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Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181129164914.GA9027@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Nov 2018 08:59:31 -0800
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
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References: <20181126171036.chcbmb35ygpxziub@treble> <20181126175624.bruqfbkngbucpvxr@treble> <20181126200801.GW2113@hirez.programming.kicks-ass.net> <20181126212628.4apztfazichxnt7r@treble> <20181127084330.GX2113@hirez.programming.kicks-ass.net> <CALCETrXam8JBtXE3S97Vyb_=+pmjgr23SoZ-Dm5pmYtwZYA+7Q@mail.gmail.com> <20181129094210.GC2131@hirez.programming.kicks-ass.net> <ECF632D9-581B-40B7-9F0A-6B5607A4D0E3@amacapital.net> <20181129143853.GO2131@hirez.programming.kicks-ass.net> <20181129163342.tp5wlfcyiazwwyoh@treble> <20181129164914.GA9027@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 29, 2018, at 8:49 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> On Thu, Nov 29, 2018 at 10:33:42AM -0600, Josh Poimboeuf wrote:
>>> can't we 'fix' that again? The alternative is moving that IRET-frame and=

>>> fixing everything up, which is going to be fragile, ugly and such
>>> things more.
>=20
>> This seems to work...
>=20
> That's almost too easy... nice!

It is indeed too easy: you=E2=80=99re putting pt_regs in the wrong place for=
 int3 from user mode, which is probably a root hole if you arrange for a ptr=
aced process to do int3 and try to write to whatever register aliases CS.

If you make it conditional on CPL, do it for 32-bit as well, add comments, a=
nd convince yourself that there isn=E2=80=99t a better solution (like pointi=
ng IP at a stub that retpolines to the target by reading the function pointe=
r, a la the unoptimizable version), then okay, I guess, with only a small am=
ount of grumbling.

>=20
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index ce25d84023c0..184523447d35 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -876,7 +876,7 @@ apicinterrupt IRQ_WORK_VECTOR            irq_work_int=
errupt        smp_irq_work_interrupt
>>  * @paranoid =3D=3D 2 is special: the stub will never switch stacks.  Thi=
s is for
>>  * #DF: if the thread stack is somehow unusable, we'll still get a useful=
 OOPS.
>>  */
>> -.macro idtentry sym do_sym has_error_code:req paranoid=3D0 shift_ist=3D-=
1
>> +.macro idtentry sym do_sym has_error_code:req paranoid=3D0 shift_ist=3D-=
1 create_gap=3D0
>> ENTRY(\sym)
>>    UNWIND_HINT_IRET_REGS offset=3D\has_error_code*8
>>=20
>> @@ -891,6 +891,12 @@ ENTRY(\sym)
>>    pushq    $-1                /* ORIG_RAX: no syscall to restart */
>>    .endif
>>=20
>> +    .if \create_gap =3D=3D 1
>> +    .rept 6
>> +    pushq 5*8(%rsp)
>> +    .endr
>> +    .endif
>> +
>>    .if \paranoid =3D=3D 1
>>    testb    $3, CS-ORIG_RAX(%rsp)        /* If coming from userspace, swi=
tch stacks */
>>    jnz    .Lfrom_usermode_switch_stack_\@
>> @@ -1126,7 +1132,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \
>> #endif /* CONFIG_HYPERV */
>>=20
>> idtentry debug            do_debug        has_error_code=3D0    paranoid=3D=
1 shift_ist=3DDEBUG_STACK
>> -idtentry int3            do_int3            has_error_code=3D0
>> +idtentry int3            do_int3            has_error_code=3D0    create=
_gap=3D1
>> idtentry stack_segment        do_stack_segment    has_error_code=3D1
>>=20
>> #ifdef CONFIG_XEN_PV