Date:   Thu, 18 Nov 2021 20:55:04 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH V5 03/50] x86/traps: Remove stack-protector from traps.c
Message-ID: <20211118195504.GM174703@worktop.programming.kicks-ass.net>
References: <20211110115736.3776-1-jiangshanlai@gmail.com>
 <20211110115736.3776-4-jiangshanlai@gmail.com>
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On Wed, Nov 10, 2021 at 07:56:49PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> When stack-protector is enabled, the compiler adds some instrument code
> at the beginning and the end of some functions. Many functions in traps.c
> are non-instrumentable.  Moreover, stack-protector code in the beginning
> of the affected function accesses the canary that might be watched by
> hardware breakpoints which also violate the non-instrumentable
> nature of some functions and might cause infinite recursive #DB because
> the canary is accessed before resetting the dr7.
> 
> So it is better to remove stack-protector from traps.c.
> 
> It is also prepared for later patches that move some entry code into
> traps.c, some of which can NOT use percpu register until gsbase is
> properly switched.  And stack-protector depends on the percpu register
> to work.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>  arch/x86/kernel/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 2ff3e600f426..8ac45801ba8b 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -50,6 +50,7 @@ KCOV_INSTRUMENT		:= n
>  
>  CFLAGS_head$(BITS).o	+= -fno-stack-protector
>  CFLAGS_cc_platform.o	+= -fno-stack-protector
> +CFLAGS_traps.o		+= -fno-stack-protector

Well, there's a lot more noinstr than just in traps. There's also real C
code in traps. This isn't really a solution.

I think GCC has recently grown __attribute__((no_stack_protector)),
which should be added to noinstr (GCC-11 and above).

Additionally we could add code to objtool to detect this problem.