Re: [PATCH 01/13] x86/entry: Fix AC assertion

["Jürgen Groß" <jgross@suse.com>]

On 02.09.20 17:58, Brian Gerst wrote:
> On Wed, Sep 2, 2020 at 9:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> From: Peter Zijlstra <peterz@infradead.org>
>>
>> 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.
>>
>> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Acked-by: Andy Lutomirski <luto@kernel.org>
>> ---
>>   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 +18,16 @@ 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) ||
>> +                   (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
>> +                       mask |= X86_EFLAGS_AC;
> 
> Is the explicit Xen check necessary?  IIRC the Xen hypervisor will
> filter out the SMAP bit in the cpuid pvop.

Right, and this test will nevertheless result in setting AC in the mask.
IIRC this was the main objective here.


Juergen