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Date: Wed, 10 Apr 2024 11:33:50 -0700
From: Kees Cook <keescook@chromium.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: "Martin K . Petersen" <martin.petersen@oracle.com>,
	Justin Stitt <justinstitt@google.com>,
	Andy Shevchenko <andy@kernel.org>, linux-hardening@vger.kernel.org,
	Charles Bertsch <cbertsch@cox.net>,
	Bart Van Assche <bvanassche@acm.org>,
	Sathya Prakash <sathya.prakash@broadcom.com>,
	Sreekanth Reddy <sreekanth.reddy@broadcom.com>,
	Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>,
	"James E.J. Bottomley" <jejb@linux.ibm.com>,
	Kashyap Desai <kashyap.desai@broadcom.com>,
	Sumit Saxena <sumit.saxena@broadcom.com>,
	Nilesh Javali <njavali@marvell.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Himanshu Madhani <himanshu.madhani@oracle.com>,
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	GR-QLogic-Storage-Upstream@marvell.com
Subject: Re: [PATCH 1/5] string.h: Introduce memtostr() and memtostr_pad()
Message-ID: <202404101132.7D845EF323@keescook>
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On Wed, Apr 10, 2024 at 07:08:10AM +0300, Andy Shevchenko wrote:
> On Wed, Apr 10, 2024 at 5:31 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > Another ambiguous use of strncpy() is to copy from strings that may not
> > be NUL-terminated. These cases depend on having the destination buffer
> > be explicitly larger than the source buffer's maximum size, having
> > the size of the copy exactly match the source buffer's maximum size,
> > and for the destination buffer to get explicitly NUL terminated.
> >
> > This usually happens when parsing protocols or hardware character arrays
> > that are not guaranteed to be NUL-terminated. The code pattern is
> > effectively this:
> >
> >         char dest[sizeof(src) + 1];
> >
> >         strncpy(dest, src, sizeof(src));
> >         dest[sizeof(dest) - 1] = '\0';
> >
> > In practice it usually looks like:
> >
> > struct from_hardware {
> >         ...
> >         char name[HW_NAME_SIZE] __nonstring;
> >         ...
> > };
> >
> >         struct from_hardware *p = ...;
> >         char name[HW_NAME_SIZE + 1];
> >
> >         strncpy(name, p->name, HW_NAME_SIZE);
> >         name[NW_NAME_SIZE] = '\0';
> >
> > This cannot be replaced with:
> >
> >         strscpy(name, p->name, sizeof(name));
> >
> > because p->name is smaller and not NUL-terminated, so FORTIFY will
> > trigger when strnlen(p->name, sizeof(name)) is used. And it cannot be
> > replaced with:
> >
> >         strscpy(name, p->name, sizeof(p->name));
> >
> > because then "name" may contain a 1 character early truncation of
> > p->name.
> >
> > Provide an unambiguous interface for converting a maybe not-NUL-terminated
> > string to a NUL-terminated string, with compile-time buffer size checking
> > so that it can never fail at runtime: memtostr() and memtostr_pad(). Also
> > add KUnit tests for both.
> 
> Obvious question, why can't strscpy() be fixed for this corner case?

We would lose the ability to detect normal out-of-bounds reads, or at
least make them ambiguous. I really want these APIs to have distinct and
dependable semantics/behaviors.

-- 
Kees Cook