Re: [PATCH] vt: Fix potential read overflow of kernel memory

[Kees Cook <keescook@chromium.org>]

On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.

Perhaps add:
"... and returns the size of the source string, not the destination
string, which can be accidentally misused."

> This read may exceed the destination size limit if
> a source string is not NUL-terminated [1].
> 
> The copy_to_user() call uses @len returned from strlcpy() directly
> without checking its value. This could potentially lead to read
> overflow.

Since the code as written today is "accidentally correct", it's worth
clarifying this: there is no existing bug, but as written it is very
fragile and specifically uses a strlcpy() result without sanity checking
and using it to copy to userspace. (This is the exact anti-pattern for
strlcpy(), and only because the source strings are known good is this
safe.)

> In an effort to remove strlcpy() completely [2], replace
> strlcpy() here with strscpy().
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> [2] https://github.com/KSPP/linux/issues/89
> 
> Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> ---
>  drivers/tty/vt/keyboard.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index 358f216c6cd6..15359c328a23 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -2079,12 +2079,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
>  			return -ENOMEM;
> 
>  		spin_lock_irqsave(&func_buf_lock, flags);
> -		len = strlcpy(kbs, func_table[kb_func] ? : "", len);
> +		len = strscpy(kbs, func_table[kb_func] ? : "", len);
>  		spin_unlock_irqrestore(&func_buf_lock, flags);
> 
> +		if (len < 0) {
> +			ret = -EFAULT;

I think this should be -ENOSPC as EFAULT implies an actual memory fault.

> +			break;
> +		}
>  		ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
>  			-EFAULT : 0;
> -
>  		break;
>  	}
>  	case KDSKBSENT:
> --
> 2.42.0.rc2.253.gd59a3bf2b4-goog
> 
> 

Thanks for sticking with these refactorings; we're almost free from
strlcpy. :)

-Kees

-- 
Kees Cook