Re: [PATCH] x86: fix early boot crash on gcc-10

[Arvind Sankar]

On Mon, Mar 16, 2020 at 02:54:21PM -0400, Arvind Sankar wrote:
> On Mon, Mar 16, 2020 at 02:20:00PM -0400, Arvind Sankar wrote:
> > On Mon, Mar 16, 2020 at 06:54:50PM +0100, Borislav Petkov wrote:
> > > On Mon, Mar 16, 2020 at 02:42:34PM +0100, Peter Zijlstra wrote:
> > > > Right I know, I looked for it recently :/ But since this is new in 10
> > > > and 10 isn't released yet, I figured someone can add the attribute
> > > > before it does get released.
> > > 
> > > Yes, that would be a good solution.
> > > 
> > > I looked at what happens briefly after building gcc10 from git and IINM,
> > > the function in question - start_secondary() - already gets the stack
> > > canary asm glue added so it checks for a stack canary.
> > > 
> > > However, the stack canary value itself gets set later in that same
> > > function:
> > > 
> > >         /* to prevent fake stack check failure in clock setup */
> > >         boot_init_stack_canary();
> > > 
> > > so the asm glue which checks for it would need to reload the newly
> > > computed canary value (it is 0 before we compute it and thus the
> > > mismatch).
> > > 
> > > So having a way to state "do not add stack canary checking to this
> > > particular function" would be optimal. And since you already have the
> > > "stack_protect" function attribute I figure adding a "no_stack_protect"
> > > one should be easy...
> > > 
> > > > > Or of course you could add noinline attribute to whatever got inlined
> > > > > and contains some array or addressable variable that whatever
> > > > > -fstack-protector* mode kernel uses triggers it.  With -fstack-protector-all
> > > > > it would never work even in the past I believe.
> > > > 
> > > > I don't think the kernel supports -fstack-protector-all, but I could be
> > > > mistaken.
> > > 
> > > The other thing I was thinking was to carve out only that function into
> > > a separate compilation unit and disable stack protector only for it.
> > > 
> > > All IMHO of course.
> > > 
> > > Thx.
> > > 
> > > -- 
> > > Regards/Gruss,
> > >     Boris.
> > > 
> > > https://people.kernel.org/tglx/notes-about-netiquette
> > 
> > With STACKPROTECTOR_STRONG, gcc9 (at least gentoo's version, not sure if
> > they have some patches that affect it) already adds stack canary into
> > start_secondary. Not sure why it doesn't panic already with gcc9?
> > 
> > 00000000000008f0 <start_secondary>:
> >      8f0:       53                      push   %rbx
> >      8f1:       48 83 ec 10             sub    $0x10,%rsp
> >      8f5:       65 48 8b 04 25 28 00    mov    %gs:0x28,%rax
> >      8fc:       00 00
> >      8fe:       48 89 44 24 08          mov    %rax,0x8(%rsp)
> >      903:       31 c0                   xor    %eax,%eax
> > ...
> >      a2e:       e8 00 00 00 00          callq  a33 <start_secondary+0x143>
> >                         a2f: R_X86_64_PLT32     cpu_startup_entry-0x4
> >      a33:       48 8b 44 24 08          mov    0x8(%rsp),%rax
> >      a38:       65 48 33 04 25 28 00    xor    %gs:0x28,%rax
> >      a3f:       00 00 
> >      a41:       75 12                   jne    a55 <start_secondary+0x165>
> >      a43:       48 83 c4 10             add    $0x10,%rsp
> >      a47:       5b                      pop    %rbx
> >      a48:       c3                      retq   
> >      a49:       0f 01 1d 00 00 00 00    lidt   0x0(%rip)        # a50 <start_secondary+0x160>
> >                         a4c: R_X86_64_PC32      debug_idt_descr-0x4
> >      a50:       e9 cb fe ff ff          jmpq   920 <start_secondary+0x30>
> >      a55:       e8 00 00 00 00          callq  a5a <start_secondary+0x16a>
> >                         a56: R_X86_64_PLT32     __stack_chk_fail-0x4
> 
> Wait a sec, this function calls cpu_startup_entry as the last thing it
> does, which should never return, and hence the stack canary value should
> never get checked.
> 
> How does the canary get checked with the gcc10 code?
> 
> boot_init_stack_canary depends on working if called from functions that
> don't return. If that doesn't work with gcc10, we need to disable stack
> protector for the other callpoints too -- start_kernel in init/main.c
> and cpu_bringup_and_idle in arch/x86/xen/smp_pv.c.
> 
> /*
>  * Initialize the stackprotector canary value.
>  *
>  * NOTE: this must only be called from functions that never return,
>  * and it must always be inlined.
>  */
> static __always_inline void boot_init_stack_canary(void)

Ugh, gcc10 tail-call optimizes the cpu_startup_entry call, and so checks
the canary before jumping out. The xen one will need to have stack
protector disabled too. It doesn't optimize the arch_call_rest_init call
in start_kernel for some reason, but we should probably disable it there
too.

     a06:       0f ae f8                sfence
     a09:       48 8b 44 24 08          mov    0x8(%rsp),%rax
     a0e:       65 48 2b 04 25 28 00    sub    %gs:0x28,%rax
     a15:       00 00
     a17:       75 1b                   jne    a34 <start_secondary+0x164>
     a19:       48 83 c4 10             add    $0x10,%rsp
     a1d:       bf 8d 00 00 00          mov    $0x8d,%edi
     a22:       5b                      pop    %rbx
     a23:       e9 00 00 00 00          jmpq   a28 <start_secondary+0x158>
                        a24: R_X86_64_PLT32     cpu_startup_entry-0x4
     a28:       0f 01 1d 00 00 00 00    lidt   0x0(%rip)        # a2f <start_secondary+0x15f>
                        a2b: R_X86_64_PC32      debug_idt_descr-0x4
     a2f:       e9 cc fe ff ff          jmpq   900 <start_secondary+0x30>
     a34:       e8 00 00 00 00          callq  a39 <start_secondary+0x169>
                        a35: R_X86_64_PLT32     __stack_chk_fail-0x4