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Message-ID: <CAJcbSZGa2=YLaFxqoZ79h1Gh794DPWpuCArSMirYVbHLYzBu5w@mail.gmail.com>
Date: Fri, 1 Feb 2019 09:11:37 -0800
From: Thomas Garnier <thgarnie@...omium.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>, kristen@...ux.intel.com,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, "the arch/x86 maintainers" <x86@...nel.org>, Mimi Zohar <zohar@...ux.ibm.com>,
Juergen Gross <jgross@...e.com>, Nayna Jain <nayna@...ux.ibm.com>,
Masahiro Yamada <yamada.masahiro@...ionext.com>, Jan Kiszka <jan.kiszka@...mens.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 13/27] x86/boot/64: Build head64.c as mcmodel large
when PIE is enabled
On Fri, Feb 1, 2019 at 3:15 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> On Thu, Jan 31, 2019 at 11:24:20AM -0800, Thomas Garnier wrote:
> > The __startup_64 function assumes all symbols have relocated addresses
> > instead of the current boot virtual address. PIE generated code favor
> > relative addresses making all virtual and physical address math incorrect.
> > If PIE is enabled, build head64.c as mcmodel large instead to ensure absolute
> > references on all memory access. Add a global __force_order variable required
> > when using a large model with read_cr* functions.
> >
> > To build head64.c as mcmodel=large, disable the retpoline gcc flags.
> > This code is used at early boot and removed later, it doesn't need
> > retpoline mitigation.
> >
> > Position Independent Executable (PIE) support will allow to extend the
> > KASLR randomization range below 0xffffffff80000000.
> >
> > Signed-off-by: Thomas Garnier <thgarnie@...omium.org>
> > ---
> > arch/x86/kernel/Makefile | 6 ++++++
> > arch/x86/kernel/head64.c | 3 +++
> > 2 files changed, 9 insertions(+)
> >
> > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> > index 00b7e27bc2b7..1f98f52eab9f 100644
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -22,6 +22,12 @@ CFLAGS_REMOVE_early_printk.o = -pg
> > CFLAGS_REMOVE_head64.o = -pg
> > endif
> >
> > +ifdef CONFIG_X86_PIE
> > +# Remove PIE and retpoline flags that are incompatible with mcmodel=large
> > +CFLAGS_REMOVE_head64.o += -fPIE -mindirect-branch=thunk-extern -mindirect-branch-register
> > +CFLAGS_head64.o = -mcmodel=large
> > +endif
> > +
> > KASAN_SANITIZE_head$(BITS).o := n
> > KASAN_SANITIZE_dumpstack.o := n
> > KASAN_SANITIZE_dumpstack_$(BITS).o := n
> > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> > index 16b1cbd3a61e..22e81275495b 100644
> > --- a/arch/x86/kernel/head64.c
> > +++ b/arch/x86/kernel/head64.c
> > @@ -63,6 +63,9 @@ EXPORT_SYMBOL(vmemmap_base);
> >
> > #define __head __section(.head.text)
> >
> > +/* Required for read_cr3 when building as PIE */
> > +unsigned long __force_order;
> > +
>
> I believe it only needed for GCC < 5. Newer GCC can eliminate the
> reference. See my comment in arch/x86/boot/compressed/pgtable_64.c.
>
> Maybe we should expand the comment here too?
Make sense, I will add a similar comment in the next iteration. Thanks
for pointing that out.
>
> --
> Kirill A. Shutemov
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