Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled

[Peter Zijlstra]

On Tue, Sep 06, 2022 at 11:00:07PM -0700, Nick Desaulniers wrote:
> On Sun, Sep 4, 2022 at 11:02 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
> >
> > On Sat, Sep 3, 2022 at 12:18 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Sep 02, 2022 at 09:37:50PM +0000, Bill Wendling wrote:
> > > > [...]
> > > >         callq   *pv_ops+536(%rip)
> > >
> > > Do you know which pv_ops function is this? I can't figure out where
> > > pte_offset_kernel() gets converted into a pv_ops call....
> > >
> > This one is _paravirt_ident_64, I believe. I think that the original
> > issue Nathan was seeing was with another seemingly innocuous function.
> 
> _paravirt_ident_64 is marked noinstr, which makes me suspect that it
> really needs to not be touched at all by the compiler for
> these...special features.

My source tree sayeth:

  u64 notrace _paravirt_ident_64(u64 x)

And that function is only ever called at boot, after alternatives runs
it's patched with:

  mov %_ASM_ARG1, %_ASM_AX

Anyway, if you want to take it away from the compiler, something like
so should do.


diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 7ca2d46c08cc..8922e2887779 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -80,11 +80,16 @@ static unsigned paravirt_patch_call(void *insn_buff, const void *target,
 }
 
 #ifdef CONFIG_PARAVIRT_XXL
-/* identity function, which can be inlined */
-u64 notrace _paravirt_ident_64(u64 x)
-{
-	return x;
-}
+extern u64 _paravirt_ident_64(u64 x);
+asm (".pushsection .entry.text, \"ax\"\n"
+     ".global _paravirt_ident_64\n"
+     "_paravirt_ident_64:\n\t"
+     ASM_ENDBR
+     "mov %" _ASM_ARG1 ", %" _ASM_AX "\n\t"
+     ASM_RET
+     ".size _paravirt_ident_64, . - _paravirt_ident_64\n\t"
+     ".type _paravirt_ident_64, @function\n\t"
+     ".popsection");
 #endif
 
 DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);