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

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Date:   Mon, 2 Aug 2021 09:13:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Len Baker <len.baker@....com>,
        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, Aug 01, 2021 at 09:55:28AM -0700, Joe Perches wrote:
> 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));				\
> })

I'm wondering, instead, if we could convert strcpy() into this instead
of adding another API? I.e. convert all the places that warn (if this
were strcpy), and then land the conversion.

-- 
Kees Cook