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

Laurent Vivier
2019-12-16T10:09:39Z

Le 16/12/2019 à 11:06, Christian Brauner a écrit :
> 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.
>

OK, thank you, I will try to add some tests here.

Thanks,
Laurent