Re: [PATCH] fs/nsfs.c: fix ioctl support of compat processes

["Dmitry V. Levin" <ldv@altlinux.org>]

On Fri, Jul 24, 2020 at 11:20:26AM +0200, Arnd Bergmann wrote:
> 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.

Note that NS_GET_OWNER_UID is provided on 32-bit architectures, too, so
this 16-bit vs 32-bit uid_t issue was exposed to userspace long time ago
when NS_GET_OWNER_UID was introduced, and making NS_GET_OWNER_UID
available for compat processes won't make any difference, as the mismatch
is not between native and compat types, but rather between 16-bit and
32-bit uid_t types.

I agree it would be correct to define NS_GET_OWNER_UID as
_IOR(NSIO, 0x4, uid_t) instead of _IO(NSIO, 0x4), but nobody Cc'ed me
on this topic when NS_GET_OWNER_UID was discussed, and that ship has long
sailed.

> > This change fixes compat strace --pidns-translation.
> > 
> > Note: when backporting this patch to stable kernels, commit
> > "compat_ioctl: add compat_ptr_ioctl()" is needed as well.
> > 
> > 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)

-- 
ldv