[Devel] [PATCH RH7 13/14] fs: add mount_setattr()
Pavel Tikhomirov
ptikhomirov at virtuozzo.com
Thu Apr 13 13:47:23 MSK 2023
From: Christian Brauner <christian.brauner at ubuntu.com>
This implements the missing mount_setattr() syscall. While the new mount
api allows to change the properties of a superblock there is currently
no way to change the properties of a mount or a mount tree using file
descriptors which the new mount api is based on. In addition the old
mount api has the restriction that mount options cannot be applied
recursively. This hasn't changed since changing mount options on a
per-mount basis was implemented in [1] and has been a frequent request
not just for convenience but also for security reasons. The legacy
mount syscall is unable to accommodate this behavior without introducing
a whole new set of flags because MS_REC | MS_REMOUNT | MS_BIND |
MS_RDONLY | MS_NOEXEC | [...] only apply the mount option to the topmost
mount. Changing MS_REC to apply to the whole mount tree would mean
introducing a significant uapi change and would likely cause significant
regressions.
The new mount_setattr() syscall allows to recursively clear and set
mount options in one shot. Multiple calls to change mount options
requesting the same changes are idempotent:
int mount_setattr(int dfd, const char *path, unsigned flags,
struct mount_attr *uattr, size_t usize);
Flags to modify path resolution behavior are specified in the @flags
argument. Currently, AT_EMPTY_PATH, AT_RECURSIVE, AT_SYMLINK_NOFOLLOW,
and AT_NO_AUTOMOUNT are supported. If useful, additional lookup flags to
restrict path resolution as introduced with openat2() might be supported
in the future.
The mount_setattr() syscall can be expected to grow over time and is
designed with extensibility in mind. It follows the extensible syscall
pattern we have used with other syscalls such as openat2(), clone3(),
sched_{set,get}attr(), and others.
The set of mount options is passed in the uapi struct mount_attr which
currently has the following layout:
struct mount_attr {
__u64 attr_set;
__u64 attr_clr;
__u64 propagation;
__u64 userns_fd;
};
The @attr_set and @attr_clr members are used to clear and set mount
options. This way a user can e.g. request that a set of flags is to be
raised such as turning mounts readonly by raising MOUNT_ATTR_RDONLY in
@attr_set while at the same time requesting that another set of flags is
to be lowered such as removing noexec from a mount tree by specifying
MOUNT_ATTR_NOEXEC in @attr_clr.
Note, since the MOUNT_ATTR_<atime> values are an enum starting from 0,
not a bitmap, users wanting to transition to a different atime setting
cannot simply specify the atime setting in @attr_set, but must also
specify MOUNT_ATTR__ATIME in the @attr_clr field. So we ensure that
MOUNT_ATTR__ATIME can't be partially set in @attr_clr and that @attr_set
can't have any atime bits set if MOUNT_ATTR__ATIME isn't set in
@attr_clr.
The @propagation field lets callers specify the propagation type of a
mount tree. Propagation is a single property that has four different
settings and as such is not really a flag argument but an enum.
Specifically, it would be unclear what setting and clearing propagation
settings in combination would amount to. The legacy mount() syscall thus
forbids the combination of multiple propagation settings too. The goal
is to keep the semantics of mount propagation somewhat simple as they
are overly complex as it is.
The @userns_fd field lets user specify a user namespace whose idmapping
becomes the idmapping of the mount. This is implemented and explained in
detail in the next patch.
