Re: [PATCH] tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN

["Günther Noack" <gnoack@google.com>]

FYI, I have tested this locally - it makes the mouse curser show up again in
Emacs.  (I do also believe that is it fine security-wise, but will also very
happily add a Reviewed-By or Tested-By from Hanno or others in the next
iteration :))

—Günther

On Mon, Dec 16, 2024 at 03:07:23PM +0000, Günther Noack wrote:
> With this, processes without CAP_SYS_ADMIN are able to use TIOCLINUX with
> subcode TIOCL_SETSEL, in the selection modes TIOCL_SETPOINTER,
> TIOCL_SELCLEAR and TIOCL_SELMOUSEREPORT.
> 
> TIOCL_SETSEL was previously changed to require CAP_SYS_ADMIN, as this IOCTL
> let callers change the selection buffer and could be used to simulate
> keypresses.  These three TIOCL_SETSEL selection modes, however, are safe to
> use, as they do not modify the selection buffer.
> 
> This fixes a mouse support regression that affected Emacs (invisible mouse
> cursor).
> 
> Link: https://lore.kernel.org/r/ee3ec63269b43b34e1c90dd8c9743bf8@finder.org
> Fixes: 8d1b43f6a6df ("tty: Restrict access to TIOCLINUX' copy-and-paste subcommands")
> Signed-off-by: Günther Noack <gnoack@google.com>
> ---
>  drivers/tty/vt/selection.c | 14 ++++++++++++++
>  drivers/tty/vt/vt.c        |  3 +--
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
> index 564341f1a74f..0bd6544e30a6 100644
> --- a/drivers/tty/vt/selection.c
> +++ b/drivers/tty/vt/selection.c
> @@ -192,6 +192,20 @@ int set_selection_user(const struct tiocl_selection __user *sel,
>  	if (copy_from_user(&v, sel, sizeof(*sel)))
>  		return -EFAULT;
>  
> +	/*
> +	 * TIOCL_SELCLEAR, TIOCL_SELPOINTER and TIOCL_SELMOUSEREPORT are OK to
> +	 * use without CAP_SYS_ADMIN as they do not modify the selection.
> +	 */
> +	switch (v.sel_mode) {
> +	case TIOCL_SELCLEAR:
> +	case TIOCL_SELPOINTER:
> +	case TIOCL_SELMOUSEREPORT:
> +		break;
> +	default:
> +		if (!capable(CAP_SYS_ADMIN))
> +			return -EPERM;
> +	}
> +
>  	return set_selection_kernel(&v, tty);
>  }
>  
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 96842ce817af..ed65b3b80fbd 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3345,8 +3345,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
>  
>  	switch (type) {
>  	case TIOCL_SETSEL:
> -		if (!capable(CAP_SYS_ADMIN))
> -			return -EPERM;
> +		/* CAP_SYS_ADMIN check happens in set_selection_user(). */
>  		return set_selection_user(param, tty);
>  	case TIOCL_PASTESEL:
>  		if (!capable(CAP_SYS_ADMIN))
> -- 
> 2.47.1.613.gc27f4b7a9f-goog
>