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Date:   Mon, 13 Jan 2020 18:20:12 -0800
From:   Kees Cook <keescook@chromium.org>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        Thomas Lendacky <Thomas.Lendacky@amd.com>,
        Mauro Rossi <issor.oruam@gmail.com>,
        Michael Matz <matz@suse.de>
Subject: Re: [PATCH v3] x86/vmlinux: Fix vmlinux.lds.S with pre-2.23 binutils
Message-ID: <202001131819.5A0031F611@keescook>
References: <20200113161310.GA191743@rani.riverdale.lan>
 <20200113195337.604646-1-nivedita@alum.mit.edu>
 <202001131750.C1B8468@keescook>
 <261ae869-4169-296e-f673-5c08ff34bdde@zytor.com>
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On Mon, Jan 13, 2020 at 05:57:23PM -0800, H. Peter Anvin wrote:
> On 2020-01-13 17:53, Kees Cook wrote:>>
> >> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> >> index 3a1a819da137..bad4e22384dc 100644
> >> --- a/arch/x86/kernel/vmlinux.lds.S
> >> +++ b/arch/x86/kernel/vmlinux.lds.S
> >> @@ -144,10 +144,12 @@ SECTIONS
> >>  		*(.text.__x86.indirect_thunk)
> >>  		__indirect_thunk_end = .;
> >>  #endif
> >> +
> >> +		/* End of text section */
> >> +		_etext = .;
> >>  	} :text =0xcccc
> >>  
> >> -	/* End of text section, which should occupy whole number of pages */
> >> -	_etext = .;
> >> +	/* .text should occupy whole number of pages */
> >>  	. = ALIGN(PAGE_SIZE);
> > 
> > NAK: linkers can add things at the end of .text that will go missing from
> > the kernel if _etext isn't _outside_ the .text section, truly beyond the
> > end of the .text section. This patch will break Control Flow Integrity
> > checking since the jump tables are at the end of .text.
> > 
> > Boris, we're always working around weird linker problems; I don't see a
> > problem with the v2 patch to fix up old binutils...
> > 
> 
> Why not add the marker into a separate section instead of leaving it as an
> absolute "floater"? Very old binutils would botch that case, but I think that
> has been long since addressed well below our current minimum version.

Can you send a patch with what you're thinking?

-- 
Kees Cook