[1]: commit 2e4b7fcd9260 ("[PATCH] r/o bind mounts: honor mount writer counts at remount")
Link: https://lore.kernel.org/r/20210121131959.646623-35-christian.brauner@ubuntu.com
Cc: David Howells <dhowells at redhat.com>
Cc: Aleksa Sarai <cyphar at cyphar.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: linux-fsdevel at vger.kernel.org
Cc: linux-api at vger.kernel.org
Reviewed-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
Changes: port syscall for x86 only, drop uapi hunks and ignore
MNT_NOSYMFOLLOW as it is not yet supported
(cherry picked from commit 2a1867219c7b27f928e2545782b86daaf9ad50bd)
https://jira.sw.ru/browse/PSBM-144416
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
arch/x86/syscalls/syscall_32.tbl | 1 +
arch/x86/syscalls/syscall_64.tbl | 1 +
fs/namespace.c | 312 +++++++++++++++++++++++++++++++
3 files changed, 314 insertions(+)
diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
index 978f07cb0ea1..7978137648ed 100644
--- a/arch/x86/syscalls/syscall_32.tbl
+++ b/arch/x86/syscalls/syscall_32.tbl
@@ -373,6 +373,7 @@
428 i386 open_tree sys_open_tree
429 i386 move_mount sys_move_mount
+442 i386 mount_setattr sys_mount_setattr
510 i386 getluid sys_getluid
511 i386 setluid sys_setluid
diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
index 3c86abac9a50..7175e94309fd 100644
--- a/arch/x86/syscalls/syscall_64.tbl
+++ b/arch/x86/syscalls/syscall_64.tbl
@@ -338,6 +338,7 @@
428 common open_tree sys_open_tree
429 common move_mount sys_move_mount
+442 common mount_setattr sys_mount_setattr
500 64 getluid sys_getluid
501 64 setluid sys_setluid
diff --git a/fs/namespace.c b/fs/namespace.c
index a40a217f9871..f37cae055dbf 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4094,6 +4094,318 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
return error;
}
+/*
+ * mount_setattr()
+ */
+struct mount_attr {
+ __u64 attr_set;
+ __u64 attr_clr;
+ __u64 propagation;
+ __u64 userns_fd;
+};
+
+struct mount_kattr {
+ unsigned int attr_set;
+ unsigned int attr_clr;
+ unsigned int propagation;
+ unsigned int lookup_flags;
+ bool recurse;
+};
+
+/* List of all mount_attr versions. */
+#define MOUNT_ATTR_SIZE_VER0 32 /* sizeof first published struct */
+
+/*
+ * Mount attributes.
+ */
+#define MOUNT_ATTR_RDONLY 0x00000001 /* Mount read-only */
+#define MOUNT_ATTR_NOSUID 0x00000002 /* Ignore suid and sgid bits */
+#define MOUNT_ATTR_NODEV 0x00000004 /* Disallow access to device special files */
+#define MOUNT_ATTR_NOEXEC 0x00000008 /* Disallow program execution */
+#define MOUNT_ATTR__ATIME 0x00000070 /* Setting on how atime should be updated */
+#define MOUNT_ATTR_RELATIME 0x00000000 /* - Update atime relative to mtime/ctime. */
+#define MOUNT_ATTR_NOATIME 0x00000010 /* - Do not update access times. */
+#define MOUNT_ATTR_STRICTATIME 0x00000020 /* - Always perform atime updates */
+#define MOUNT_ATTR_NODIRATIME 0x00000080 /* Do not update directory access times */
+#define MOUNT_ATTR_IDMAP 0x00100000 /* Idmap mount to @userns_fd in struct mount_attr. */
+#define MOUNT_ATTR_NOSYMFOLLOW 0x00200000 /* Do not follow symlinks */
+
