Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Nadav Amit <namit@vmware.com>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.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 Cartwright <julia@ni.com>,
	Jessica Yu <jeyu@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Edward Cree <ecree@solarflare.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible
Date: Fri, 11 Jan 2019 10:57:52 -0600	[thread overview]
Message-ID: <20190111165752.z6e2dfktj2caqi4n@treble> (raw)
In-Reply-To: <ad6fae93-4122-7854-aa4c-2facd67ca839@oracle.com>

On Fri, Jan 11, 2019 at 05:46:36PM +0100, Alexandre Chartre wrote:
> 
> 
> On 01/11/2019 04:28 PM, Josh Poimboeuf wrote:
> > On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote:
> > > To avoid any issue with live patching the call instruction, what about
> > > toggling between two call instructions: one would be the currently active
> > > call, while the other would currently be inactive but to be used after a
> > > change is made. You can safely patch the inactive call and then toggle
> > > the call flow (using a jump label) between the active and inactive calls.
> > > 
> > > So instead of having a single call instruction:
> > > 
> > > 	call function
> > > 
> > > You would have:
> > > 
> > > 	STATIC_JUMP_IF_TRUE label, key
> > > 	call function1
> > > 	jmp done
> > > label:
> > > 	call function2
> > > done:
> > > 
> > > If the key is set so that function1 is currently called then you can
> > > safely update the call instruction for function2. Once this is done,
> > > just flip the key to make the function2 call active. On a next update,
> > > you would, of course, have to switch and update the call for function1.
> > 
> > What about the following race?
> > 
> > CPU1						CPU2
> > static key is false, doesn't jump
> > task gets preempted before calling function1
> > 						change static key to true
> > 						start patching "call function1"
> > task resumes, sees inconsistent call instruction
> > 
> 
> If the function1 call is active then it won't be changed, you will change
> function2. However, I presume you can still have a race but if the function
> is changed twice before calling function1:
> 
> CPU1						CPU2
> static key is false, doesn't jump
> task gets preempted before calling function1
>                                                 -- first function change --
>                                                 patch "call function2"
>                                                 change static key to true
>                                                 -- second function change --
>                                                 start patching "call function1"
> task resumes, sees inconsistent call instruction
> 
> So right, that's a problem.

Right, that's what I meant to say :-)

-- 
Josh