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

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Date:   Mon, 7 Sep 2020 21:17:50 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Serge Hallyn <serge@...lyn.com>,
        Andrei Vagin <avagin@...il.com>,
        Ákos Uzonyi <uzonyi.akos@...il.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-man@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/nsfs.c: fix ioctl support of compat processes

Hi,

On Fri, Jul 24, 2020 at 02:31:19PM -0500, Eric W. Biederman wrote:
> Michael,
> 
> As the original author of NS_GET_OWNER_UID can you take a look at this?

This is a gentle reminder that my patch hasn't been applied,
the problem reported by Ákos Uzonyi hasn't been fixed,
and the example in ioctl_ns(2) manual page doesn't work
when e.g. it's compiled with -m32 on a 64-bit kernel.

> "Dmitry V. Levin" <ldv@...linux.org> writes:
> > 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 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@...il.com>
> >> > Fixes: 6786741dbf99 ("nsfs: add ioctl to get an owning user namespace for ns file descriptor")
> >> > Cc: stable@...r.kernel.org # v4.9+
> >> > Signed-off-by: Dmitry V. Levin <ldv@...linux.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)
> 
> Thank you,
> Eric

-- 
ldv