linux-hardening - Re: [PATCH] HID: uhid: refactor deprecated strncpy

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Date:   Thu, 14 Sep 2023 22:13:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     David Rheinsberg <david@...dahead.eu>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        David Herrmann <dh.herrmann@...il.com>
Subject: Re: [PATCH] HID: uhid: refactor deprecated strncpy

On Thu, Sep 14, 2023 at 10:47:21PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We should prefer more robust and less ambiguous string interfaces.
> 
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on the destination buffer without
> unnecessarily NUL-padding.
> 
> Looking at: Commit 4d26d1d1e806 ("Revert "HID: uhid: use strlcpy() instead of strncpy()"")
> we see referenced the fact that many attempts have been made to change
> these strncpy's into strlcpy to no success. I think strscpy is an
> objectively better interface here as it doesn't unnecessarily NUL-pad
> the destination buffer whilst allowing us to drop the `len = min(...)`
> pattern as strscpy will implicitly limit the number of bytes copied by
> the smaller of its dest and src arguments.

I think the worry here is that ev->u.create2.name may not be %NUL
terminated. But I think that doesn't matter, see below...

Also, note, this should CC David Herrmann <dh.herrmann@...il.com>
(now added).

> So while the existing code may not have a bug (i.e: overread problems)
> we should still favor strscpy due to readability (plus a very slight
> performance boost).
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
>  drivers/hid/uhid.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 4588d2cd4ea4..00e1566ad37b 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -490,7 +490,7 @@ static int uhid_dev_create2(struct uhid_device *uhid,
>  			    const struct uhid_event *ev)
>  {
>  	struct hid_device *hid;
> -	size_t rd_size, len;
> +	size_t rd_size;
>  	void *rd_data;
>  	int ret;
>  
> @@ -514,13 +514,9 @@ static int uhid_dev_create2(struct uhid_device *uhid,
>  		goto err_free;
>  	}
>  
> -	/* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
> -	len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
> -	strncpy(hid->name, ev->u.create2.name, len);
> -	len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
> -	strncpy(hid->phys, ev->u.create2.phys, len);
> -	len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
> -	strncpy(hid->uniq, ev->u.create2.uniq, len);

ev->u.create2 is:
struct uhid_create2_req {
        __u8 name[128];
        __u8 phys[64];
        __u8 uniq[64];
	...

hid is:
struct hid_device { /* device report descriptor */
	...
        char name[128]; /* Device name */
        char phys[64]; /* Device physical location */
        char uniq[64]; /* Device unique identifier (serial #) */

So these "min" calls are redundant -- it wants to copy at most 1 less so
it can be %NUL terminated. Which is what strscpy() already does. And
source and dest are the same size, so we can't over-read source if it
weren't terminated (since strscpy won't overread like strlcpy).

> +	strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
> +	strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
> +	strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

>  
>  	hid->ll_driver = &uhid_hid_driver;
>  	hid->bus = ev->u.create2.bus;
> 
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-strncpy-drivers-hid-uhid-c-a465f3e784dd
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook