linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Date:   Thu, 29 Nov 2018 08:59:31 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, julia@...com, jeyu@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64



> On Nov 29, 2018, at 8:49 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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.
> 
>> This seems to work...
> 
> That's almost too easy... nice!

It is indeed too easy: you’re putting pt_regs in the wrong place for int3 from user mode, which is probably a root hole if you arrange for a ptraced 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, and convince yourself that there isn’t a better solution (like pointing IP at a stub that retpolines to the target by reading the function pointer, a la the unoptimizable version), then okay, I guess, with only a small amount of grumbling.

> 
>> 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_interrupt        smp_irq_work_interrupt
>>  * @paranoid == 2 is special: the stub will never switch stacks.  This 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=0 shift_ist=-1
>> +.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0
>> ENTRY(\sym)
>>    UNWIND_HINT_IRET_REGS offset=\has_error_code*8
>> 
>> @@ -891,6 +891,12 @@ ENTRY(\sym)
>>    pushq    $-1                /* ORIG_RAX: no syscall to restart */
>>    .endif
>> 
>> +    .if \create_gap == 1
>> +    .rept 6
>> +    pushq 5*8(%rsp)
>> +    .endr
>> +    .endif
>> +
>>    .if \paranoid == 1
>>    testb    $3, CS-ORIG_RAX(%rsp)        /* If coming from userspace, switch stacks */
>>    jnz    .Lfrom_usermode_switch_stack_\@
>> @@ -1126,7 +1132,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \
>> #endif /* CONFIG_HYPERV */
>> 
>> idtentry debug            do_debug        has_error_code=0    paranoid=1 shift_ist=DEBUG_STACK
>> -idtentry int3            do_int3            has_error_code=0
>> +idtentry int3            do_int3            has_error_code=0    create_gap=1
>> idtentry stack_segment        do_stack_segment    has_error_code=1
>> 
>> #ifdef CONFIG_XEN_PV