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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Borislav Petkov <bp@alien8.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Jakub Jelinek <jakub@redhat.com>,
	Sergei Trofimovich <slyfox@gentoo.org>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org
Subject: Re: [PATCH] x86: fix early boot crash on gcc-10
Date: Mon, 16 Mar 2020 15:53:41 -0400
Message-ID: <20200316195340.GA768497@rani.riverdale.lan> (raw)
In-Reply-To: <20200316185418.GA372474@rani.riverdale.lan>

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

  reply index

Thread overview: 79+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-14 16:44 Sergei Trofimovich
2020-03-16 13:04 ` Peter Zijlstra
2020-03-16 13:26   ` Jakub Jelinek
2020-03-16 13:42     ` Peter Zijlstra
2020-03-16 17:54       ` Borislav Petkov
2020-03-16 18:03         ` Jakub Jelinek
2020-03-17 14:36           ` Borislav Petkov
2020-03-17 14:39             ` Jakub Jelinek
2020-03-17 14:49               ` Borislav Petkov
2020-03-17 16:35                 ` David Laight
2020-03-25 13:31                 ` Borislav Petkov
2020-03-26 21:54                   ` Sergei Trofimovich
2020-03-26 22:35                     ` Borislav Petkov
2020-03-28  8:48                       ` [PATCH v2] " Sergei Trofimovich
2020-04-13 14:15                         ` [tip: x86/urgent] x86: Fix " tip-bot2 for Sergei Trofimovich
2020-04-13 16:35                         ` [PATCH v2] x86: fix " Borislav Petkov
2020-04-14 13:50                           ` Michael Matz
2020-04-15  7:48                             ` Borislav Petkov
2020-04-15 14:53                               ` Michael Matz
2020-04-15 22:19                                 ` Sergei Trofimovich
2020-04-17  7:57                                   ` Borislav Petkov
2020-04-17  8:07                                     ` Jakub Jelinek
2020-04-17  8:42                                       ` Borislav Petkov
2020-04-17  8:58                                         ` Jakub Jelinek
2020-04-17  9:09                                           ` Borislav Petkov
2020-04-17 18:15                                             ` Nick Desaulniers
2020-04-17 18:22                                               ` Nick Desaulniers
2020-04-17 19:06                                                 ` Jakub Jelinek
2020-04-17 19:49                                                   ` Nick Desaulniers
2020-04-17 19:53                                                     ` Nick Desaulniers
2020-04-20 14:04                                                     ` Michael Matz
2020-04-22 10:23                                                       ` Borislav Petkov
2020-04-22 11:40                                                         ` Peter Zijlstra
2020-04-22 13:49                                                           ` Borislav Petkov
2020-04-22 13:55                                                             ` Jakub Jelinek
2020-04-22 14:16                                                               ` Martin Liška
2020-04-22 15:06                                                                 ` Michael Matz
2020-04-22 16:53                                                                 ` Borislav Petkov
2020-04-22 17:02                                                                   ` Jakub Jelinek
2020-04-22 18:47                                                                   ` Nick Desaulniers
2020-04-22 18:55                                                         ` Nick Desaulniers
2020-04-22 19:21                                                           ` Borislav Petkov
2020-04-22 21:05                                                             ` Nick Desaulniers
2020-04-22 21:26                                                               ` Borislav Petkov
2020-04-22 22:57                                                                 ` Nick Desaulniers
2020-04-23 12:53                                                                   ` Borislav Petkov
2020-04-23 16:12                                                                     ` [PATCH] x86: Fix early boot crash on gcc-10, next try Borislav Petkov
2020-04-23 17:30                                                                       ` Borislav Petkov
2020-04-23 18:02                                                                         ` Nick Desaulniers
2020-04-23 18:27                                                                           ` Borislav Petkov
2020-04-27 11:37                                                                         ` [tip: x86/build] x86/build: Check whether the compiler is sane tip-bot2 for Borislav Petkov
2020-04-23 19:40                                                                       ` [PATCH] x86: Fix early boot crash on gcc-10, next try Kees Cook
2020-04-25  1:46                                                                       ` Arvind Sankar
2020-04-25  8:57                                                                         ` Borislav Petkov
2020-04-25 11:09                                                                           ` Jürgen Groß
2020-04-25 15:04                                                                           ` Arvind Sankar
2020-04-25 17:31                                                                             ` Borislav Petkov
2020-04-25 17:52                                                                               ` Borislav Petkov
2020-04-27 17:07                                                                                 ` David Laight
2020-04-25 18:37                                                                               ` Segher Boessenkool
2020-04-25 18:53                                                                                 ` Borislav Petkov
2020-04-25 19:15                                                                                   ` Segher Boessenkool
2020-04-25 22:17                                                                                     ` Borislav Petkov
2020-04-25 22:25                                                                                     ` Arvind Sankar
2020-04-17 10:38                                           ` [PATCH v2] x86: fix early boot crash on gcc-10 Peter Zijlstra
2020-04-18 13:12                                             ` David Laight
2020-04-17 10:41                                     ` Peter Zijlstra
2020-03-16 18:20         ` [PATCH] " Arvind Sankar
2020-03-16 18:54           ` Arvind Sankar
2020-03-16 19:53             ` Arvind Sankar [this message]
2020-03-16 20:08               ` Jakub Jelinek
2020-03-16 20:40                 ` Arvind Sankar
2020-03-16 22:12     ` Sergei Trofimovich
2020-03-17 11:46       ` Jakub Jelinek
2020-03-17 18:10         ` Sergei Trofimovich
2020-03-16 18:22 ` Arvind Sankar
2020-03-26 23:16 ` [PATCH v2] " Sergei Trofimovich
2020-04-27 11:37 ` [tip: x86/build] x86: Fix early boot crash on gcc-10, next try tip-bot2 for Borislav Petkov
2020-05-15 11:20 ` [tip: x86/urgent] x86: Fix early boot crash on gcc-10, third try tip-bot2 for Borislav Petkov

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