linux-kernel - Re: [PATCH] x86/build: Move _etext to actual end of .text

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Message-ID: <201905140842.21066115C5@keescook>
Date:   Tue, 14 May 2019 08:43:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Johannes Hirte <johannes.hirte@...enkhaos.de>
Cc:     Borislav Petkov <bp@...e.de>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/build: Move _etext to actual end of .text

On Tue, May 14, 2019 at 02:04:21PM +0200, Johannes Hirte wrote:
> On 2019 Apr 23, Kees Cook wrote:
> > When building x86 with Clang LTO and CFI, CFI jump regions are
> > automatically added to the end of the .text section late in linking. As a
> > result, the _etext position was being labelled before the appended jump
> > regions, causing confusion about where the boundaries of the executable
> > region actually are in the running kernel, and broke at least the fault
> > injection code. This moves the _etext mark to outside (and immediately
> > after) the .text area, as it already the case on other architectures
> > (e.g. arm64, arm).
> > 
> > Reported-and-tested-by: Sami Tolvanen <samitolvanen@...gle.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  arch/x86/kernel/vmlinux.lds.S | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index bad8c51fee6e..de94da2366e7 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -141,11 +141,11 @@ SECTIONS
> >  		*(.text.__x86.indirect_thunk)
> >  		__indirect_thunk_end = .;
> >  #endif
> > -
> > -		/* End of text section */
> > -		_etext = .;
> >  	} :text = 0x9090
> >  
> > +	/* End of text section */
> > +	_etext = .;
> > +
> >  	NOTES :text :note
> >  
> >  	EXCEPTION_TABLE(16) :text = 0x9090
> > -- 
> > 2.17.1
> 
> This breaks the build on my system:
> 
>   RELOCS  arch/x86/boot/compressed/vmlinux.relocs
>   CC      arch/x86/boot/compressed/early_serial_console.o
>   CC      arch/x86/boot/compressed/kaslr.o
>   AS      arch/x86/boot/compressed/mem_encrypt.o
>   CC      arch/x86/boot/compressed/kaslr_64.o
> Invalid absolute R_X86_64_32S relocation: _etext
> make[2]: *** [arch/x86/boot/compressed/Makefile:130: arch/x86/boot/compressed/vmlinux.relocs] Error 1
> make[2]: *** Deleting file 'arch/x86/boot/compressed/vmlinux.relocs'
> make[2]: *** Waiting for unfinished jobs....
> make[1]: *** [arch/x86/boot/Makefile:112: arch/x86/boot/compressed/vmlinux] Error 2
> make: *** [arch/x86/Makefile:283: bzImage] Error 2

Interesting! Can you send along your .config and compiler details?

-- 
Kees Cook