This capability is exposed currently through the proposed new bind
API. To mark a mount for shifting, you add the allow-shift flag to the
properties, either by a reconfigure or a rebind. Only real root on
the system can do this. Once this is done, admin in a user namespace
(i.e. an unprivileged user) can take that mount point and bind it with
a shift in effect.
The way an admin marks a mount is:
pathfd = open("/path/to/shift", O_PATH);
fd = configfd_open("bind", O_CLOEXEC);
configfd_action(fd, CONFIGFD_SET_FD, "pathfd", NULL, pathfd);
configfd_action(fd, CONFIGFD_SET_FLAG, "allow-shift", NULL, 0);
configfd_action(fd, CONFIGFD_SET_FLAG, "detached", NULL, 0);
configfd_action(fd, CONFIGFD_CMD_CREATE, NULL, NULL, 0);
configfd_action(fd, CONFIGFD_GET_FD, "bindfd", &bindfd, O_CLOEXEC);
move_mount(bindfd, "", AT_FDCWD, "/path/to/allow", MOVE_MOUNT_F_EMPTY_PATH);
Technically /path/to/shift and /path/to/allow can be the same, which
basically installs a mnt at the path that allows onward traversal.
Then any mount namespace in a user namespace can do:
pathfd = open("/path/to/allow", O_PATH);
fd = configfd_open("bind", O_CLOEXEC);
configfd_action(fd, CONFIGFD_SET_FD, "pathfd", NULL, pathfd);
configfd_action(fd, CONFIGFD_SET_FLAG, "shift", NULL, 0);
configfd_action(fd, CONFIGFD_SET_FLAG, "detached", NULL, 0);
configfd_action(fd, CONFIGFD_CMD_CREATE, NULL, NULL, 0);
configfd_action(fd, CONFIGFD_GET_FD, "bindfd", &bindfd, O_CLOEXEC);
move_mount(bindfd, "", AT_FDCWD, "/path/to/mount", MOVE_MOUNT_F_EMPTY_PATH);
And /path/to/mount will have the uid/gid shifting bind mount installed.
Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
---
fs/bind.c | 35 +++++++++++++++++++++++++++++++++++
fs/mount.h | 2 ++
fs/namespace.c | 1 +
fs/proc_namespace.c | 4 ++++
4 files changed, 42 insertions(+)
diff --git a/fs/bind.c b/fs/bind.c
index eea4e6cd5108..6b4668041248 100644
--- a/fs/bind.c
+++ b/fs/bind.c
@@ -21,6 +21,8 @@ struct bind_data {
bool nodev:1;
bool detached:1;
bool recursive:1;
+ bool shift:1;
+ bool allow_shift:1;
struct file *file;
struct file *retfile;
};
@@ -66,6 +68,25 @@ static int bind_set_flag(const struct configfd_context *cfc,
bd->nodev = true;
} else if (strcmp(p->key, "noexec") == 0) {
bd->noexec = true;
+ } else if (strcmp(p->key, "shift") == 0) {
+ struct mount *m;
+
+ if (!bd->file) {
+ logger_err(cfc->log, "can't shift without setting pathfd");
+ return -EINVAL;
+ }
+ m = real_mount(bd->file->f_path.mnt);
+ if (!m->allow_shift) {
+ logger_err(cfc->log, "pathfd doesn't allow shifting");
+ return -EINVAL;
+ }
+ bd->shift = true;
+ } else if (strcmp(p->key, "allow-shift") == 0) {
+ if (!capable(CAP_SYS_ADMIN)) {
+ logger_err(cfc->log, "must be root to set allow-shift");
+ return -EPERM;
+ }
+ bd->allow_shift = true;
} else if (strcmp(p->key, "recursive") == 0 &&
cfc->op == CONFIGFD_CMD_CREATE) {
bd->recursive = true;
@@ -126,6 +147,8 @@ static int bind_get_mnt_flags(struct bind_data *bd, int mnt_flags)
mnt_flags |= MNT_NODEV;
if (bd->noexec)
mnt_flags |= MNT_NOEXEC;
+ if (bd->shift)
+ mnt_flags |= MNT_SHIFT;
return mnt_flags;
}
@@ -143,6 +166,13 @@ static int bind_reconfigure(const struct configfd_context *cfc)
mnt_flags = bd->file->f_path.mnt->mnt_flags & MNT_ATIME_MASK;
mnt_flags = bind_get_mnt_flags(bd, mnt_flags);
+ if (bd->allow_shift) {
+ struct mount *m = real_mount(bd->file->f_path.mnt);
+
+ /* FIXME: this should be set with the reconfigure locking */
+ m->allow_shift = true;
+ }
+
return do_reconfigure_mnt(&bd->file->f_path, mnt_flags);
}
@@ -183,6 +213,11 @@ static int bind_create(const struct configfd_context *cfc)
/* since this is a detached copy, we can do without locking */
f->f_path.mnt->mnt_flags |= mnt_flags;
+ if (bd->allow_shift) {
+ struct mount *m = real_mount(f->f_path.mnt);
+
+ m->allow_shift = true;
+ }
}
bd->retfile = f;
diff --git a/fs/mount.h b/fs/mount.h
index 711a4093e475..1da13decf93b 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -72,6 +72,8 @@ struct mount {
int mnt_expiry_mark; /* true if marked for expiry */
struct hlist_head mnt_pins;
struct hlist_head mnt_stuck_children;
+ /* shifting bind mount parameters */
+ bool allow_shift:1;
} __randomize_layout;
#define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
diff --git a/fs/namespace.c b/fs/namespace.c
index 3ae0124e9783..8266e9540595 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1038,6 +1038,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
mnt->mnt.mnt_flags = old->mnt.mnt_flags;
mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);
+ mnt->allow_shift = old->allow_shift;
atomic_inc(&sb->s_active);
mnt->mnt.mnt_sb = sb;
diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
index 273ee82d8aa9..bdf8d23cf42e 100644
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -70,14 +70,18 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
{ MNT_NOATIME, ",noatime" },
{ MNT_NODIRATIME, ",nodiratime" },
{ MNT_RELATIME, ",relatime" },
+ { MNT_SHIFT, ",shift" },
{ 0, NULL }
};
const struct proc_fs_info *fs_infop;
+ struct mount *rm = real_mount(mnt);
for (fs_infop = mnt_info; fs_infop->flag; fs_infop++) {
if (mnt->mnt_flags & fs_infop->flag)
seq_puts(m, fs_infop->str);
}
+ if (rm->allow_shift)
+ seq_puts(m, ",allow-shift");
}
static inline void mangle(struct seq_file *m, const char *s)
--
2.16.4
