Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init

From: Nadav Amit <namit@vmware.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andrew Lutomirski <luto@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
Date: Mon, 5 Nov 2018 21:31:25 +0000	[thread overview]
Message-ID: <5308A7F7-1C69-495D-A20B-C8B0A2470F8D@vmware.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1811052126310.1659@nanos.tec.linutronix.de>

From: Thomas Gleixner
Sent: November 5, 2018 at 8:28:29 PM GMT
> To: Andy Lutomirski <luto@amacapital.net>
> Cc: Nadav Amit <namit@vmware.com>, Linus Torvalds <torvalds@linux-foundation.org>, H. Peter Anvin <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Andrew Lutomirski <luto@kernel.org>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.com>, Masami Hiramatsu <mhiramat@kernel.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@vmware.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.