linux-kernel - Re: [PATCH v2 4/4] x86/boot: Check that there are no runtime relocations

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Message-ID: <20200526151623.GB2190602@rani.riverdale.lan>
Date:   Tue, 26 May 2020 11:16:23 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] x86/boot: Check that there are no runtime
 relocations

On Tue, May 26, 2020 at 08:11:56AM +0200, Ard Biesheuvel wrote:
> On Tue, 26 May 2020 at 00:59, Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >  # Compressed kernel should be built as PIE since it may be loaded at any
> >  # address by the bootloader.
> > -KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> > +KBUILD_LDFLAGS += -pie $(call ld-option, --no-dynamic-linker)
> 
> Do we still need -pie linking with these changes applied?
> 

I think it's currently not strictly necessary -- eg the 64bit kernel
doesn't get linked as pie right now with LLD or old binutils. However,
it is safer to do so to ensure that the result remains PIC with future
versions of the linker. There are linker optimizations that can convert
certain PIC instructions when PIE is disabled. While I think they
currently all focus on eliminating indirection through the GOT (and thus
wouldn't be applicable any more), it's easy to imagine that they could
get extended to, for eg, convert
	leaq	foo(%rip), %rax
to
	movl	$foo, %eax
with some nop padding, etc.

Also, the relocation check that's being added here would only work with
PIE linking.