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Date:   Wed, 8 Apr 2020 09:27:26 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        hch@infradead.org, sean.j.christopherson@intel.com,
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Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
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        <20200407111007.429362016@infradead.org>
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On Tue, 07 Apr 2020 13:02:40 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> Since we now have infrastructure to analyze module text, disallow
> modules that write to CRn and DRn registers.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/module.c |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -266,6 +266,22 @@ static bool insn_is_vmx(struct insn *ins
>  	return false;
>  }
>  
> +static bool insn_is_mov_CRn(struct insn *insn)
> +{
> +	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x22)
> +		return true;
> +
> +	return false;
> +}
> +
> +static bool insn_is_mov_DRn(struct insn *insn)
> +{
> +	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x23)
> +		return true;
> +
> +	return false;
> +}
> +
>  static int decode_module(struct module *mod, void *text, void *text_end, bool sld_safe)
>  {
>  	bool allow_vmx = sld_safe || !split_lock_enabled();
> @@ -285,6 +301,11 @@ static int decode_module(struct module *
>  			return -ENOEXEC;
>  		}
>  
> +		if (insn_is_mov_CRn(&insn) || insn_is_mov_DRn(&insn)) {
> +			pr_err("Module writes to CRn or DRn, please use the proper accessors: %s\n", mod->name);
> +			return -ENOEXEC;
> +		}
> +

Something like this should be done for all modules, not just out of tree
modules.

-- Steve