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 18:56:03 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On Tue, Nov 14, 2017 at 08:34:08AM +0100, Ingo Molnar wrote:
> 
> * 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 ...

It made sense to me to capitalize code and changelogs but keep the printed warnings
in lower case. Thus, the format would look like:

    umip: smsw instruction cannot be used by applications.

I do see that this could be confusing with STR. I will capitalize these as well.
> 
> > +/*
> > + * 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!

I need to concatenate the instruction mnemonic with the a string. Does something like
this is more acceptable?

	unsigned char warn[50];

	...

	strcpy(warn, umip_insns[umip_inst]);
	strcat(warn, " instruction cannot be used by applications.");
	umip_pr_warn(regs, warn, 0);

In this manner I use the string literal directly but I still have a buffer that might
overflow. Code looks more clear to me. I could #defines for the string lengths or
set a maximum length.

Thanks and BR,
Ricardo