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From: Ard Biesheuvel <ardb@kernel.org>
Date: Wed, 31 Jan 2024 15:07:50 +0100
Message-ID: <CAMj1kXGk1QivyoK4H5CVp7p-tfYoQSTuethpkm8bWBcTO_UMGw@mail.gmail.com>
Subject: Re: [PATCH v3 03/19] x86/startup_64: Drop long return to initial_code pointer
To: Borislav Petkov <bp@alien8.de>
Cc: Ard Biesheuvel <ardb+git@google.com>, linux-kernel@vger.kernel.org, 
	Kevin Loughlin <kevinloughlin@google.com>, Tom Lendacky <thomas.lendacky@amd.com>, 
	Dionna Glaze <dionnaglaze@google.com>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, 
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>, Nathan Chancellor <nathan@kernel.org>, 
	Nick Desaulniers <ndesaulniers@google.com>, Justin Stitt <justinstitt@google.com>, 
	Kees Cook <keescook@chromium.org>, Brian Gerst <brgerst@gmail.com>, linux-arch@vger.kernel.org, 
	llvm@lists.linux.dev
Content-Type: text/plain; charset="UTF-8"

On Wed, 31 Jan 2024 at 14:57, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Wed, 31 Jan 2024 at 14:45, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Mon, Jan 29, 2024 at 07:05:06PM +0100, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@kernel.org>
> > >
> > > Since commit 866b556efa12 ("x86/head/64: Install startup GDT"), the
> > > primary startup sequence sets the code segment register (CS) to __KERNEL_CS
> > > before calling into the startup code shared between primary and
> > > secondary boot.
> > >
> > > This means a simple indirect call is sufficient here.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/x86/kernel/head_64.S | 35 ++------------------
> > >  1 file changed, 3 insertions(+), 32 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> > > index d4918d03efb4..4017a49d7b76 100644
> > > --- a/arch/x86/kernel/head_64.S
> > > +++ b/arch/x86/kernel/head_64.S
> > > @@ -428,39 +428,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> > >       movq    %r15, %rdi
> > >
> > >  .Ljump_to_C_code:
> > > -     /*
> > > -      * Jump to run C code and to be on a real kernel address.
> > > -      * Since we are running on identity-mapped space we have to jump
> > > -      * to the full 64bit address, this is only possible as indirect
> > > -      * jump.  In addition we need to ensure %cs is set so we make this
> > > -      * a far return.
> > > -      *
> > > -      * Note: do not change to far jump indirect with 64bit offset.
> > > -      *
> > > -      * AMD does not support far jump indirect with 64bit offset.
> > > -      * AMD64 Architecture Programmer's Manual, Volume 3: states only
> > > -      *      JMP FAR mem16:16 FF /5 Far jump indirect,
> > > -      *              with the target specified by a far pointer in memory.
> > > -      *      JMP FAR mem16:32 FF /5 Far jump indirect,
> > > -      *              with the target specified by a far pointer in memory.
> > > -      *
> > > -      * Intel64 does support 64bit offset.
> > > -      * Software Developer Manual Vol 2: states:
> > > -      *      FF /5 JMP m16:16 Jump far, absolute indirect,
> > > -      *              address given in m16:16
> > > -      *      FF /5 JMP m16:32 Jump far, absolute indirect,
> > > -      *              address given in m16:32.
> > > -      *      REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
> > > -      *              address given in m16:64.
> > > -      */
> > > -     pushq   $.Lafter_lret   # put return address on stack for unwinder
> > >       xorl    %ebp, %ebp      # clear frame pointer
> > > -     movq    initial_code(%rip), %rax
> > > -     pushq   $__KERNEL_CS    # set correct cs
> > > -     pushq   %rax            # target address in negative space
> > > -     lretq
> > > -.Lafter_lret:
> > > -     ANNOTATE_NOENDBR
> > > +     ANNOTATE_RETPOLINE_SAFE
> > > +     callq   *initial_code(%rip)
> > > +     int3
> > >  SYM_CODE_END(secondary_startup_64)
> > >
> > >  #include "verify_cpu.S"
> >
> > objtool doesn't like it yet:
> >
> > vmlinux.o: warning: objtool: verify_cpu+0x0: stack state mismatch: cfa1=4+8 cfa2=-1+0
> >
> > Once we've solved this, I'll take this one even now - very nice cleanup!
> >
>
> s/int3/RET seems to do the trick.
>

or ud2, even better,