen.org/xen-devel
> 

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Re: [PATCH v2] xen/arm32: Distinguish guest SError from Xen data aborts

[Julien Grall]

Hi,

On 05/04/2017 07:32 PM, Stefano Stabellini wrote:
> On Thu, 4 May 2017, Wei Chen wrote:
>> ARM32 doesn't have an exception similar to hyp_sync of ARM64 to catch
>> the synchronous data abort (For example, a NULL pointer has been referenced).
>> Hence the SError and sync data abort will be caught by the same data abort
>> exception.
>>
>> Since commit "3f16c8cb" we treat all data aborts caught by this excetpion

s/excetpion/exception/

>> as SError. This means, we will forward Xen synchronous data abort to guest,
>> if the serror_op=FORWARD. This is obviously incorrect. But we don't have
>> any method to distinguish SError from Xen data aborts.
>>
>> But we can distinguish guest generated SError from Xen data aborts. So we
>> want to change the policy to handle data aborts for ARM32:
>> 1. If this data abort is guest generated SError, we will handle this data
>>    abort follow the SError handle option setting.
>> 2. If this data abort is synchronous data abort or Xen generate SError, we
>>    will PANIC the whole system.
>>
>> Signed-off-by: Wei Chen <Wei.Chen@arm.com>
>
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

This is basically reverting the patch 3f16c "xen/arm: Replace 
do_trap_guest_serror with new helpers" with a tweak in the name.

I think this is the right solution as we don't want Xen legit error to 
be forwarded to the guest.

Release-acked-by: Julien Grall <julien.grall@arm.com>

Cheers,

>
>
>> v1->v2:
>> Add an in-code comment to describe this change.
>>
>> ---
>>  xen/arch/arm/arm32/traps.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/arch/arm/arm32/traps.c b/xen/arch/arm/arm32/traps.c
>> index 5bc5f64..48baa64 100644
>> --- a/xen/arch/arm/arm32/traps.c
>> +++ b/xen/arch/arm/arm32/traps.c
>> @@ -62,7 +62,18 @@ asmlinkage void do_trap_prefetch_abort(struct cpu_user_regs *regs)
>>
>>  asmlinkage void do_trap_data_abort(struct cpu_user_regs *regs)
>>  {
>> -    do_trap_hyp_serror(regs);
>> +    /*
>> +     * We cannot distinguish Xen SErrors from synchronous data aborts. We
>> +     * want to avoid treating any Xen synchronous aborts as SErrors and
>> +     * forwarding them to the guest. Instead, crash the system in all
>> +     * cases when the abort comes from Xen. Even if they are Xen SErrors
>> +     * it would be a reasonable thing to do, and the default behavior with
>> +     * serror_op == DIVERSE.
>> +     */
>> +    if ( VABORT_GEN_BY_GUEST(regs) )
>> +        do_trap_guest_serror(regs);
>> +    else
>> +        do_unexpected_trap("Data Abort", regs);
>>  }
>>
>>  /*
>> --
>> 2.7.4
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org
>> https://lists.xen.org/xen-devel
>>

-- 
Julien Grall

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