linux-kernel - Re: [PATCH v2 5/5] x86/head/64: Disable stack protection for head$(BITS).o

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Date:   Fri, 9 Oct 2020 16:49:36 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86@...nel.org, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] x86/head/64: Disable stack protection for
 head$(BITS).o

On Thu, Oct 08, 2020 at 03:16:23PM -0400, Arvind Sankar wrote:
> On 64-bit, the startup_64_setup_env() function added in
>   866b556efa12 ("x86/head/64: Install startup GDT")
> has stack protection enabled because of set_bringup_idt_handler().
> 
> At this point, %gs is not yet initialized, and this doesn't cause a
> crash only because the #PF handler from the decompressor stub is still
> installed and handles the page fault.
> 
> Disable stack protection for the whole file, and do it on 32-bit as
> well to avoid surprises.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  arch/x86/kernel/Makefile | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 04ceea8f4a89..68608bd892c0 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -47,6 +47,8 @@ endif
>  # non-deterministic coverage.
>  KCOV_INSTRUMENT		:= n
>  
> +CFLAGS_head$(BITS).o	+= -fno-stack-protector
> +
>  CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
>  
>  obj-y			:= process_$(BITS).o signal.o
> -- 
> 2.26.2

Reviewed-by: Joerg Roedel <jroedel@...e.de>