ext4: support idmapped mounts

Enable idmapped mounts for ext4. All dedicated helpers we need for this
exist. So this basically just means we're passing down the
user_namespace argument from the VFS methods to the relevant helpers.

Let's create simple example where we idmap an ext4 filesystem:

 root@f2-vm:~# truncate -s 5G ext4.img

 root@f2-vm:~# mkfs.ext4 ./ext4.img
 mke2fs 1.45.5 (07-Jan-2020)
 Discarding device blocks: done
 Creating filesystem with 1310720 4k blocks and 327680 inodes
 Filesystem UUID: 3fd91794-c6ca-4b0f-9964-289a000919cf
 Superblock backups stored on blocks:
         32768, 98304, 163840, 229376, 294912, 819200, 884736

 Allocating group tables: done
 Writing inode tables: done
 Creating journal (16384 blocks): done
 Writing superblocks and filesystem accounting information: done

 root@f2-vm:~# losetup -f --show ./ext4.img
 /dev/loop0

 root@f2-vm:~# mount /dev/loop0 /mnt

 root@f2-vm:~# ls -al /mnt/
 total 24
 drwxr-xr-x  3 root root  4096 Oct 28 13:34 .
 drwxr-xr-x 30 root root  4096 Oct 28 13:22 ..
 drwx------  2 root root 16384 Oct 28 13:34 lost+found

 # Let's create an idmapped mount at /idmapped1 where we map uid and gid
 # 0 to uid and gid 1000
 root@f2-vm:/# ./mount-idmapped --map-mount b:0:1000:1 /mnt/ /idmapped1/

 root@f2-vm:/# ls -al /idmapped1/
 total 24
 drwxr-xr-x  3 ubuntu ubuntu  4096 Oct 28 13:34 .
 drwxr-xr-x 30 root   root    4096 Oct 28 13:22 ..
 drwx------  2 ubuntu ubuntu 16384 Oct 28 13:34 lost+found

 # Let's create an idmapped mount at /idmapped2 where we map uid and gid
 # 0 to uid and gid 2000
 root@f2-vm:/# ./mount-idmapped --map-mount b:0:2000:1 /mnt/ /idmapped2/

 root@f2-vm:/# ls -al /idmapped2/
 total 24
 drwxr-xr-x  3 2000 2000  4096 Oct 28 13:34 .
 drwxr-xr-x 31 root root  4096 Oct 28 13:39 ..
 drwx------  2 2000 2000 16384 Oct 28 13:34 lost+found

Let's create another example where we idmap the rootfs filesystem
without a mapping for uid 0 and gid 0:

 # Create an idmapped mount of for a full POSIX range of rootfs under
 # /mnt but without a mapping for uid 0 to reduce attack surface

 root@f2-vm:/# ./mount-idmapped --map-mount b:1:1:65536 / /mnt/

 # Since we don't have a mapping for uid and gid 0 all files owned by
 # uid and gid 0 should show up as uid and gid 65534:
 root@f2-vm:/# ls -al /mnt/
 total 664
 drwxr-xr-x 31 nobody nogroup   4096 Oct 28 13:39 .
 drwxr-xr-x 31 root   root      4096 Oct 28 13:39 ..
 lrwxrwxrwx  1 nobody nogroup      7 Aug 25 07:44 bin -> usr/bin
 drwxr-xr-x  4 nobody nogroup   4096 Oct 28 13:17 boot
 drwxr-xr-x  2 nobody nogroup   4096 Aug 25 07:48 dev
 drwxr-xr-x 81 nobody nogroup   4096 Oct 28 04:00 etc
 drwxr-xr-x  4 nobody nogroup   4096 Oct 28 04:00 home
 lrwxrwxrwx  1 nobody nogroup      7 Aug 25 07:44 lib -> usr/lib
 lrwxrwxrwx  1 nobody nogroup      9 Aug 25 07:44 lib32 -> usr/lib32
 lrwxrwxrwx  1 nobody nogroup      9 Aug 25 07:44 lib64 -> usr/lib64
 lrwxrwxrwx  1 nobody nogroup     10 Aug 25 07:44 libx32 -> usr/libx32
 drwx------  2 nobody nogroup  16384 Aug 25 07:47 lost+found
 drwxr-xr-x  2 nobody nogroup   4096 Aug 25 07:44 media
 drwxr-xr-x 31 nobody nogroup   4096 Oct 28 13:39 mnt
 drwxr-xr-x  2 nobody nogroup   4096 Aug 25 07:44 opt
 drwxr-xr-x  2 nobody nogroup   4096 Apr 15  2020 proc
 drwx--x--x  6 nobody nogroup   4096 Oct 28 13:34 root
 drwxr-xr-x  2 nobody nogroup   4096 Aug 25 07:46 run
 lrwxrwxrwx  1 nobody nogroup      8 Aug 25 07:44 sbin -> usr/sbin
 drwxr-xr-x  2 nobody nogroup   4096 Aug 25 07:44 srv
 drwxr-xr-x  2 nobody nogroup   4096 Apr 15  2020 sys
 drwxrwxrwt 10 nobody nogroup   4096 Oct 28 13:19 tmp
 drwxr-xr-x 14 nobody nogroup   4096 Oct 20 13:00 usr
 drwxr-xr-x 12 nobody nogroup   4096 Aug 25 07:45 var

