linux-kernel - Re: [PATCH] tty: replace capable() with file_ns

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Message-ID: <20250617130431.50f761dc@pumpkin>
Date: Tue, 17 Jun 2025 13:04:31 +0100
From: David Laight <david.laight.linux@...il.com>
To: Pranav Tyagi <pranav.tyagi03@...il.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, kees@...nel.org,
 skhan@...uxfoundation.org, linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev
Subject: Re: [PATCH] tty: replace capable() with file_ns_capable()

On Sat,  7 Jun 2025 19:11:14 +0530
Pranav Tyagi <pranav.tyagi03@...il.com> wrote:

> The TIOCCONS ioctl currently uses capable(CAP_SYS_ADMIN) to check for
> privileges, which validates the current task's credentials. Since this
> ioctl acts on an open file descriptor, the check should instead use the
> file opener's credentials.

Is that right?
A terminal will have been opened before the login sequence changed the user id.

The 'best practise' might be to check both!

	David

> 
> Replace capable() with file_ns_capable() to ensure the capability is
> checked against file->f_cred in the correct user namespace. This
> prevents unintended privilege escalation and aligns with best practices
> for secure ioctl implementations.
> 
> Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>
> Link: https://github.com/KSPP/linux/issues/156
> ---
>  drivers/tty/tty_io.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e2d92cf70eb7..ee0df35d65c3 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -102,6 +102,9 @@
>  #include <linux/uaccess.h>
>  #include <linux/termios_internal.h>
>  #include <linux/fs.h>
> +#include <linux/cred.h>
> +#include <linux/user_namespace.h>
> +#include <linux/capability.h>
>  
>  #include <linux/kbd_kern.h>
>  #include <linux/vt_kern.h>
> @@ -2379,7 +2382,7 @@ static int tiocswinsz(struct tty_struct *tty, struct winsize __user *arg)
>   */
>  static int tioccons(struct file *file)
>  {
> -	if (!capable(CAP_SYS_ADMIN))
> +	if (!file_ns_capable(file, file->f_cred->user_ns, CAP_SYS_ADMIN))
>  		return -EPERM;
>  	if (file->f_op->write_iter == redirected_tty_write) {
>  		struct file *f;