linux-hardening - Re: [PATCH] drivers/input: Remove all strcpy() uses in favor of strscpy()

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Date:   Sun, 01 Aug 2021 09:55:28 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Len Baker <len.baker@....com>
Cc:     Kees Cook <keescook@...omium.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-hardening@...r.kernel.org,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/input: Remove all strcpy() uses in favor of
 strscpy()

On Sun, 2021-08-01 at 09:39 -0700, Joe Perches wrote:
> On Sun, 2021-08-01 at 16:00 +0100, Russell King (Oracle) wrote:
> > On Sun, Aug 01, 2021 at 04:43:16PM +0200, Len Baker wrote:
> > > strcpy() performs no bounds checking on the destination buffer. This
> > > could result in linear overflows beyond the end of the buffer, leading
> > > to all kinds of misbehaviors. The safe replacement is strscpy().
[]
> > So if the string doesn't fit, it's fine to silently truncate it?
> > 
> > Rather than converting every single strcpy() in the kernel to
> > strscpy(), maybe there should be some consideration given to how the
> > issue of a strcpy() that overflows the buffer should be handled.
> > E.g. in the case of a known string such as the above, if it's longer
> > than the destination, should we find a way to make the compiler issue
> > a warning at compile time?

(apologies for the earlier blank reply, sometimes I dislike my email client)

stracpy could do that with a trivial addition like below:

Old lkml references:

https://lore.kernel.org/lkml/cover.1563889130.git.joe@perches.com/
and
https://lore.kernel.org/lkml/56dc4de7e0db153cb10954ac251cb6c27c33da4a.camel@perches.com/

But Linus T wants a copy_string mechanism instead:
https://lore.kernel.org/lkml/CAHk-=wgqQKoAnhmhGE-2PBFt7oQs9LLAATKbYa573UO=DPBE0Q@mail.gmail.com/

/**
 * stracpy - Copy a C-string into an array of char/u8/s8 or equivalent
 * @dest: Where to copy the string, must be an array of char and not a pointer
 * @src: String to copy, may be a pointer or const char array
 *
 * Helper for strscpy().
 * Copies a maximum of sizeof(@dest) bytes of @src with %NUL termination.
 *
 * A BUILD_BUG_ON is used for cases where @dest is not a char array or
 * @src is a char array and is larger than @dest.
 *
 * Returns:
 * * The number of characters copied (not including the trailing %NUL)
 * * -E2BIG if @dest is a zero size array or @src was truncated.
 */
#define stracpy(dest, src)						\
({									\
	BUILD_BUG_ON(!(__same_type(dest, char[]) ||			\
		       __same_type(dest, unsigned char[]) ||		\
		       __same_type(dest, signed char[])));		\
	BUILD_BUG_ON((__same_type(src, char[]) ||			\
		      __same_type(src, unsigned char[]) ||		\
		      __same_type(src, signed char[])) &&		\
		     ARRAY_SIZE(src) > ARRAY_SIZE(dest));		\
									\
	strscpy(dest, src, ARRAY_SIZE(dest));				\
})