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Date: Fri, 9 Feb 2024 23:19:18 -0800
From: Kees Cook <keescook@chromium.org>
To: Andy Shevchenko <andy@black.fi.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Justin Stitt <justinstitt@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] x86/tdx: replace deprecated strncpy with strtomem_pad
Message-ID: <202402092318.F5C569D7C@keescook>
References: <20231003-strncpy-arch-x86-coco-tdx-tdx-c-v2-1-0bd21174a217@google.com>
 <ZR0VJsgA6g0Wk4dq@gmail.com>
 <ZcONt_he_08batik@black.fi.intel.com>
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In-Reply-To: <ZcONt_he_08batik@black.fi.intel.com>

On Wed, Feb 07, 2024 at 04:03:35PM +0200, Andy Shevchenko wrote:
> 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.

It does agree -- the comment could be more clear.

> How does the previous code work if message indeed takes 64 bytes?
> By luck?

It's saying "the non-existent 65th byte is assumed to be %NUL". As in,
this is treated as a C string, even if it uses all 64 bytes.

-- 
Kees Cook