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

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Date:   Wed, 30 Aug 2023 19:57:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Azeem Shaikh <azeemshaikh38@...il.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] vt: Fix potential read overflow of kernel memory

On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote:
> strlcpy() reads the entire source buffer first.
> This read may exceed the destination size limit if
> a source string is not NUL-terminated [1].

But that's not the case here, right?  So your "potential read overflow"
isn't relevant here.

> The copy_to_user() call uses @len returned from strlcpy() directly
> without checking its value. This could potentially lead to read
> overflow.

But can it?  How?

Those are all hard-coded strings, in the kernel source, there is no
potential overflow here.

And you know the buffer size is correct as well.

So why even check?

> 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@...il.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;
> +			break;
> +		}

Don't check for impossible things please.

thanks,

greg k-h