Re: [PATCH v8 0/1] ns: introduce binfmt_misc namespace

[Christian Brauner <christian.brauner@ubuntu.com>]

On Mon, Dec 16, 2019 at 10:53:28AM +0100, Laurent Vivier wrote:
> Le 16/12/2019 à 10:46, Christian Brauner a écrit :
> > On Mon, Dec 16, 2019 at 10:12:19AM +0100, Laurent Vivier wrote:
> >> v8: s/file->f_path.dentry/file_dentry(file)/
> >>
> >> v7: Use the new mount API
> >>
> >>     Replace
> >>
> >>       static struct dentry *bm_mount(struct file_system_type *fs_type,
> >>                             int flags, const char *dev_name, void *data)
> >>       {
> >>                struct user_namespace *ns = current_user_ns();
> >>
> >>                return mount_ns(fs_type, flags, data, ns, ns,
> >>                                bm_fill_super);
> >>       }
> >>
> >>     by
> >>
> >>       static void bm_free(struct fs_context *fc)
> >>       {
> >>              if (fc->s_fs_info)
> >>                      put_user_ns(fc->s_fs_info);
> >>       }
> >>
> >>       static int bm_get_tree(struct fs_context *fc)
> >>       {
> >>               return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
> >>       }
> >>
> >>       static const struct fs_context_operations bm_context_ops = {
> >>               .free           = bm_free,
> >>               .get_tree       = bm_get_tree,
> >>       };
> >>
> >>       static int bm_init_fs_context(struct fs_context *fc)
> >>       {
> >>               fc->ops = &bm_context_ops;
> >>               return 0;
> >>       }
> >>
> >> v6: Return &init_binfmt_ns instead of NULL in binfmt_ns()
> >>     This should never happen, but to stay safe return a
> >>     value we can use.
> >>     change subject from "RFC" to "PATCH"
> >>
> >> v5: Use READ_ONCE()/WRITE_ONCE()
> >>     move mount pointer struct init to bm_fill_super() and add smp_wmb()
> >>     remove useless NULL value init
> >>     add WARN_ON_ONCE()
> >>
> >> v4: first user namespace is initialized with &init_binfmt_ns,
> >>     all new user namespaces are initialized with a NULL and use
> >>     the one of the first parent that is not NULL. The pointer
> >>     is initialized to a valid value the first time the binfmt_misc
> >>     fs is mounted in the current user namespace.
> >>     This allows to not change the way it was working before:
> >>     new ns inherits values from its parent, and if parent value is modified
> >>     (or parent creates its own binfmt entry by mounting the fs) child
> >>     inherits it (unless it has itself mounted the fs).
> >>
> >> v3: create a structure to store binfmt_misc data,
> >>     add a pointer to this structure in the user_namespace structure,
> >>     in init_user_ns structure this pointer points to an init_binfmt_ns
> >>     structure. And all new user namespaces point to this init structure.
> >>     A new binfmt namespace structure is allocated if the binfmt_misc
> >>     filesystem is mounted in a user namespace that is not the initial
> >>     one but its binfmt namespace pointer points to the initial one.
> >>     add override_creds()/revert_creds() around open_exec() in
> >>     bm_register_write()
> >>
> >> v2: no new namespace, binfmt_misc data are now part of
> >>     the mount namespace
> >>     I put this in mount namespace instead of user namespace
> >>     because the mount namespace is already needed and
> >>     I don't want to force to have the user namespace for that.
> >>     As this is a filesystem, it seems logic to have it here.
> >>
> >> This allows to define a new interpreter for each new container.
> >>
> >> But the main goal is to be able to chroot to a directory
> >> using a binfmt_misc interpreter without being root.
> >>
> >> I have a modified version of unshare at:
> >>
> >>   https://github.com/vivier/util-linux.git branch unshare-chroot
> >>
