Linux Virtualization - Jun 2020 - [PATCH v3 35/75] x86/head/64: Build k/head64.c with -fno-stack-protector

On Tue, Apr 28, 2020 at 11:28 AM Joerg Roedel <joro at 8bytes.org>
wrote:
>
> From: Joerg Roedel <jroedel at suse.de>
>
> The code inserted by the stack protector does not work in the early
> boot environment because it uses the GS segment, at least with memory
> encryption enabled. Make sure the early code is compiled without this
> feature enabled.
>
> Signed-off-by: Joerg Roedel <jroedel at suse.de>
> ---
>  arch/x86/kernel/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index ba89cabe5fcf..1192de38fa56 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -35,6 +35,10 @@ ifdef CONFIG_FRAME_POINTER
>  OBJECT_FILES_NON_STANDARD_ftrace_$(BITS).o             := y
>  endif
>
> +# make sure head64.c is built without stack protector
> +nostackp := $(call cc-option, -fno-stack-protector)
> +CFLAGS_head64.o                := $(nostackp)
> +
>  # If instrumentation of this dir is enabled, boot hangs during first
second.
>  # Probably could be more selective here, but note that files related to
irqs,
>  # boot, dumpstack/stacktrace, etc are either non-interesting or can lead
to
The proper fix would be to initialize MSR_GS_BASE earlier.

--
Brian Gerst

On Tue, May 19, 2020 at 09:58:18AM -0400, Brian Gerst
wrote:
> On Tue, Apr 28, 2020 at 11:28 AM Joerg Roedel <joro at 8bytes.org>
wrote:
> The proper fix would be to initialize MSR_GS_BASE earlier.
That'll mean to initialize it two times during boot, as the first C
function with stack-protection is called before the kernel switches to
its high addresses (early_idt_setup call-path). But okay, I can do that.

On the other side, which value does the stack protector have in the early
boot code?


	Joerg

On Wed, Jun 3, 2020 at 11:18 AM Joerg Roedel <joro at 8bytes.org>
wrote:
>
> On Tue, May 19, 2020 at 09:58:18AM -0400, Brian Gerst wrote:
> > On Tue, Apr 28, 2020 at 11:28 AM Joerg Roedel <joro at
8bytes.org> wrote:
>
> > The proper fix would be to initialize MSR_GS_BASE earlier.
>
> That'll mean to initialize it two times during boot, as the first C
> function with stack-protection is called before the kernel switches to
> its high addresses (early_idt_setup call-path). But okay, I can do that.
Good point.  Since this is boot code which isn't subject to stack
smashing attacks, disabling stack protector is probably the simpler
option.

--
Brian Gerst