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

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Date:   Mon, 24 Aug 2020 18:27:46 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>, peterz@...radead.org
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/entry: Fix AC assertion

On 24.08.20 17:58, Andrew Cooper wrote:
> 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.)

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


Juergen