On Mon, Aug 03, 2020 at 02:37:08PM +0100, David Howells wrote:
> Allow the fsinfo() syscall to look up a mount object by ID rather than by
> pathname. This is necessary as there can be multiple mounts stacked up at
> the same pathname and there's no way to look through them otherwise.
>
> This is done by passing FSINFO_FLAGS_QUERY_MOUNT to fsinfo() in the
> parameters and then passing the mount ID as a string to fsinfo() in place
> of the filename:
>
> struct fsinfo_params params = {
> .flags = FSINFO_FLAGS_QUERY_MOUNT,
> .request = FSINFO_ATTR_IDS,
> };
>
> ret = fsinfo(AT_FDCWD, "21", ¶ms, buffer, sizeof(buffer));
>
> The caller is only permitted to query a mount object if the root directory
> of that mount connects directly to the current chroot if dfd == AT_FDCWD[*]
> or the directory specified by dfd otherwise. Note that this is not
> available to the pathwalk of any other syscall.
>
> [*] This needs to be something other than AT_FDCWD, perhaps AT_FDROOT.
>
> [!] This probably needs an LSM hook.
>
> [!] This might want to check the permissions on all the intervening dirs -
> but it would have to do that under RCU conditions.
>
> [!] This might want to check a CAP_* flag.
Was this reviewed by security folks?
>
> Signed-off-by: David Howells <[email protected]>
> ---
>
> fs/fsinfo.c | 53 +++++++++++++++++++
> fs/internal.h | 1
> fs/namespace.c | 117
> ++++++++++++++++++++++++++++++++++++++++++-
> include/uapi/linux/fsinfo.h | 1
> samples/vfs/test-fsinfo.c | 7 ++-
> 5 files changed, 175 insertions(+), 4 deletions(-)
>
> diff --git a/fs/fsinfo.c b/fs/fsinfo.c
> index aef7a736e8fc..8ccbcddb4f16 100644
> --- a/fs/fsinfo.c
> +++ b/fs/fsinfo.c
> @@ -563,6 +563,56 @@ static int vfs_fsinfo_fd(unsigned int fd, struct
> fsinfo_context *ctx)
> return ret;
> }
>
> +/*
> + * Look up the root of a mount object. This allows access to mount objects
> + * (and their attached superblocks) that can't be retrieved by path because
> + * they're entirely covered.
> + *
> + * We only permit access to a mount that has a direct path between either the
> + * dentry pointed to by dfd or to our chroot (if dfd is AT_FDCWD).
> + */
> +static int vfs_fsinfo_mount(int dfd, const char __user *filename,
> + struct fsinfo_context *ctx)
> +{
> + struct path path;
> + struct fd f = {};
> + char *name;
> + unsigned long mnt_id;
> + int ret;
> +
> + if (!filename)
> + return -EINVAL;
> +
> + name = strndup_user(filename, 32);
> + if (IS_ERR(name))
> + return PTR_ERR(name);
> + ret = kstrtoul(name, 0, &mnt_id);
> + if (ret < 0)
> + goto out_name;
> + if (mnt_id > INT_MAX)
> + goto out_name;
> +
> + if (dfd != AT_FDCWD) {
> + ret = -EBADF;
> + f = fdget_raw(dfd);
> + if (!f.file)
> + goto out_name;
> + }
> +
> + ret = lookup_mount_object(f.file ? &f.file->f_path : NULL,
> + mnt_id, &path);
> + if (ret < 0)
> + goto out_fd;
> +
> + ret = vfs_fsinfo(&path, ctx);
> + path_put(&path);
> +out_fd:
> + fdput(f);
> +out_name:
> + kfree(name);
> + return ret;
> +}
> +
> /**
> * sys_fsinfo - System call to get filesystem information
> * @dfd: Base directory to pathwalk from or fd referring to filesystem.
