[PATCH 02/34] vfs: syscall: Add move_mount(2) to move mounts around [ver #12]

From: David Howells
Date: Fri Sep 21 2018 - 12:30:32 EST

Add a move_mount() system call that will move a mount from one place to
another and, in the next commit, allow to attach an unattached mount tree.

The new system call looks like the following:

int move_mount(int from_dfd, const char *from_path,
int to_dfd, const char *to_path,
unsigned int flags);

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: linux-api@xxxxxxxxxxxxxxx
---

arch/x86/entry/syscalls/syscall_32.tbl | 1
arch/x86/entry/syscalls/syscall_64.tbl | 1
fs/namespace.c | 102 ++++++++++++++++++++++++++------
include/linux/lsm_hooks.h | 6 ++
include/linux/security.h | 7 ++
include/linux/syscalls.h | 3 +
include/uapi/linux/mount.h | 11 +++
security/security.c | 5 ++
8 files changed, 118 insertions(+), 18 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index ea1b413afd47..76d092b7d1b0 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -399,3 +399,4 @@
385 i386 io_pgetevents sys_io_pgetevents __ia32_compat_sys_io_pgetevents
386 i386 rseq sys_rseq __ia32_sys_rseq
387 i386 open_tree sys_open_tree __ia32_sys_open_tree
+388 i386 move_mount sys_move_mount __ia32_sys_move_mount
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 0545bed581dc..37ba4e65eee6 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -344,6 +344,7 @@
333 common io_pgetevents __x64_sys_io_pgetevents
334 common rseq __x64_sys_rseq
335 common open_tree __x64_sys_open_tree
+336 common move_mount __x64_sys_move_mount

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/namespace.c b/fs/namespace.c
index ded1a970ec40..dd38141b1723 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2386,43 +2386,37 @@ static inline int tree_contains_unbindable(struct mount *mnt)
return 0;
}

-static int do_move_mount(struct path *path, const char *old_name)
+static int do_move_mount(struct path *old_path, struct path *new_path)
{
- struct path old_path, parent_path;
+ struct path parent_path = {.mnt = NULL, .dentry = NULL};
struct mount *p;
struct mount *old;
struct mountpoint *mp;
int err;
- if (!old_name || !*old_name)
- return -EINVAL;
- err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
- if (err)
- return err;

- mp = lock_mount(path);
+ mp = lock_mount(new_path);
err = PTR_ERR(mp);
if (IS_ERR(mp))
goto out;

- old = real_mount(old_path.mnt);
- p = real_mount(path->mnt);
+ old = real_mount(old_path->mnt);
+ p = real_mount(new_path->mnt);

err = -EINVAL;
if (!check_mnt(p) || !check_mnt(old))
goto out1;

- if (old->mnt.mnt_flags & MNT_LOCKED)
+ if (!mnt_has_parent(old))
goto out1;

- err = -EINVAL;
- if (old_path.dentry != old_path.mnt->mnt_root)
+ if (old->mnt.mnt_flags & MNT_LOCKED)
goto out1;

- if (!mnt_has_parent(old))
+ if (old_path->dentry != old_path->mnt->mnt_root)
goto out1;

- if (d_is_dir(path->dentry) !=
- d_is_dir(old_path.dentry))
+ if (d_is_dir(new_path->dentry) !=
+ d_is_dir(old_path->dentry))
goto out1;
/*
* Don't move a mount residing in a shared parent.
@@ -2440,7 +2434,8 @@ static int do_move_mount(struct path *path, const char *old_name)
if (p == old)
goto out1;

- err = attach_recursive_mnt(old, real_mount(path->mnt), mp, &parent_path);
+ err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
+ &parent_path);
if (err)
goto out1;

@@ -2452,6 +2447,22 @@ static int do_move_mount(struct path *path, const char *old_name)
out:
if (!err)
path_put(&parent_path);
+ return err;
+}
+
+static int do_move_mount_old(struct path *path, const char *old_name)
+{
+ struct path old_path;
+ int err;
+
+ if (!old_name || !*old_name)
+ return -EINVAL;
+
+ err = kern_path(old_name, LOOKUP_FOLLOW, &old_path);
+ if (err)
+ return err;
+
+ err = do_move_mount(&old_path, path);
path_put(&old_path);
return err;
}
@@ -2873,7 +2884,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
retval = do_change_type(&path, flags);
else if (flags & MS_MOVE)
- retval = do_move_mount(&path, dev_name);
+ retval = do_move_mount_old(&path, dev_name);
else
retval = do_new_mount(&path, type_page, sb_flags, mnt_flags,
dev_name, data_page, data_size);
@@ -3108,6 +3119,61 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
return ksys_mount(dev_name, dir_name, type, flags, data);
}