+#define MOUNT_SETATTR_VALID_FLAGS \
+(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | \
+ MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME | \
+ MOUNT_ATTR_NOSYMFOLLOW)
+
+#define MOUNT_SETATTR_PROPAGATION_FLAGS \
+(MS_UNBINDABLE | MS_PRIVATE | MS_SLAVE | MS_SHARED)
+
+static unsigned int attr_flags_to_mnt_flags(u64 attr_flags)
+{
+ unsigned int mnt_flags = 0;
+
+ if (attr_flags & MOUNT_ATTR_RDONLY)
+ mnt_flags |= MNT_READONLY;
+ if (attr_flags & MOUNT_ATTR_NOSUID)
+ mnt_flags |= MNT_NOSUID;
+ if (attr_flags & MOUNT_ATTR_NODEV)
+ mnt_flags |= MNT_NODEV;
+ if (attr_flags & MOUNT_ATTR_NOEXEC)
+ mnt_flags |= MNT_NOEXEC;
+ if (attr_flags & MOUNT_ATTR_NODIRATIME)
+ mnt_flags |= MNT_NODIRATIME;
+
+ return mnt_flags;
+}
+
+static unsigned int recalc_flags(struct mount_kattr *kattr, struct mount *mnt)
+{
+ unsigned int flags = mnt->mnt.mnt_flags;
+
+ /* flags to clear */
+ flags &= ~kattr->attr_clr;
+ /* flags to raise */
+ flags |= kattr->attr_set;
+
+ return flags;
+}
+
+static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
+ struct mount *mnt, int *err)
+{
+ struct mount *m = mnt, *last = NULL;
+
+ if (!is_mounted(&m->mnt)) {
+ *err = -EINVAL;
+ goto out;
+ }
+
+ if (!(mnt_has_parent(m) ? check_mnt(m) : is_anon_ns(m->mnt_ns))) {
+ *err = -EINVAL;
+ goto out;
+ }
+
+ do {
+ unsigned int flags;
+
+ flags = recalc_flags(kattr, m);
+ if (!can_change_locked_flags(m, flags)) {
+ *err = -EPERM;
+ goto out;
+ }
+
+ last = m;
+
+ if ((kattr->attr_set & MNT_READONLY) &&
+ !(m->mnt.mnt_flags & MNT_READONLY)) {
+ *err = mnt_hold_writers(m);
+ if (*err)
+ goto out;
+ }
+ } while (kattr->recurse && (m = next_mnt(m, mnt)));
+
+out:
+ return last;
+}
+
+static void mount_setattr_commit(struct mount_kattr *kattr,
+ struct mount *mnt, struct mount *last,
+ int err)
+{
+ struct mount *m = mnt;
+
+ do {
+ if (!err) {
+ unsigned int flags;
+
+ flags = recalc_flags(kattr, m);
+ WRITE_ONCE(m->mnt.mnt_flags, flags);
+ }
+
+ /*
+ * We either set MNT_READONLY above so make it visible
+ * before ~MNT_WRITE_HOLD or we failed to recursively
+ * apply mount options.
+ */
+ if ((kattr->attr_set & MNT_READONLY) &&
+ (m->mnt.mnt_flags & MNT_WRITE_HOLD))
+ mnt_unhold_writers(m);
+
+ if (!err && kattr->propagation)
+ change_mnt_propagation(m, kattr->propagation);
+
+ /*
+ * On failure, only cleanup until we found the first mount
+ * we failed to handle.
+ */
+ if (err && m == last)
+ break;
+ } while (kattr->recurse && (m = next_mnt(m, mnt)));
+
+ if (!err)
+ touch_mnt_namespace(mnt->mnt_ns);
+}
+
+static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
+{
+ struct mount *mnt = real_mount(path->mnt), *last = NULL;
+ int err = 0;
+
+ if (path->dentry != mnt->mnt.mnt_root)
+ return -EINVAL;
+
+ if (kattr->propagation) {
+ /*
+ * Only take namespace_lock() if we're actually changing
+ * propagation.
+ */
+ namespace_lock();
+ if (kattr->propagation == MS_SHARED) {
+ err = invent_group_ids(mnt, kattr->recurse);
+ if (err) {
+ namespace_unlock();
+ return err;
+ }
+ }
+ }
+
+ lock_mount_hash();
+
+ /*
+ * Get the mount tree in a shape where we can change mount
+ * properties without failure.
