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Date:   Fri, 31 Jul 2020 22:32:58 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        James Morse <james.morse@arm.com>,
        Borislav Petkov <bp@suse.de>, Ingo Molnar <mingo@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>, x86@kernel.org,
        clang-built-linux@googlegroups.com, linux-arch@vger.kernel.org,
        linux-efi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section
Message-ID: <202007312231.60719F7C4@keescook>
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 <20200801021248.GB2700342@rani.riverdale.lan>
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On Fri, Jul 31, 2020 at 10:12:48PM -0400, Arvind Sankar wrote:
> On Fri, Jul 31, 2020 at 04:08:13PM -0700, Kees Cook wrote:
> > The .got.plt section should always be zero (or filled only with the
> > linker-generated lazy dispatch entry). Enforce this with an assert and
> > mark the section as NOLOAD. This is more sensitive than just blindly
> > discarding the section.
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  arch/x86/kernel/vmlinux.lds.S | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> > index 0cc035cb15f1..7faffe7414d6 100644
> > --- a/arch/x86/kernel/vmlinux.lds.S
> > +++ b/arch/x86/kernel/vmlinux.lds.S
> > @@ -414,8 +414,20 @@ SECTIONS
> >  	ELF_DETAILS
> >  
> >  	DISCARDS
> > -}
> >  
> > +	/*
> > +	 * Make sure that the .got.plt is either completely empty or it
> > +	 * contains only the lazy dispatch entries.
> > +	 */
> > +	.got.plt (NOLOAD) : { *(.got.plt) }
> > +	ASSERT(SIZEOF(.got.plt) == 0 ||
> > +#ifdef CONFIG_X86_64
> > +	       SIZEOF(.got.plt) == 0x18,
> > +#else
> > +	       SIZEOF(.got.plt) == 0xc,
> > +#endif
> > +	       "Unexpected GOT/PLT entries detected!")
> > +}
> >  
> >  #ifdef CONFIG_X86_32
> >  /*
> > -- 
> > 2.25.1
> > 
> 
> Is this actually needed? vmlinux is a position-dependent executable, and
> it doesn't get linked with any shared libraries, so it should never have
> a .got or .got.plt at all I think? Does it show up as an orphan without
> this?

When I switched from DISCARD to 0-assert, I tested all of these, but given
so many combinations, perhaps I made a mistake. I will double-check and
report back.

-- 
Kees Cook