Re: [PATCH Part1 v5 23/38] x86/head/64: set up a startup %gs for stack protector

[Michael Roth]

On Wed, Aug 25, 2021 at 04:29:13PM +0200, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 10:19:18AM -0500, Brijesh Singh wrote:
> > From: Michael Roth <michael.roth@xxxxxxx>
> > 
> > As of commit 103a4908ad4d ("x86/head/64: Disable stack protection for
> > head$(BITS).o") kernel/head64.c is compiled with -fno-stack-protector
> > to allow a call to set_bringup_idt_handler(), which would otherwise
> > have stack protection enabled with CONFIG_STACKPROTECTOR_STRONG. While
> > sufficient for that case, this will still cause issues if we attempt to
> > call out to any external functions that were compiled with stack
> > protection enabled that in-turn make stack-protected calls, or if the
> > exception handlers set up by set_bringup_idt_handler() make calls to
> > stack-protected functions.
> > 
> > Subsequent patches for SEV-SNP CPUID validation support will introduce
> > both such cases. Attempting to disable stack protection for everything
> > in scope to address that is prohibitive since much of the code, like
> > SEV-ES #VC handler, is shared code that remains in use after boot and
> > could benefit from having stack protection enabled. Attempting to inline
> > calls is brittle and can quickly balloon out to library/helper code
> > where that's not really an option.
> > 
> > Instead, set up %gs to point a buffer that stack protector can use for
> > canary values when needed.
> > 
> > In doing so, it's likely we can stop using -no-stack-protector for
> > head64.c, but that hasn't been tested yet, and head32.c would need a
> > similar solution to be safe, so that is left as a potential follow-up.
> 
> That...

Argh! I had this fixed up but I think it got clobbered in the patch
shuffle. I'll make sure to fix this, and remember to actually test
without CONFIG_STACKPTROTECTOR this time. Sorry for the screw-up.

> 
> > Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > ---
> >  arch/x86/kernel/Makefile |  2 +-
> >  arch/x86/kernel/head64.c | 20 ++++++++++++++++++++
> >  2 files changed, 21 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> > index 3e625c61f008..5abdfd0dbbc3 100644
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -46,7 +46,7 @@ endif
> >  # non-deterministic coverage.
> >  KCOV_INSTRUMENT		:= n
> >  
> > -CFLAGS_head$(BITS).o	+= -fno-stack-protector
> > +CFLAGS_head32.o		+= -fno-stack-protector
> 
> ... and that needs to be taken care of too.

I didn't realize the the 32-bit path was something you were suggesting
to have added in this patch, but I'll take a look at that as well.