Re: [PATCH] x86/entry: Fix AC assertion

[Jürgen Groß]

On 24.08.20 17:58, Andrew Cooper wrote:

On 24/08/2020 16:21, pet...@infradead.org wrote:

On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:

On 24/08/2020 11:14, pet...@infradead.org wrote:

The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
improve user entry sanity checks") unconditionally triggers on my IVB
machine because it does not support SMAP.

For !SMAP hardware we patch out CLAC/STAC instructions and thus if
userspace sets AC, we'll still have it set after entry.

Technically, you don't patch in, rather than patch out.

True.


Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity 
checks")
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Andy Lutomirski 
---
  arch/x86/include/asm/entry-common.h |   11 +--
  1 file changed, 9 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -18,8 +18,15 @@ static __always_inline void arch_check_u
 * state, not the interrupt state as imagined by Xen.
 */
unsigned long flags = native_save_fl();
-   WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
- X86_EFLAGS_NT));
+   unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
+
+   /*
+* For !SMAP hardware we patch out CLAC on entry.
+*/
+   if (boot_cpu_has(X86_FEATURE_SMAP))
+   mask |= X86_EFLAGS_AC;

The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
actually running in Ring 3, and therefore susceptible to #AC's which
wouldn't occur natively.

So do you then want it to be something like:

if (boot_cpu_has(X86_FEATURE_SMAP) ||
(IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))

? Or are you fine with the proposed?


Dealers choice, but this option would be slightly better overall.

(Are there any other cases where Linux will be running in Ring 3?  I
haven't been paying attention to recent changes in PVOps.)


I'm not aware of any other case running kernel code in ring 3.


Juergen

Re: [PATCH] x86/entry: Fix AC assertion

[Andrew Cooper]

On 24/08/2020 16:21, pet...@infradead.org wrote:
> On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
>> On 24/08/2020 11:14, pet...@infradead.org wrote:
>>> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
>>> improve user entry sanity checks") unconditionally triggers on my IVB
>>> machine because it does not support SMAP.
>>>
>>> For !SMAP hardware we patch out CLAC/STAC instructions and thus if
>>> userspace sets AC, we'll still have it set after entry.
>> Technically, you don't patch in, rather than patch out.
> True.
>
>>> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry 
>>> sanity checks")
>>> Signed-off-by: Peter Zijlstra (Intel) 
>>> Acked-by: Andy Lutomirski 
>>> ---
>>>  arch/x86/include/asm/entry-common.h |   11 +--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> --- a/arch/x86/include/asm/entry-common.h
>>> +++ b/arch/x86/include/asm/entry-common.h
>>> @@ -18,8 +18,15 @@ static __always_inline void arch_check_u
>>>  * state, not the interrupt state as imagined by Xen.
>>>  */
>>> unsigned long flags = native_save_fl();
>>> -   WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
>>> - X86_EFLAGS_NT));
>>> +   unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
>>> +
>>> +   /*
>>> +* For !SMAP hardware we patch out CLAC on entry.
>>> +*/
>>> +   if (boot_cpu_has(X86_FEATURE_SMAP))
>>> +   mask |= X86_EFLAGS_AC;
>> The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
>> actually running in Ring 3, and therefore susceptible to #AC's which
>> wouldn't occur natively.
> So do you then want it to be something like:
>
>   if (boot_cpu_has(X86_FEATURE_SMAP) ||
>   (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
>
> ? Or are you fine with the proposed?

Dealers choice, but this option would be slightly better overall.

(Are there any other cases where Linux will be running in Ring 3?  I
haven't been paying attention to recent changes in PVOps.)

~Andrew

Re: [PATCH] x86/entry: Fix AC assertion

[peterz]

On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
> On 24/08/2020 11:14, pet...@infradead.org wrote:
> > The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
> > improve user entry sanity checks") unconditionally triggers on my IVB
> > machine because it does not support SMAP.
> >
> > For !SMAP hardware we patch out CLAC/STAC instructions and thus if
> > userspace sets AC, we'll still have it set after entry.
> 
> Technically, you don't patch in, rather than patch out.

True.

> > Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry 
> > sanity checks")
> > Signed-off-by: Peter Zijlstra (Intel) 
> > Acked-by: Andy Lutomirski 
> > ---
> >  arch/x86/include/asm/entry-common.h |   11 +--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > --- a/arch/x86/include/asm/entry-common.h
> > +++ b/arch/x86/include/asm/entry-common.h
> > @@ -18,8 +18,15 @@ static __always_inline void arch_check_u
> >  * state, not the interrupt state as imagined by Xen.
> >  */
> > unsigned long flags = native_save_fl();
> > -   WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
> > - X86_EFLAGS_NT));
> > +   unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
> > +
> > +   /*
> > +* For !SMAP hardware we patch out CLAC on entry.
> > +*/
> > +   if (boot_cpu_has(X86_FEATURE_SMAP))
> > +   mask |= X86_EFLAGS_AC;
> 
> The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
> actually running in Ring 3, and therefore susceptible to #AC's which
> wouldn't occur natively.

So do you then want it to be something like:

if (boot_cpu_has(X86_FEATURE_SMAP) ||
(IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))

? Or are you fine with the proposed?

Re: [PATCH] x86/entry: Fix AC assertion

[Andrew Cooper]

On 24/08/2020 11:14, pet...@infradead.org wrote:
> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
> improve user entry sanity checks") unconditionally triggers on my IVB
> machine because it does not support SMAP.
>
> For !SMAP hardware we patch out CLAC/STAC instructions and thus if
> userspace sets AC, we'll still have it set after entry.

Technically, you don't patch in, rather than patch out.

>
> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity 
> checks")
> Signed-off-by: Peter Zijlstra (Intel) 
> Acked-by: Andy Lutomirski 
> ---
>  arch/x86/include/asm/entry-common.h |   11 +--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -18,8 +18,15 @@ static __always_inline void arch_check_u
>* state, not the interrupt state as imagined by Xen.
>*/
>   unsigned long flags = native_save_fl();
> - WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
> -   X86_EFLAGS_NT));
> + unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
> +
> + /*
> +  * For !SMAP hardware we patch out CLAC on entry.
> +  */
> + if (boot_cpu_has(X86_FEATURE_SMAP))
> + mask |= X86_EFLAGS_AC;

The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
actually running in Ring 3, and therefore susceptible to #AC's which
wouldn't occur natively.

~Andrew