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SubjectRe: [PATCH v4 1/7] fs: Add user namesapace member to struct super_block
Seth Forshee <seth.forshee@canonical.com> writes:

> On Thu, Sep 24, 2015 at 04:14:33PM -0500, Eric W. Biederman wrote:
>> Seth Forshee <seth.forshee@canonical.com> writes:
>>
>> > Initially this will be used to eliminate the implicit MNT_NODEV
>> > flag for mounts from user namespaces. In the future it will also
>> > be used for translating ids and checking capabilities for
>> > filesystems mounted from user namespaces.
>> >
>> > s_user_ns is initialized in alloc_super() and is generally set to
>> > current_user_ns(). To avoid security and corruption issues, two
>> > additional mount checks are also added:
>> >
>> > - do_new_mount() gains a check that the user has CAP_SYS_ADMIN
>> > in current_user_ns().
>> >
>> > - sget() will fail with EBUSY when the filesystem it's looking
>> > for is already mounted from another user namespace.
>> >
>> > proc requires some special handling. The user namespace of
>> > current isn't appropriate when forking as a result of clone (2)
>> > with CLONE_NEWPID|CLONE_NEWUSER, as it will set s_user_ns to the
>> > namespace of the parent and make proc unmountable in the new user
>> > namespace. Instead, the user namespace which owns the new pid
>> > namespace is used. sget_userns() is allowed to allow passing in
>> > a namespace other than that of current, and sget becomes a
>> > wrapper around sget_userns() which passes current_user_ns().
>>
>> Minor nits below. I have fixed them up.
>>
>> > Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
>> > ---
>> > fs/namespace.c | 3 +++
>> > fs/proc/root.c | 3 ++-
>> > fs/super.c | 38 +++++++++++++++++++++++++++++++++-----
>> > include/linux/fs.h | 9 ++++++++-
>> > 4 files changed, 46 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/fs/namespace.c b/fs/namespace.c
>> > index 0570729c87fd..d023a353dc63 100644
>> > --- a/fs/namespace.c
>> > +++ b/fs/namespace.c
>> > @@ -2381,6 +2381,9 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
>> > struct vfsmount *mnt;
>> > int err;
>> >
>> > + if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
>> > + return -EPERM;
>> > +
>> > if (!fstype)
>> > return -EINVAL;
>> >
>> > diff --git a/fs/proc/root.c b/fs/proc/root.c
>> > index 361ab4ee42fc..4b302cbf13f9 100644
>> > --- a/fs/proc/root.c
>> > +++ b/fs/proc/root.c
>> > @@ -117,7 +117,8 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
>> > return ERR_PTR(-EPERM);
>> > }
>> >
>> > - sb = sget(fs_type, proc_test_super, proc_set_super, flags, ns);
>> > + sb = sget_userns(fs_type, proc_test_super, proc_set_super, flags,
>> > + ns->user_ns, ns);
>> > if (IS_ERR(sb))
>> > return ERR_CAST(sb);
>> >
>> > diff --git a/fs/super.c b/fs/super.c
>> > index 954aeb80e202..42837da7d641 100644
>> > --- a/fs/super.c
>> > +++ b/fs/super.c
>> > @@ -33,6 +33,7 @@
>> > #include <linux/cleancache.h>
>> > #include <linux/fsnotify.h>
>> > #include <linux/lockdep.h>
>> > +#include <linux/user_namespace.h>
>> > #include "internal.h"
>> >
>> >
>> > @@ -163,6 +164,7 @@ static void destroy_super(struct super_block *s)
>> > {
>> > list_lru_destroy(&s->s_dentry_lru);
>> > list_lru_destroy(&s->s_inode_lru);
>> > + put_user_ns(s->s_user_ns);
>> > security_sb_free(s);
>> > WARN_ON(!list_empty(&s->s_mounts));
>> > kfree(s->s_subtype);
>> > @@ -178,7 +180,8 @@ static void destroy_super(struct super_block *s)
>> > * Allocates and initializes a new &struct super_block. alloc_super()
>> > * returns a pointer new superblock or %NULL if allocation had failed.
>> > */
>> > -static struct super_block *alloc_super(struct file_system_type *type, int flags)
>> > +static struct super_block *alloc_super(struct file_system_type *type, int flags,
>> > + struct user_namespace *user_ns)
>> > {
>> > struct super_block *s = kzalloc(sizeof(struct super_block), GFP_USER);
>> > static const struct super_operations default_op;
>> > @@ -246,6 +249,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
>> > s->s_shrink.count_objects = super_cache_count;
>> > s->s_shrink.batch = 1024;
>> > s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
>> > +
>> > + s->s_user_ns = get_user_ns(user_ns);
>> > return s;
>> >
>> > fail:
>> > @@ -442,17 +447,17 @@ void generic_shutdown_super(struct super_block *sb)
>> > EXPORT_SYMBOL(generic_shutdown_super);
>> >
>> > /**
>> > - * sget - find or create a superblock
>> > + * sget_userns - find or create a superblock
>> > * @type: filesystem type superblock should belong to
>> > * @test: comparison callback
>> > * @set: setup callback
>> > * @flags: mount flags
>>
>> You don't mention the user namespace parameter here. I have fixed that
>> as.
>>
>> + * @user_ns: User namespace you need CAP_SYS_ADMIN over to mount this fs.
>
> Looks good, thanks. Seems I also missed it in alloc_super though.

FYI I have placed everything that I has made it through my review in my
for-testing branch up on kernel.org. So you can see what I have merged,
and the build test bots can look and see if they find anything to
complain about.

git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing

Eric



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