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

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Message-ID: <878sf8ogl4.fsf@x220.int.ebiederm.org>
Date:   Fri, 24 Jul 2020 14:31:19 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Michael Kerrisk (man-pages) <mtk.manpages@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: Re: [PATCH] fs/nsfs.c: fix ioctl support of compat processes


Michael,

As the original author of NS_GET_OWNER_UID can you take a look at this?

"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 <ldv@...linux.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@...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