linux-kernel - Re: [GIT PULL] x86/urgent for v5.11-rc7

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Date:   Tue, 9 Feb 2021 08:55:51 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...el.com>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        live-patching@...r.kernel.org
Subject: Re: [GIT PULL] x86/urgent for v5.11-rc7



> On Feb 9, 2021, at 8:45 AM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Tue, Feb 9, 2021 at 6:49 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>> 
>>> On Tue, 9 Feb 2021 09:32:34 +0100 (CET)
>>> Miroslav Benes <mbenes@...e.cz> wrote:
>>> 
>>> powerpc has this
>>> 
>>> static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
>>> {
>>>        /*
>>>         * Live patch works only with -mprofile-kernel on PPC. In this case,
>>>         * the ftrace location is always within the first 16 bytes.
>>>         */
>>>        return ftrace_location_range(faddr, faddr + 16);
>>> }
>>> 
>>>>> I suppose the trivial fix is to see if it points to endbr64 and if so,
>>>>> increment the addr by the length of that.
>>>> 
>>>> I thought of that too. But one thing that may be possible, is to use
>>>> kallsym. I believe you can get the range of a function (start and end of
>>>> the function) from kallsyms. Then ask ftrace for the addr in that range
>>>> (there should only be one).
>>> 
>>> And we can do this if a hard-coded value live above is not welcome. If I
>>> remember correctly, we used to have exactly this in the old versions of
>>> kGraft. We walked through all ftrace records, called
>>> kallsyms_lookup_size_offset() on every record's ip and if the offset+ip
>>> matched faddr (in this case), we returned the ip.
>> 
>> Either way is fine. Question is, should we just wait till CET is
>> implemented for the kernel before making any of these changes? Just knowing
>> that we have a solution to handle it may be good enough for now.
> 
> I think the issue is more fundamental than what appears on the surface.
> According to endbr64 documentation it's not just any instruction.
> The cpu will wait for it and if it's replaced with int3 or not seen at
> the branch target the cpu will throw an exception.
> If I understood the doc correctly it means that endbr64 can never be
> replaced with a breakpoint. If that's the case text_poke_bp and kprobe
> need to do extra safety checks.

Ugh.

Or we hack up #CP to handle this case. I don’t quite know how I feel about this.