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From: Arnd Bergmann <arnd@arndb.de>
To: "Dmitry V. Levin" <ldv@altlinux.org>
Cc: "Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Serge Hallyn" <serge@hallyn.com>,
	"Andrei Vagin" <avagin@gmail.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Ákos Uzonyi" <uzonyi.akos@gmail.com>,
	"Linux FS-devel Mailing List" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fs/nsfs.c: fix ioctl support of compat processes
Date: Fri, 24 Jul 2020 11:20:26 +0200	[thread overview]
Message-ID: <CAK8P3a0JM8dytW6C8P9HoPcGksg0d5JCut1yT7JzBcUCAm-WcQ@mail.gmail.com> (raw)
In-Reply-To: <20200724001248.GC25522@altlinux.org>

On Fri, Jul 24, 2020 at 2:12 AM Dmitry V. Levin <ldv@altlinux.org> wrote:
>
> According to Documentation/driver-api/ioctl.rst, in order to support
> 32-bit user space running on a 64-bit kernel, each subsystem or driver
> that implements an ioctl callback handler must also implement the
> corresponding compat_ioctl handler.  The compat_ptr_ioctl() helper can
> be used in place of a custom compat_ioctl file operation for drivers
> that only take arguments that are pointers to compatible data
> structures.
>
> In case of NS_* ioctls only NS_GET_OWNER_UID accepts an argument, and
> this argument is a pointer to uid_t type, which is universally defined
> to __kernel_uid32_t.

This is potentially dangerous to rely on, as there are two parts that
are mismatched:

- user space does not see the kernel's uid_t definition, but has its own,
  which may be either the 16-bit or the 32-bit type. 32-bit uid_t was
  introduced with linux-2.3.39 in back in 2000. glibc was already
  using 32-bit uid_t at the time in user space, but uclibc only changed
  in 2003, and others may have been even later.

- the ioctl command number is defined (incorrectly) as if there was no
  argument, so if there is any user space that happens to be built with
  a 16-bit uid_t, this does not get caught.

       Arnd

> Reported-by: Ákos Uzonyi <uzonyi.akos@gmail.com>
> Fixes: 6786741dbf99 ("nsfs: add ioctl to get an owning user namespace for ns file descriptor")
> Cc: stable@vger.kernel.org # v4.9+
> Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
> ---
>  fs/nsfs.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 800c1d0eb0d0..a00236bffa2c 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -21,6 +21,7 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
>  static const struct file_operations ns_file_operations = {
>         .llseek         = no_llseek,
>         .unlocked_ioctl = ns_ioctl,
> +       .compat_ioctl   = compat_ptr_ioctl,
>  };
>
>  static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)

  reply	other threads:[~2020-07-24  9:20 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24  0:12 Dmitry V. Levin
2020-07-24  9:20 ` Arnd Bergmann [this message]
2020-07-24 10:28   ` Dmitry V. Levin
2020-07-24 19:31     ` Eric W. Biederman
2020-09-07 18:17       ` Dmitry V. Levin

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