+/*
+ * Move a mount from one place to another.
+ *
+ * Note the flags value is a combination of MOVE_MOUNT_* flags.
+ */
+SYSCALL_DEFINE5(move_mount,
+ int, from_dfd, const char *, from_pathname,
+ int, to_dfd, const char *, to_pathname,
+ unsigned int, flags)
+{
+ struct path from_path, to_path;
+ unsigned int lflags;
+ int ret = 0;
+
+ if (!may_mount())
+ return -EPERM;
+
+ if (flags & ~MOVE_MOUNT__MASK)
+ return -EINVAL;
+
+ /* If someone gives a pathname, they aren't permitted to move
+ * from an fd that requires unmount as we can't get at the flag
+ * to clear it afterwards.
+ */
+ lflags = 0;
+ if (flags & MOVE_MOUNT_F_SYMLINKS) lflags |= LOOKUP_FOLLOW;
+ if (flags & MOVE_MOUNT_F_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
+ if (flags & MOVE_MOUNT_F_EMPTY_PATH) lflags |= LOOKUP_EMPTY;
+
+ ret = user_path_at(from_dfd, from_pathname, lflags, &from_path);
+ if (ret < 0)
+ return ret;
+
+ lflags = 0;
+ if (flags & MOVE_MOUNT_T_SYMLINKS) lflags |= LOOKUP_FOLLOW;
+ if (flags & MOVE_MOUNT_T_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
+ if (flags & MOVE_MOUNT_T_EMPTY_PATH) lflags |= LOOKUP_EMPTY;
+
+ ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
+ if (ret < 0)
+ goto out_from;
+
+ ret = security_move_mount(&from_path, &to_path);
+ if (ret < 0)
+ goto out_to;
+
+ ret = do_move_mount(&from_path, &to_path);
+
+out_to:
+ path_put(&to_path);
+out_from:
+ path_put(&from_path);
+ return ret;
+}
+
/*
* Return true if path is reachable from root
*
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 8a44075347ac..d052db1a1565 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -147,6 +147,10 @@
* Parse a string of security data filling in the opts structure
* @options string containing all mount options known by the LSM
* @opts binary data structure usable by the LSM
+ * @move_mount:
+ * Check permission before a mount is moved.
+ * @from_path indicates the mount that is going to be moved.
+ * @to_path indicates the mountpoint that will be mounted upon.
* @dentry_init_security:
* Compute a context for a dentry as the inode is not yet available
* since NFSv4 has no label backed by an EA anyway.
@@ -1484,6 +1488,7 @@ union security_list_options {
unsigned long kern_flags,
unsigned long *set_kern_flags);
int (*sb_parse_opts_str)(char *options, struct security_mnt_opts *opts);
+ int (*move_mount)(const struct path *from_path, const struct path *to_path);
int (*dentry_init_security)(struct dentry *dentry, int mode,
const struct qstr *name, void **ctx,
u32 *ctxlen);
@@ -1816,6 +1821,7 @@ struct security_hook_heads {
struct hlist_head sb_set_mnt_opts;
struct hlist_head sb_clone_mnt_opts;
struct hlist_head sb_parse_opts_str;
+ struct hlist_head move_mount;
struct hlist_head dentry_init_security;
struct hlist_head dentry_create_files_as;
#ifdef CONFIG_SECURITY_PATH
diff --git a/include/linux/security.h b/include/linux/security.h
index 30a3db9f284b..a306061d2197 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -266,6 +266,7 @@ int security_sb_clone_mnt_opts(const struct super_block *oldsb,
unsigned long kern_flags,
unsigned long *set_kern_flags);
int security_sb_parse_opts_str(char *options, struct security_mnt_opts *opts);
+int security_move_mount(const struct path *from_path, const struct path *to_path);
int security_dentry_init_security(struct dentry *dentry, int mode,
const struct qstr *name, void **ctx,
u32 *ctxlen);
@@ -621,6 +622,12 @@ static inline int security_sb_parse_opts_str(char *options, struct security_mnt_
return 0;
}

+static inline int security_move_mount(const struct path *from_path,
+ const struct path *to_path)
+{
+ return 0;
+}
+
static inline int security_inode_alloc(struct inode *inode)
{
return 0;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 6978f3c76d41..79042396f7e5 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -907,6 +907,9 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
int flags, uint32_t sig);
asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
+asmlinkage long sys_move_mount(int from_dfd, const char __user *from_path,
+ int to_dfd, const char __user *to_path,
+ unsigned int ms_flags);

/*
* Architecture-specific system calls
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index e8db2911adca..89adf0d731ab 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -7,4 +7,15 @@
#define OPEN_TREE_CLONE 1 /* Clone the target tree and attach the clone */
#define OPEN_TREE_CLOEXEC O_CLOEXEC /* Close the file on execve() */

+/*
+ * move_mount() flags.
+ */
+#define MOVE_MOUNT_F_SYMLINKS 0x00000001 /* Follow symlinks on from path */
+#define MOVE_MOUNT_F_AUTOMOUNTS 0x00000002 /* Follow automounts on from path */
+#define MOVE_MOUNT_F_EMPTY_PATH 0x00000004 /* Empty from path permitted */
+#define MOVE_MOUNT_T_SYMLINKS 0x00000010 /* Follow symlinks on to path */
+#define MOVE_MOUNT_T_AUTOMOUNTS 0x00000020 /* Follow automounts on to path */
+#define MOVE_MOUNT_T_EMPTY_PATH 0x00000040 /* Empty to path permitted */
+#define MOVE_MOUNT__MASK 0x00000077
+
#endif /* _UAPI_LINUX_MOUNT_H */
diff --git a/security/security.c b/security/security.c
index 3d99ed8d9ddd..96a061cecb39 100644
--- a/security/security.c
+++ b/security/security.c
@@ -444,6 +444,11 @@ int security_sb_parse_opts_str(char *options, struct security_mnt_opts *opts)
}
EXPORT_SYMBOL(security_sb_parse_opts_str);

+int security_move_mount(const struct path *from_path, const struct path *to_path)
+{
+ return call_int_hook(move_mount, 0, from_path, to_path);
+}
+
int security_inode_alloc(struct inode *inode)
{
inode->i_security = NULL;