+ */
+ last = mount_setattr_prepare(kattr, mnt, &err);
+ if (last) /* Commit all changes or revert to the old state. */
+ mount_setattr_commit(kattr, mnt, last, err);
+
+ unlock_mount_hash();
+
+ if (kattr->propagation) {
+ namespace_unlock();
+ if (err)
+ cleanup_group_ids(mnt, NULL);
+ }
+
+ return err;
+}
+
+static int build_mount_kattr(const struct mount_attr *attr,
+ struct mount_kattr *kattr, unsigned int flags)
+{
+ unsigned int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
+
+ if (flags & AT_NO_AUTOMOUNT)
+ lookup_flags &= ~LOOKUP_AUTOMOUNT;
+ if (flags & AT_SYMLINK_NOFOLLOW)
+ lookup_flags &= ~LOOKUP_FOLLOW;
+ if (flags & AT_EMPTY_PATH)
+ lookup_flags |= LOOKUP_EMPTY;
+
+ *kattr = (struct mount_kattr) {
+ .lookup_flags = lookup_flags,
+ .recurse = !!(flags & AT_RECURSIVE),
+ };
+
+ if (attr->propagation & ~MOUNT_SETATTR_PROPAGATION_FLAGS)
+ return -EINVAL;
+ if (hweight32(attr->propagation & MOUNT_SETATTR_PROPAGATION_FLAGS) > 1)
+ return -EINVAL;
+ kattr->propagation = attr->propagation;
+
+ if ((attr->attr_set | attr->attr_clr) & ~MOUNT_SETATTR_VALID_FLAGS)
+ return -EINVAL;
+
+ if (attr->userns_fd)
+ return -EINVAL;
+
+ kattr->attr_set = attr_flags_to_mnt_flags(attr->attr_set);
+ kattr->attr_clr = attr_flags_to_mnt_flags(attr->attr_clr);
+
+ /*
+ * Since the MOUNT_ATTR_<atime> values are an enum, not a bitmap,
+ * users wanting to transition to a different atime setting cannot
+ * simply specify the atime setting in @attr_set, but must also
+ * specify MOUNT_ATTR__ATIME in the @attr_clr field.
+ * So ensure that MOUNT_ATTR__ATIME can't be partially set in
+ * @attr_clr and that @attr_set can't have any atime bits set if
+ * MOUNT_ATTR__ATIME isn't set in @attr_clr.
+ */
+ if (attr->attr_clr & MOUNT_ATTR__ATIME) {
+ if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
+ return -EINVAL;
+
+ /*
+ * Clear all previous time settings as they are mutually
+ * exclusive.
+ */
+ kattr->attr_clr |= MNT_RELATIME | MNT_NOATIME;
+ switch (attr->attr_set & MOUNT_ATTR__ATIME) {
+ case MOUNT_ATTR_RELATIME:
+ kattr->attr_set |= MNT_RELATIME;
+ break;
+ case MOUNT_ATTR_NOATIME:
+ kattr->attr_set |= MNT_NOATIME;
+ break;
+ case MOUNT_ATTR_STRICTATIME:
+ break;
+ default:
+ return -EINVAL;
+ }
+ } else {
+ if (attr->attr_set & MOUNT_ATTR__ATIME)
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path,
+ unsigned int, flags, struct mount_attr __user *, uattr,
+ size_t, usize)
+{
+ int err;
+ struct path target;
+ struct mount_attr attr;
+ struct mount_kattr kattr;
+
+ BUILD_BUG_ON(sizeof(struct mount_attr) != MOUNT_ATTR_SIZE_VER0);
+
+ if (flags & ~(AT_EMPTY_PATH |
+ AT_RECURSIVE |
+ AT_SYMLINK_NOFOLLOW |
+ AT_NO_AUTOMOUNT))
+ return -EINVAL;
+
+ if (unlikely(usize > PAGE_SIZE))
+ return -E2BIG;
+ if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
+ return -EINVAL;
+
+ if (!may_mount())
+ return -EPERM;
+
+ err = copy_struct_from_user(&attr, sizeof(attr), uattr, usize);
+ if (err)
+ return err;
+
+ /* Don't bother walking through the mounts if this is a nop. */
+ if (attr.attr_set == 0 &&
+ attr.attr_clr == 0 &&
+ attr.propagation == 0)
+ return 0;
+
+ err = build_mount_kattr(&attr, &kattr, flags);
+ if (err)
+ return err;
+
+ err = user_path_at(dfd, path, kattr.lookup_flags, &target);
+ if (err)
+ return err;
+
+ err = do_mount_setattr(&target, &kattr);
+ path_put(&target);
+ return err;
+}
+
static void __init init_mount_tree(void)
{
struct vfsmount *mnt;
--
2.39.2
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