linux-kernel - Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying shstk

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Message-ID: <8031fb5f-ca53-d89e-1e29-262514552708@citrix.com>
Date:   Wed, 5 Oct 2022 22:58:05 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "hpa@...or.com" <hpa@...or.com>,
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        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
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        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
CC:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 26/39] x86/cet/shstk: Introduce routines modifying
 shstk

On 05/10/2022 23:47, Edgecombe, Rick P wrote:
> On Wed, 2022-10-05 at 02:43 +0000, Andrew Cooper wrote:
>> On 29/09/2022 23:29, Rick Edgecombe wrote:
>>> diff --git a/arch/x86/include/asm/special_insns.h
>>> b/arch/x86/include/asm/special_insns.h
>>> index 35f709f619fb..f096f52bd059 100644
>>> --- a/arch/x86/include/asm/special_insns.h
>>> +++ b/arch/x86/include/asm/special_insns.h
>>> @@ -223,6 +223,19 @@ static inline void clwb(volatile void *__p)
>>>                : [pax] "a" (p));
>>>   }
>>>   
>>> +#ifdef CONFIG_X86_SHADOW_STACK
>>> +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
>>> +{
>>> +     asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
>>> +                       _ASM_EXTABLE(1b, %l[fail])
>>> +                       :: [addr] "r" (addr), [val] "r" (val)
>>> +                       :: fail);
>> "1: wrssq %[val], %[addr]\n"
>> _ASM_EXTABLE(1b, %l[fail])
>> : [addr] "+m" (*addr)
>> : [val] "r" (val)
>> :: fail
>>
>> Otherwise you've failed to tell the compiler that you wrote to *addr.
>>
>> With that fixed, it's not volatile because there are no unexpressed
>> side
>> effects.
> Ok, thanks!

On further consideration, it should be "=m" not "+m", which is even less
constrained, and even easier for an enterprising optimiser to try and do
something useful with.

~Andrew