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Date: Mon, 16 Mar 2020 15:53:41 -0400
From: Arvind Sankar <nivedita@...m.mit.edu>
To: Arvind Sankar <nivedita@...m.mit.edu>
Cc: Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <peterz@...radead.org>,
Jakub Jelinek <jakub@...hat.com>,
Sergei Trofimovich <slyfox@...too.org>,
linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Subject: Re: [PATCH] x86: fix early boot crash on gcc-10
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
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