linux-kernel - Re: [RESEND PATCH v2 4/4] x86/umip: Warn if UMIP-protected instructions are used

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Date:   Tue, 14 Nov 2017 08:34:08 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        ricardo.neri@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2 4/4] x86/umip: Warn if UMIP-protected
 instructions are used


* Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:

> +const char * const umip_insns[5] = {
> +	[UMIP_INST_SGDT] = "sgdt",
> +	[UMIP_INST_SIDT] = "sidt",
> +	[UMIP_INST_SMSW] = "smsw",
> +	[UMIP_INST_SLDT] = "sldt",
> +	[UMIP_INST_STR] = "str",
> +};

Sigh ...

> +/*
> + * If you change these strings, ensure that buffers using them are sufficiently
> + * large.
> + */
> +static const char umip_warn_use[] = "cannot be used by applications.";
> +static const char umip_warn_emu[] = "For now, expensive software emulation returns result.";

Please use the string literals directly, don't add an extra obfuscation layer.

Plus:

> +	unsigned char buf[MAX_INSN_SIZE], warn[128];

> +	snprintf(warn, sizeof(warn), "%s %s", umip_insns[umip_inst],
> +		 umip_warn_use);

This is incredibly fragile against future buffer overflows, and warning about it 
in comments does not make it less fragile!

Thanks,

	Ingo