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

Laurent Vivier
2019-12-16T09:54:05Z

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?

Thanks,
Laurent