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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Mon, 16 Mar 2020 14:20:00 -0400
To:     Borislav Petkov <bp@alien8.de>
Cc:     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
Message-ID: <20200316181957.GA348193@rani.riverdale.lan>
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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