linux-hardening - Re: [PATCH v2] x86/tdx: replace deprecated strncpy with strtomem

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Date: Wed, 7 Feb 2024 16:03:35 +0200
From: Andy Shevchenko <andy@...ck.fi.intel.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Justin Stitt <justinstitt@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] x86/tdx: replace deprecated strncpy with strtomem_pad

On Wed, Oct 04, 2023 at 09:32:54AM +0200, Ingo Molnar wrote:

...

> > Note: Ingo Molnar has some concerns about the comment being out of sync
> > [1] but I believe the comment still has a place as we can still
> > theoretically copy 64 bytes into our destination buffer without a
> > NUL-byte. The extra information about the 65th byte being NUL may serve
> > helpful to future travelers of this code. What do we think? I can drop
> > the comment in a v3 if needed.
> 
> >  	/* VMM assumes '\0' in byte 65, if the message took all 64 bytes */
> > -	strncpy(message.str, msg, 64);
> > +	strtomem_pad(message.str, msg, '\0');
> 
> My concern was that with the old code it was obvious that the size
> of message.str was 64 bytes - but I judged this based on the
> patch context alone, which seemingly lost context due to the change.
> 
> In reality it's easy to see it when reading the code, because the
> length definition is right before the code:
> 
>         union {
>                 /* Define register order according to the GHCI */
>                 struct { u64 r14, r15, rbx, rdi, rsi, r8, r9, rdx; };
> 
>                 char str[64];
>                 ^^^^^^^^^^^^^
>         } message;
> 
>         /* VMM assumes '\0' in byte 65, if the message took all 64 bytes */
>         strtomem_pad(message.str, msg, '\0');

This comment and size of union seems not in agreement.
How does the previous code work if message indeed takes 64 bytes?
By luck?

-- 
With Best Regards,
Andy Shevchenko