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Message-ID: <YyHn+UbfC7e1XIT3@dev-arch.thelio-3990X> Date: Wed, 14 Sep 2022 07:40:57 -0700 From: Nathan Chancellor <nathan@...nel.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Kees Cook <keescook@...omium.org>, Juergen Gross <jgross@...e.com>, "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>, Alexey Makhalov <amakhalov@...are.com>, VMware PV-Drivers Reviewers <pv-drivers@...are.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, virtualization@...ts.linux-foundation.org, LKML <linux-kernel@...r.kernel.org>, clang-built-linux <llvm@...ts.linux.dev>, linux-hardening@...r.kernel.org Subject: Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled On Wed, Sep 07, 2022 at 10:50:03AM +0200, Peter Zijlstra wrote: > 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@...gle.com> wrote: > > > > > > On Sat, Sep 3, 2022 at 12:18 AM Kees Cook <keescook@...omium.org> 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. This appears to work fine for me in QEMU, as I can still boot with CONFIG_ZERO_CALL_USED_REGS and spawn a nested guest without any issues. > 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); >
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