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Date:   Tue, 1 Feb 2022 15:59:39 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Alexander Popov <alex.popov@linux.com>
Subject: Re: Stackleak vs noinstr (Was: [GIT pull] objtool/core for v5.16-rc1)
Message-ID: <202202011558.DAAF17D@keescook>
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On Tue, Nov 02, 2021 at 11:03:44AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 10:05:50AM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 02, 2021 at 09:00:36AM +0100, Peter Zijlstra wrote:
> > > On Mon, Nov 01, 2021 at 01:44:39PM -0700, Linus Torvalds wrote:
> > 
> > > >     do_machine_check()+0x27: call to stackleak_track_stack ...
> > > >     do_syscall_64()+0x9: call to stackleak_track_stack ...
> > > >     do_int80_syscall_32()+0x9: call to stackleak_track_stack ...
> > > >     exc_general_protection()+0x22: call to stackleak_track_stack ...
> > > >     fixup_bad_iret()+0x20: call to stackleak_track_stack ...
> > > >     .entry.text+0x10e6: call to stackleak_erase ...
> > > >     .entry.text+0x143: call to stackleak_erase ...
> > > >     .entry.text+0x17d9: call to stackleak_erase ...
> > > > 
> > > > most seem to be about the stackleak thing,
> > > 
> > > Right, I recently ran into this and hacen't yet had time to look into
> > > it. I suspect my normal build box doesn't have the GCC plugin crud
> > > enabled or somesuch.
> > > 
> > > I think the GCC stackleak plugin needs fixing, specifically it needs a
> > > function attribute such that it will not emit instrumentation in noinstr
> > > functions. I'll go chase down the developer of that thing.
> > 
> > Alexander, is there any way to make this plugin grow a function
> > attribute which we can add to noinstr ? There's a strict requirement the
> > compiler doesn't add extra code to noinstr functions these days.
> > 
> > We'll 'soon' be running noinstr C code before switching to kernel page
> > tables even.
> 
> Using my pre-release GCC-12 compiler (the only one I have with plugin
> crud enabled apparently), the below seems to work.
> 
> Having the plugin gate on section name seems a lot hacky, but given it's
> already doing that, one more doesn't hurt.
> 
> ---
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index ce161a8e8d97..135866ca8878 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -48,7 +48,7 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
>  #define skip_erasing()	false
>  #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
>  
> -asmlinkage void notrace stackleak_erase(void)
> +asmlinkage noinstr void stackleak_erase(void)
>  {
>  	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
>  	unsigned long kstack_ptr = current->lowest_stack;
> @@ -102,7 +102,6 @@ asmlinkage void notrace stackleak_erase(void)
>  	/* Reset the 'lowest_stack' value for the next syscall */
>  	current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
>  }
> -NOKPROBE_SYMBOL(stackleak_erase);
>  
>  void __used __no_caller_saved_registers notrace stackleak_track_stack(void)
>  {
> diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
> index e9db7dcb3e5f..07688a1c686b 100644
> --- a/scripts/gcc-plugins/stackleak_plugin.c
> +++ b/scripts/gcc-plugins/stackleak_plugin.c
> @@ -446,6 +446,8 @@ static bool stackleak_gate(void)
>  			return false;
>  		if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13))
>  			return false;
> +		if (!strncmp(TREE_STRING_POINTER(section), ".noinstr.text", 13))
> +			return false;
>  	}
>  
>  	return track_frame_size >= 0;

Did this ever turn into a real patch? I don't see anything in -next for
it, so I assume it's still needed.

-- 
Kees Cook