Re: [PATCH] arch: x86: power: cpu: init %gs before __restore_processor_state (clang)

[Arvind Sankar <nivedita@alum.mit.edu>]

On Tue, Sep 15, 2020 at 11:00:30AM -0700, Nick Desaulniers wrote:
> On Tue, Sep 15, 2020 at 10:46 AM Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Tue, Sep 15, 2020 at 10:26:58AM -0700, rkir@google.com wrote:
> > > From: Haitao Shan <hshan@google.com>
> > >
> > > This is a workaround which fixes triple fault
> > > in __restore_processor_state on clang when
> > > built with LTO.
> > >
> > > When load_TR_desc and load_mm_ldt are inlined into
> > > fix_processor_context due to LTO, they cause

Does this apply to load_TR_desc()? That is an inline function even
without LTO, no?

> > > fix_processor_context (or in this case __restore_processor_state,
> > > as fix_processor_context was inlined into __restore_processor_state)
> > > to access the stack canary through %gs, but before
> > > __restore_processor_state has restored the previous value
> > > of %gs properly. LLVM appears to be inlining functions with stack
> > > protectors into functions compiled with -fno-stack-protector,
> > > which is likely a bug in LLVM's inliner that needs to be fixed.
> > >
> > > The LLVM bug is here: https://bugs.llvm.org/show_bug.cgi?id=47479
> > >
> > > Signed-off-by: Haitao Shan <hshan@google.com>
> > > Signed-off-by: Roman Kiryanov <rkir@google.com>
> >
> > Ok, google guys, pls make sure you Cc LKML too as this is where *all*
> > patches and discussions are archived. Adding it now to Cc.
> 
> Roman, please use ./scripts/get_maintainer.pl (in the kernel tree) for that.
> 
> >
> > > ---
> > >  arch/x86/power/cpu.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > >
> > > diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> > > index db1378c6ff26..e5677adb2d28 100644
> > > --- a/arch/x86/power/cpu.c
> > > +++ b/arch/x86/power/cpu.c
> > > @@ -274,6 +274,16 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> > >  /* Needed by apm.c */
> > >  void notrace restore_processor_state(void)
> > >  {
> > > +#ifdef __clang__
> 
> Should be CONFIG_CC_IS_CLANG; is more canonical throughout the tree.
> Or if this is only a bug when doing builds with LTO, and LTO is not
> yet upstream, then maybe Sami should carry this in his series, at
> least until I can fix the bug in Clang.  Or guard this with the
> CONFIG_LTO_CLANG config (not upstream yet; see Sami's series).
> 
> > > +     // The following code snippet is copied from __restore_processor_state.
> > > +     // Its purpose is to prepare GS segment before the function is called.
> > > +#ifdef CONFIG_X86_64
> > > +     wrmsrl(MSR_GS_BASE, saved_context.kernelmode_gs_base);
> > > +#else
> > > +     loadsegment(fs, __KERNEL_PERCPU);
> > > +     loadsegment(gs, __KERNEL_STACK_CANARY);
> > > +#endif
> > > +#endif
> >
> > Ok, so why is the kernel supposed to take yet another ugly workaround
> > because there's a bug in the compiler?
> 
> This is exactly the same code from __restore_processor_state.  If it's
> ugly, talk to the author of 7ee18d677989e. ;)  All this patch is doing
> is moving this up a call frame (though now this is effectively being
> run twice).
> 

Possibly dumb question: why does this fix anything? Won't
__restore_processor_state(), which is a static function with only one
caller, in turn get inlined into restore_processor_state(), so that
restore_processor_state() will also have stack protection enabled, and
the canary will be accessed before the MSR or segment register is
loaded?

Thanks.