linux-kernel - Re: [PATCH] x86/entry: Fix AC assertion

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Date:   Mon, 24 Aug 2020 16:58:43 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     <peterz@...radead.org>
CC:     <x86@...nel.org>, Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/entry: Fix AC assertion

On 24/08/2020 16:21, peterz@...radead.org wrote:
> On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
>> On 24/08/2020 11:14, peterz@...radead.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) <peterz@...radead.org>
>>> Acked-by: Andy Lutomirski <luto@...nel.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