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Date:   Sat, 25 Apr 2020 10:57:59 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Michael Matz <matz@suse.de>, Jakub Jelinek <jakub@redhat.com>,
        Sergei Trofimovich <slyfox@gentoo.org>,
        LKML <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>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>,
        Martin =?utf-8?B?TGnFoWth?= <mliska@suse.cz>,
        =?utf-8?Q?Fr=C3=A9d=C3=A9ric_Pierret_=28fepitre=29?= 
        <frederic.pierret@qubes-os.org>, boris.ostrovsky@oracle.com,
        jgross@suse.com
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try
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On Fri, Apr 24, 2020 at 09:46:57PM -0400, Arvind Sankar wrote:
> The comment above boot_init_stack_canary's definition should be updated
> to note that it needs to be called from a function that, in addition to
> not returning, either has stackprotector disabled or avoids ending in a
> tail call.

How's that?

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 91e29b6a86a5..237a54f60d6b 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -55,8 +55,12 @@
 /*
  * Initialize the stackprotector canary value.
  *
- * NOTE: this must only be called from functions that never return,
- * and it must always be inlined.
+ * NOTE: this must only be called from functions that never return, it must
+ * always be inlined and it should be called from a compilation unit for
+ * which stack protector is disabled.
+ *
+ * Alternatively, the caller should not end with a function call which gets
+ * tail-call optimized as that would lead to checking a modified canary value.
  */
 static __always_inline void boot_init_stack_canary(void)
 {

> There are also other calls that likely need to be fixed as well -- in
> init/main.c, arch/x86/xen/smp_pv.c, and there is a powerpc version of
> start_secondary in arch/powerpc/kernel/smp.c which may also be affected.

Yes, there was an attempt to fix former:

https://lkml.kernel.org/r/20200413123535.10884-1-frederic.pierret@qubes-os.org

I probably should point the folks to this thread. CCed.

Boris O, Jürgen, I'm guessing I should fix cpu_bringup_and_idle() too,
see:

https://lkml.kernel.org/r/20200423161126.GD26021@zn.tnic

or do you prefer a separate patch?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette