linux-hardening - Re: [PATCH v2] x86/stackprotector: Work around strict Clang TLS symbol requirements

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Message-ID: <20241016021045.GA1000009@thelio-3990X>
Date: Tue, 15 Oct 2024 19:10:45 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Kees Cook <kees@...nel.org>, Ard Biesheuvel <ardb+git@...gle.com>,
	x86@...nel.org, llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org, stable@...r.kernel.org,
	Fangrui Song <i@...kray.me>, Brian Gerst <brgerst@...il.com>,
	Uros Bizjak <ubizjak@...il.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] x86/stackprotector: Work around strict Clang TLS
 symbol requirements

On Tue, Oct 15, 2024 at 12:56:57PM +0200, Ard Biesheuvel wrote:
> On Mon, 14 Oct 2024 at 22:59, Kees Cook <kees@...nel.org> wrote:
> >
> > On Wed, Oct 09, 2024 at 02:43:53PM +0200, Ard Biesheuvel wrote:
> > > However, if a non-TLS definition of the symbol in question is visible in
> > > the same compilation unit (which amounts to the whole of vmlinux if LTO
> > > is enabled), it will drop the per-CPU prefix and emit a load from a
> > > bogus address.
> >
> > I take this to mean that x86 32-bit kernels built with the stack
> > protector and using Clang LTO will crash very quickly?
> >
> 
> Yeah. The linked issue is not quite clear, but it does suggest things
> are pretty broken in that case.

Yeah, i386_defconfig with CONFIG_LTO_CLANG_FULL=y explodes on boot for
me without this change:

  [    0.000000] Linux version 6.12.0-rc3-00044-g2f87d0916ce0 (nathan@...lio-3990X) (ClangBuiltLinux clang version 19.1.2 (https://github.com/llvm/llvm-project.git 7ba7d8e2f7b6445b60679da826210cdde29eaf8b), ClangBuiltLinux LLD 19.1.2 (https://github.com/llvm/llvm-project.git 7ba7d8e2f7b6445b60679da826210cdde29eaf8b)) #1 SMP PREEMPT_DYNAMIC Tue Oct 15 19:00:21 MST 2024
  ...
  [    0.631002] Freeing unused kernel image (initmem) memory: 936K
  [    0.631613] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: free_initmem+0x95/0x98
  [    0.632606] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.12.0-rc3-00044-g2f87d0916ce0 #1
  [    0.633467] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
  [    0.634583] Call Trace:
  [    0.634837]  panic+0xd4/0x2cc
  [    0.635146]  ? _vdso_rng_data+0xd80/0xd80
  [    0.635550]  ? _vdso_rng_data+0xd80/0xd80
  [    0.635965]  ? rest_init+0xb0/0xb0
  [    0.636312]  __stack_chk_fail+0x10/0x10
  [    0.636701]  ? free_initmem+0x95/0x98
  [    0.637074]  free_initmem+0x95/0x98
  [    0.637434]  ? _vdso_rng_data+0xd80/0xd80
  [    0.637838]  ? rest_init+0xb0/0xb0
  [    0.638196]  kernel_init+0x42/0x1e4
  [    0.638558]  ret_from_fork+0x2b/0x40
  [    0.638922]  ret_from_fork_asm+0x12/0x18
  [    0.639331]  entry_INT80_32+0x108/0x108
  [    0.639864] Kernel Offset: disabled
  [    0.640224] ---[ end Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: free_initmem+0x95/0x98 ]---

I can confirm that this patch resolves that issue for me and LKDTM's
REPORT_STACK_CANARY test passes with that configuration.

Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Tested-by: Nathan Chancellor <nathan@...nel.org>

I presume the '#ifndef CONFIG_X86_64' in arch/x86/entry/entry.S is
present because only X86_32 uses '-mstack-protector-guard-reg='? I
assume that will disappear when X86_64 supports this option (IIRC that
was the plan)?

Cheers,
Nathan