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Subject[PATCH 08/14] Implement fsmount() to effect a pre-configured mount
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Provide a system call by which a filesystem opened with fsopen() and
configured by a series of writes can be mounted:

int ret = fsmount(int fsfd, int dfd, const char *path);

where fsfd is the fd returned by fsopen(), dfd and path describe the
mountpoint. dfd can be AT_FDCWD or an fd open to a directory.

In the event that fsmount() fails, it may be possible to get an error
message by calling read(). If no message is available, ENODATA will be
reported.

Signed-off-by: David Howells <dhowells@redhat.com>
---

arch/x86/entry/syscalls/syscall_32.tbl | 1
arch/x86/entry/syscalls/syscall_64.tbl | 1
fs/namespace.c | 87 ++++++++++++++++++++++++++++++++
include/linux/lsm_hooks.h | 6 ++
include/linux/security.h | 6 ++
include/linux/syscalls.h | 1
kernel/sys_ni.c | 1
security/security.c | 7 +++
security/selinux/hooks.c | 13 +++++
9 files changed, 123 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 9bf8d4c62f85..abe6ea95e0e6 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -392,3 +392,4 @@
383 i386 statx sys_statx
384 i386 arch_prctl sys_arch_prctl compat_sys_arch_prctl
385 i386 fsopen sys_fsopen
+386 i386 fsmount sys_fsmount
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 9b198c5fc412..0977c5079831 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -340,6 +340,7 @@
331 common pkey_free sys_pkey_free
332 common statx sys_statx
333 common fsopen sys_fsopen
+334 common fsmount sys_fsmount

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/namespace.c b/fs/namespace.c
index 554e99c980e0..49d630a4fbd4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3261,6 +3261,93 @@ vfs_submount_sc(const struct dentry *mountpoint, struct sb_config *sc)
EXPORT_SYMBOL_GPL(vfs_submount_sc);

/*
+ * Mount a new, prepared superblock (specified by fs_fd) on the location
+ * specified by dfd and dir_name. dfd can be AT_FDCWD, a dir fd or a container
+ * fd. This cannot be used for binding, moving or remounting mounts.
+ */
+SYSCALL_DEFINE3(fsmount, int, fs_fd, int, dfd, const char __user *, dir_name)
+{
+ struct sb_config *sc;
+ struct inode *inode;
+ struct path mountpoint;
+ struct fd f = fdget(fs_fd);
+ unsigned int mnt_flags = 0;
+ long ret;
+
+ if (!f.file)
+ return -EBADF;
+
+ ret = -EINVAL;
+ if (f.file->f_op != &fs_fs_fops)
+ goto err_fsfd;
+
+ sc = f.file->private_data;
+
+ ret = -EPERM;
+ if (!may_mount() ||
+ ((sc->ms_flags & MS_MANDLOCK) && !may_mandlock()))
+ goto err_fsfd;
+
+ /* Prevent further changes. */
+ inode = file_inode(f.file);
+ ret = inode_lock_killable(inode);
+ if (ret < 0)
+ goto err_fsfd;
+ ret = -EBUSY;
+ if (!sc->mounted) {
+ sc->mounted = true;
+ ret = 0;
+ }
+ inode_unlock(inode);
+ if (ret < 0)
+ goto err_fsfd;
+
+ /* Find the mountpoint. A container can be specified in dfd. */
+ ret = user_path_at(dfd, dir_name, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
+ &mountpoint);
+ if (ret < 0) {
+ sb_cfg_error(sc, "VFS: Mountpoint lookup failed");
+ goto err_fsfd;
+ }
+
+ ret = security_sb_config_mountpoint(sc, &mountpoint);
+ if (ret < 0)
+ goto err_mp;
+
+ /* Default to relatime unless overriden */
+ if (!(sc->ms_flags & MS_NOATIME))
+ mnt_flags |= MNT_RELATIME;
+
+ /* Separate the per-mountpoint flags */
+ if (sc->ms_flags & MS_NOSUID)
+ mnt_flags |= MNT_NOSUID;
+ if (sc->ms_flags & MS_NODEV)
+ mnt_flags |= MNT_NODEV;
+ if (sc->ms_flags & MS_NOEXEC)
+ mnt_flags |= MNT_NOEXEC;
+ if (sc->ms_flags & MS_NOATIME)
+ mnt_flags |= MNT_NOATIME;
+ if (sc->ms_flags & MS_NODIRATIME)
+ mnt_flags |= MNT_NODIRATIME;
+ if (sc->ms_flags & MS_STRICTATIME)
+ mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
+ if (sc->ms_flags & MS_RDONLY)
+ mnt_flags |= MNT_READONLY;
+
+ sc->ms_flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
+ MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
+ MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);
+
+ ret = do_new_mount_sc(sc, &mountpoint, mnt_flags);
+
+err_mp:
+ path_put(&mountpoint);
+err_fsfd:
+ fdput(f);
+ return ret;
+}
+
+/*
* Return true if path is reachable from root
*
* namespace_sem or mount_lock is held
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 6238fccaf5a4..e8e473ce5ddd 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -101,6 +101,10 @@
* Equivalent of sb_kern_mount, but with a superblock configuration context.
* @sc indicates the superblock configuration context.
* @src_sb indicates the new superblock.
+ * @sb_config_mountpoint:
+ * Equivalent of sb_mount, but with an sb_config.
+ * @sc indicates the superblock configuration context.
+ * @mountpoint indicates the path on which the mount will take place.
*
* Security hooks for filesystem operations.
*
@@ -1390,6 +1394,7 @@ union security_list_options {
void (*sb_config_free)(struct sb_config *sc);
int (*sb_config_parse_option)(struct sb_config *sc, char *opt);
int (*sb_config_kern_mount)(struct sb_config *sc, struct super_block *sb);
+ int (*sb_config_mountpoint)(struct sb_config *sc, struct path *mountpoint);

int (*sb_alloc_security)(struct super_block *sb);
void (*sb_free_security)(struct super_block *sb);
@@ -1704,6 +1709,7 @@ struct security_hook_heads {
struct list_head sb_config_free;
struct list_head sb_config_parse_option;
struct list_head sb_config_kern_mount;
+ struct list_head sb_config_mountpoint;
struct list_head sb_alloc_security;
struct list_head sb_free_security;
struct list_head sb_copy_data;
diff --git a/include/linux/security.h b/include/linux/security.h
index 49a7254aa30a..f95dc555cf29 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -226,6 +226,7 @@ int security_sb_config_dup(struct sb_config *sc, struct sb_config *src_sc);
void security_sb_config_free(struct sb_config *sc);
int security_sb_config_parse_option(struct sb_config *sc, char *opt);
int security_sb_config_kern_mount(struct sb_config *sc, struct super_block *sb);
+int security_sb_config_mountpoint(struct sb_config *sc, struct path *mountpoint);
int security_sb_alloc(struct super_block *sb);
void security_sb_free(struct super_block *sb);
int security_sb_copy_data(char *orig, char *copy);
@@ -541,6 +542,11 @@ static inline int security_sb_config_kern_mount(struct sb_config *sc,
{
return 0;
}
+static inline int security_sb_config_mountpoint(struct sb_config *sc,
+ struct path *mountpoint)
+{
+ return 0;
+}

static inline int security_sb_alloc(struct super_block *sb)
{
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 91ec8802ad5d..9ac7d8ca8c2e 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -906,5 +906,6 @@ asmlinkage long sys_pkey_free(int pkey);
asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);
asmlinkage long sys_fsopen(const char *fs_name, int containerfd, unsigned int flags);
+asmlinkage long sys_fsmount(int fsfd, int dfd, const char *path);

#endif
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index de1dc63e7e47..a0fe764bd5dd 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -261,3 +261,4 @@ cond_syscall(sys_pkey_free);

/* fd-based mount */
cond_syscall(sys_fsopen);
+cond_syscall(sys_fsmount);
diff --git a/security/security.c b/security/security.c
index fa9037186634..8b2b7e6b3112 100644
--- a/security/security.c
+++ b/security/security.c
@@ -334,6 +334,11 @@ int security_sb_config_kern_mount(struct sb_config *sc, struct super_block *sb)
return call_int_hook(sb_config_kern_mount, 0, sc, sb);
}

+int security_sb_config_mountpoint(struct sb_config *sc, struct path *mountpoint)
+{
+ return call_int_hook(sb_config_mountpoint, 0, sc, mountpoint);
+}
+
int security_sb_alloc(struct super_block *sb)
{
return call_int_hook(sb_alloc_security, 0, sb);
@@ -1691,6 +1696,8 @@ struct security_hook_heads security_hook_heads = {
LIST_HEAD_INIT(security_hook_heads.sb_config_parse_option),
.sb_config_kern_mount =
LIST_HEAD_INIT(security_hook_heads.sb_config_kern_mount),
+ .sb_config_mountpoint =
+ LIST_HEAD_INIT(security_hook_heads.sb_config_mountpoint),
.sb_alloc_security =
LIST_HEAD_INIT(security_hook_heads.sb_alloc_security),
.sb_free_security =
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 142be91888c9..e1bf18af72f5 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2990,6 +2990,18 @@ static int selinux_sb_config_kern_mount(struct sb_config *sc,
return rc;
}

+static int selinux_sb_config_mountpoint(struct sb_config *sc,
+ struct path *mountpoint)
+{
+ const struct cred *cred = current_cred();
+ int ret;
+
+ ret = path_has_perm(cred, mountpoint, FILE__MOUNTON);
+ if (ret < 0)
+ sb_cfg_error(sc, "SELinux: Mount on mountpoint not permitted");
+ return ret;
+}
+
/* inode security operations */

static int selinux_inode_alloc_security(struct inode *inode)
@@ -6300,6 +6312,7 @@ static struct security_hook_list selinux_hooks[] = {
LSM_HOOK_INIT(sb_config_free, selinux_sb_config_free),
LSM_HOOK_INIT(sb_config_parse_option, selinux_sb_config_parse_option),
LSM_HOOK_INIT(sb_config_kern_mount, selinux_sb_config_kern_mount),
+ LSM_HOOK_INIT(sb_config_mountpoint, selinux_sb_config_mountpoint),

LSM_HOOK_INIT(sb_alloc_security, selinux_sb_alloc_security),
LSM_HOOK_INIT(sb_free_security, selinux_sb_free_security),
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