> @@ -636,6 +686,9 @@ SYSCALL_DEFINE6(fsinfo,
> return -EINVAL;
> ret = vfs_fsinfo_fd(dfd, &ctx);
> break;
> + case FSINFO_FLAGS_QUERY_MOUNT:
> + ret = vfs_fsinfo_mount(dfd, pathname, &ctx);
> + break;
> default:
> return -EINVAL;
> }
> diff --git a/fs/internal.h b/fs/internal.h
> index 0b57da498f06..84bbb743a5ac 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -89,6 +89,7 @@ extern int __mnt_want_write_file(struct file *);
> extern void __mnt_drop_write_file(struct file *);
>
> extern void dissolve_on_fput(struct vfsmount *);
> +extern int lookup_mount_object(struct path *, unsigned int, struct path *);
> extern int fsinfo_generic_mount_source(struct path *, struct fsinfo_context
> *);
>
> /*
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ead8d1a16610..b2b9920ffd3c 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -64,7 +64,7 @@ static int __init set_mphash_entries(char *str)
> __setup("mphash_entries=", set_mphash_entries);
>
> static u64 event;
> -static DEFINE_IDA(mnt_id_ida);
> +static DEFINE_IDR(mnt_id_ida);
> static DEFINE_IDA(mnt_group_ida);
>
> static struct hlist_head *mount_hashtable __read_mostly;
> @@ -105,17 +105,27 @@ static inline struct hlist_head *mp_hash(struct dentry
> *dentry)
>
> static int mnt_alloc_id(struct mount *mnt)
> {
> - int res = ida_alloc(&mnt_id_ida, GFP_KERNEL);
> + int res;
>
> + /* Allocate an ID, but don't set the pointer back to the mount until
> + * later, as once we do that, we have to follow RCU protocols to get
> + * rid of the mount struct.
> + */
> + res = idr_alloc(&mnt_id_ida, NULL, 0, INT_MAX, GFP_KERNEL);
This needs to be a separate patch.
> if (res < 0)
> return res;
> mnt->mnt_id = res;
> return 0;
> }
>
> +static void mnt_publish_id(struct mount *mnt)
> +{
> + idr_replace(&mnt_id_ida, mnt, mnt->mnt_id);
> +}
> +
> static void mnt_free_id(struct mount *mnt)
> {
> - ida_free(&mnt_id_ida, mnt->mnt_id);
> + idr_remove(&mnt_id_ida, mnt->mnt_id);
> }
>
> /*
> @@ -975,6 +985,7 @@ struct vfsmount *vfs_create_mount(struct fs_context *fc)
> lock_mount_hash();
> list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
> unlock_mount_hash();
> + mnt_publish_id(mnt);
> return &mnt->mnt;
> }
> EXPORT_SYMBOL(vfs_create_mount);
> @@ -1068,6 +1079,7 @@ static struct mount *clone_mnt(struct mount *old,
> struct dentry *root,
> lock_mount_hash();
> list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
> unlock_mount_hash();
> + mnt_publish_id(mnt);
>
> if ((flag & CL_SLAVE) ||
> ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
> @@ -4151,4 +4163,103 @@ int fsinfo_generic_mount_source(struct path *path,
> struct fsinfo_context *ctx)
> return m.count + 1;
> }
>
> +/*
> + * See if one path point connects directly to another by ancestral
> relationship
> + * across mountpoints. Must call with the RCU read lock held.
> + */
> +static bool are_paths_connected(struct path *ancestor, struct path *to_check)
> +{
> + struct mount *mnt, *parent;
> + struct path cursor;
> + unsigned seq;
> + bool connected;
> +
> + seq = 0;
> +restart:
> + cursor = *to_check;
> +
> + read_seqbegin_or_lock(&rename_lock, &seq);
> + while (cursor.mnt != ancestor->mnt) {
> + mnt = real_mount(cursor.mnt);
> + parent = READ_ONCE(mnt->mnt_parent);
> + if (mnt == parent)
> + goto failed;
> + cursor.dentry = READ_ONCE(mnt->mnt_mountpoint);
> + cursor.mnt = &parent->mnt;
> + }
> +
> + while (cursor.dentry != ancestor->dentry) {
> + if (cursor.dentry == cursor.mnt->mnt_root ||
> + IS_ROOT(cursor.dentry))
> + goto failed;
> + cursor.dentry = READ_ONCE(cursor.dentry->d_parent);
> + }
> +
> + connected = true;
> +out:
> + done_seqretry(&rename_lock, seq);
> + return connected;
> +
> +failed:
> + if (need_seqretry(&rename_lock, seq)) {
> + seq = 1;
> + goto restart;
> + }
> + connected = false;
> + goto out;
> +}
> +
> +/**
> + * lookup_mount_object - Look up a vfsmount object by ID
> + * @root: The mount root must connect backwards to this point (or chroot if
> NULL).
