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Date:   Wed, 26 Feb 2020 02:13:50 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Steven Rostedt <rostedt@goodmis.org>,
        Brian Gerst <brgerst@gmail.com>,
        Juergen Gross <jgross@suse.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [patch 02/10] x86/mce: Disable tracing and kprobes on
 do_machine_check()
Message-ID: <20200226011349.GH9599@lenoir>
References: <20200225213636.689276920@linutronix.de>
 <20200225220216.315548935@linutronix.de>
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On Tue, Feb 25, 2020 at 10:36:38PM +0100, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@kernel.org>
> 
> do_machine_check() can be raised in almost any context including the most
> fragile ones. Prevent kprobes and tracing.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  arch/x86/include/asm/traps.h   |    3 ---
>  arch/x86/kernel/cpu/mce/core.c |   12 ++++++++++--
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -88,9 +88,6 @@ dotraplinkage void do_page_fault(struct
>  dotraplinkage void do_spurious_interrupt_bug(struct pt_regs *regs, long error_code);
>  dotraplinkage void do_coprocessor_error(struct pt_regs *regs, long error_code);
>  dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code);
> -#ifdef CONFIG_X86_MCE
> -dotraplinkage void do_machine_check(struct pt_regs *regs, long error_code);
> -#endif
>  dotraplinkage void do_simd_coprocessor_error(struct pt_regs *regs, long error_code);
>  #ifdef CONFIG_X86_32
>  dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code);
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -1213,8 +1213,14 @@ static void __mc_scan_banks(struct mce *
>   * On Intel systems this is entered on all CPUs in parallel through
>   * MCE broadcast. However some CPUs might be broken beyond repair,
>   * so be always careful when synchronizing with others.
> + *
> + * Tracing and kprobes are disabled: if we interrupted a kernel context
> + * with IF=1, we need to minimize stack usage.  There are also recursion
> + * issues: if the machine check was due to a failure of the memory
> + * backing the user stack, tracing that reads the user stack will cause
> + * potentially infinite recursion.
>   */
> -void do_machine_check(struct pt_regs *regs, long error_code)
> +void notrace do_machine_check(struct pt_regs *regs, long error_code)
>  {
>  	DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
>  	DECLARE_BITMAP(toclear, MAX_NR_BANKS);
> @@ -1360,6 +1366,7 @@ void do_machine_check(struct pt_regs *re
>  	ist_exit(regs);
>  }
>  EXPORT_SYMBOL_GPL(do_machine_check);
> +NOKPROBE_SYMBOL(do_machine_check);

That won't protect all the function called by do_machine_check(), right?
There are lots of them.