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) <pet...@infradead.org>
>>> Acked-by: Andy Lutomirski <l...@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 +18,15 @@ 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
