linux-kernel - Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early

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Message-ID: <5308A7F7-1C69-495D-A20B-C8B0A2470F8D@vmware.com>
Date:   Mon, 5 Nov 2018 21:31:25 +0000
From:   Nadav Amit <namit@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during
 early_init

From: Thomas Gleixner
Sent: November 5, 2018 at 8:28:29 PM GMT
> To: Andy Lutomirski <luto@...capital.net>
> Cc: Nadav Amit <namit@...are.com>, Linus Torvalds <torvalds@...ux-foundation.org>, H. Peter Anvin <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, Andrew Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>, Dave Hansen <dave.hansen@...el.com>, Masami Hiramatsu <mhiramat@...nel.org>
> Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
> 
> 
> On Mon, 5 Nov 2018, Andy Lutomirski wrote:
>> On Mon, Nov 5, 2018 at 11:25 AM Nadav Amit <namit@...are.com> wrote:
>> Linus, hpa, or Dave, a question for you: suppose I map some page
>> writably, write to it, then upgrade permissions to allow execute.
>> Must I force all CPUs that might execute from it without first
>> serializing to serialize?  I suspect this doesn't really affect user
>> code, but it may affect the module loader.
>> 
>> To be safe, shouldn't the module loader broadcast an IPI to
>> sync_core() everywhere after loading a module and before making it
>> runnable, regardless of alternative patching?
>> 
>> IOW, the right sequence of events probably ought to me:
>> 
>> 1. Allocate the memory and map it.
>> 2. Copy in the text.
>> 3. Patch alternatives, etc.  This is logically just like (2) from an
>> architectural perspective -- we're just writing to memory that won't
>> be executed.
>> 4. Serialize everything.
>> 5. Run it!
> 
> I'd make that:
> 
> 1. Allocate the memory and map it RW
> 2. Copy in the text.
> 3. Patch alternatives, etc.  This is logically just like (2) from an
>   architectural perspective -- we're just writing to memory that won't
>   be executed.
> 4. Map it RX
> 5. Serialize everything.
> 6. Run it!

Thanks. I will do something along these lines. This can improve module
loading time (saving IRQ save/restore time), but it will not make things
much prettier, since two code-paths for “early init kernel” and “early init
module” would be needed.