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

[Laurent Vivier <laurent@vivier.eu>]

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