> + * @id: The ID of the mountpoint.
> + * @_mntpt: Where to return the resulting mountpoint path.
> + *
> + * Look up the root of the mount with the corresponding ID. This is only
> + * permitted if that mount connects directly to the specified root/chroot.
> + */
> +int lookup_mount_object(struct path *root, unsigned int mnt_id, struct path
> *_mntpt)
> +{
> + struct mount *mnt;
> + struct path stop, mntpt = {};
> + int ret = -EPERM;
> +
> + if (!root)
> + get_fs_root(current->fs, &stop);
> + else
> + stop = *root;
> +
> + rcu_read_lock();
> + lock_mount_hash();
> + mnt = idr_find(&mnt_id_ida, mnt_id);
> + if (!mnt)
> + goto out_unlock_mh;
> + if (mnt->mnt.mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED))
> + goto out_unlock_mh;
> + if (mnt_get_count(mnt) == 0)
> + goto out_unlock_mh;
> + mnt_add_count(mnt, 1);
> + mntpt.mnt = &mnt->mnt;
> + mntpt.dentry = dget(mnt->mnt.mnt_root);
> + unlock_mount_hash();
> +
> + if (are_paths_connected(&stop, &mntpt)) {
> + *_mntpt = mntpt;
> + mntpt.mnt = NULL;
> + mntpt.dentry = NULL;
> + ret = 0;
> + }
> +
> +out_unlock:
> + rcu_read_unlock();
> + if (!root)
> + path_put(&stop);
> + path_put(&mntpt);
> + return ret;
> +
> +out_unlock_mh:
> + unlock_mount_hash();
> + goto out_unlock;
> +}
> +
> #endif /* CONFIG_FSINFO */
> diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
> index a27e92b68266..d24e47762a07 100644
> --- a/include/uapi/linux/fsinfo.h
> +++ b/include/uapi/linux/fsinfo.h
> @@ -44,6 +44,7 @@ struct fsinfo_params {
> #define FSINFO_FLAGS_QUERY_MASK 0x0007 /* What object should fsinfo()
> query? */
> #define FSINFO_FLAGS_QUERY_PATH 0x0000 /* - path, specified by
> dirfd,pathname,AT_EMPTY_PATH */
> #define FSINFO_FLAGS_QUERY_FD 0x0001 /* - fd specified by dirfd */
> +#define FSINFO_FLAGS_QUERY_MOUNT 0x0002 /* - mount object
> (path=>mount_id, dirfd=>subtree) */
> __u32 request; /* ID of requested attribute */
> __u32 Nth; /* Instance of it (some may have multiple) */
> __u32 Mth; /* Subinstance of Nth instance */
> diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
> index 634f30b7e67f..dfa44bba8bbd 100644
> --- a/samples/vfs/test-fsinfo.c
> +++ b/samples/vfs/test-fsinfo.c
> @@ -593,7 +593,7 @@ int main(int argc, char **argv)
> bool meta = false;
> int raw = 0, opt, Nth, Mth;
>
> - while ((opt = getopt(argc, argv, "Madlr"))) {
> + while ((opt = getopt(argc, argv, "Madmlr"))) {
> switch (opt) {
> case 'M':
> meta = true;
> @@ -609,6 +609,10 @@ int main(int argc, char **argv)
> params.at_flags &= ~AT_SYMLINK_NOFOLLOW;
> params.flags = FSINFO_FLAGS_QUERY_PATH;
> continue;
> + case 'm':
> + params.resolve_flags = 0;
> + params.flags = FSINFO_FLAGS_QUERY_MOUNT;
> + continue;
> case 'r':
> raw = 1;
> continue;
> @@ -621,6 +625,7 @@ int main(int argc, char **argv)
>
> if (argc != 1) {
> printf("Format: test-fsinfo [-Madlr] <path>\n");
> + printf("Format: test-fsinfo [-Mdr] -m <mnt_id>\n");
> exit(2);
> }
>
>
>