> >> with some new options to unshare binfmt_misc namespace and to chroot
> >> to a directory.
> >>
> >> If you have a directory /chroot/powerpc/jessie containing debian for powerpc
> >> binaries and a qemu-ppc interpreter, you can do for instance:
> >>
> >>  $ uname -a
> >>  Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 x86_64 x86_64 x86_64 GNU/Linux
> >>  $ ./unshare --map-root-user --fork --pid \
> >>    --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/qemu-ppc:OC" \
> >>    --root=/chroot/powerpc/jessie /bin/bash -l
> >>  # uname -a
> >>  Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 ppc GNU/Linux
> >>  # id
> >> uid=0(root) gid=0(root) groups=0(root),65534(nogroup)
> >>  # ls -l
> >> total 5940
> >> drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:58 bin
> >> drwxr-xr-x.   2 nobody nogroup    4096 Jun 17 20:26 boot
> >> drwxr-xr-x.   4 nobody nogroup    4096 Aug 12 00:08 dev
> >> drwxr-xr-x.  42 nobody nogroup    4096 Sep 28 07:25 etc
> >> drwxr-xr-x.   3 nobody nogroup    4096 Sep 28 07:25 home
> >> drwxr-xr-x.   9 nobody nogroup    4096 Aug 12 00:58 lib
> >> drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 media
> >> drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 mnt
> >> drwxr-xr-x.   3 nobody nogroup    4096 Aug 12 13:09 opt
> >> dr-xr-xr-x. 143 nobody nogroup       0 Sep 30 23:02 proc
> >> -rwxr-xr-x.   1 nobody nogroup 6009712 Sep 28 07:22 qemu-ppc
> >> drwx------.   3 nobody nogroup    4096 Aug 12 12:54 root
> >> drwxr-xr-x.   3 nobody nogroup    4096 Aug 12 00:08 run
> >> drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:58 sbin
> >> drwxr-xr-x.   2 nobody nogroup    4096 Aug 12 00:08 srv
> >> drwxr-xr-x.   2 nobody nogroup    4096 Apr  6  2015 sys
> >> drwxrwxrwt.   2 nobody nogroup    4096 Sep 28 10:31 tmp
> >> drwxr-xr-x.  10 nobody nogroup    4096 Aug 12 00:08 usr
> >> drwxr-xr-x.  11 nobody nogroup    4096 Aug 12 00:08 var
> >>
> >> If you want to use the qemu binary provided by your distro, you can use
> >>
> >>     --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/bin/qemu-ppc-static:OCF"
> >>
> >> With the 'F' flag, qemu-ppc-static will be then loaded from the main root
> >> filesystem before switching to the chroot.
> >>
> >> Another example is to use the 'P' flag in one chroot and not in another one (useful in a test
> >> environment to test different configurations of the same interpreter):
> >>
> >> ./unshare --fork --pid --mount-proc --map-root-user --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff://usr/bin/qemu-ppc-noargv0:OCF" --root=/chroot/powerpc/jessie /bin/bash -l
> >> root@localhost:/# sh -c 'echo $0'
> >> /bin/sh
> >>
> >> ./unshare --fork --pid --mount-proc --map-root-user --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff://usr/bin/qemu-ppc-argv0:OCFP" --root=/chroot/powerpc/jessie /bin/bash -l
> >> root@localhost:/# sh -c 'echo $0'
> >> sh
> > 
> > Hey Laurent,
> > 
> > We have quite some time before the v5.6 merge window opens. So I would
> > really like for this new feature to come with proper testing!
> 
> Are there some already existing tests for binfmt_misc or namespace I can
> update to test the new feature?

I don't think so but there are tests for other namespace-aware
filesystem. For example, I've added basic tests for binderfs in
tools/testing/selftests/filesystems/binderfs/ and there are some devpts
tests in there (Though the devpts tests don't actually make use of the
kselftest framework so they aren't a great example. I'm not claiming
binderfs is either tbh. :))

You can just place the binfmt_misc tests in there. Helpers for setting
up user namespace and mappings are in there as well. I think you can
just place them in a separate file/header and include it for both
binderfs and binfmt_misc.
I'm happy to review this/answer questions.

Thanks!
Christian