On Thu, Sep 24, 2015 at 04:14:33PM -0500, Eric W. Biederman wrote:
> Seth Forshee <seth.forshee@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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.
Seth
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