 # Since we do have a mapping for uid and gid 1000 all files owned by
 # uid and gid 1000 should simply show up as uid and gid 1000:
 root@f2-vm:/# ls -al /mnt/home/ubuntu/
 total 40
 drwxr-xr-x 3 ubuntu ubuntu  4096 Oct 28 00:43 .
 drwxr-xr-x 4 nobody nogroup 4096 Oct 28 04:00 ..
 -rw------- 1 ubuntu ubuntu  2936 Oct 28 12:26 .bash_history
 -rw-r--r-- 1 ubuntu ubuntu   220 Feb 25  2020 .bash_logout
 -rw-r--r-- 1 ubuntu ubuntu  3771 Feb 25  2020 .bashrc
 -rw-r--r-- 1 ubuntu ubuntu   807 Feb 25  2020 .profile
 -rw-r--r-- 1 ubuntu ubuntu     0 Oct 16 16:11 .sudo_as_admin_successful
 -rw------- 1 ubuntu ubuntu  1144 Oct 28 00:43 .viminfo

Link: https://lore.kernel.org/r/20210121131959.646623-39-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-ext4@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

7 files changed, 46 insertions, 39 deletions

diff --git a/fs/ext4/acl.c b/fs/ext4/acl.c
index 059434e0f36c..c5eaffccecc3 100644
--- a/fs/ext4/acl.c
+++ b/fs/ext4/acl.c
@@ -246,8 +246,7 @@ retry:
 	ext4_fc_start_update(inode);
 
 	if ((type == ACL_TYPE_ACCESS) && acl) {
-		error = posix_acl_update_mode(&init_user_ns, inode, &mode,
-					      &acl);
+		error = posix_acl_update_mode(mnt_userns, inode, &mode, &acl);
 		if (error)
 			goto out_stop;
 		if (mode != inode->i_mode)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 3c750f5e8ebd..644fd69185d3 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2755,18 +2755,19 @@ extern int ext4fs_dirhash(const struct inode *dir, const char *name, int len,
 
 /* ialloc.c */
 extern int ext4_mark_inode_used(struct super_block *sb, int ino);
-extern struct inode *__ext4_new_inode(handle_t *, struct inode *, umode_t,
+extern struct inode *__ext4_new_inode(struct user_namespace *, handle_t *,
+				      struct inode *, umode_t,
 				      const struct qstr *qstr, __u32 goal,
 				      uid_t *owner, __u32 i_flags,
 				      int handle_type, unsigned int line_no,
 				      int nblocks);
 
-#define ext4_new_inode(handle, dir, mode, qstr, goal, owner, i_flags) \
-	__ext4_new_inode((handle), (dir), (mode), (qstr), (goal), (owner), \
-			 i_flags, 0, 0, 0)
-#define ext4_new_inode_start_handle(dir, mode, qstr, goal, owner, \
+#define ext4_new_inode(handle, dir, mode, qstr, goal, owner, i_flags)          \
+	__ext4_new_inode(&init_user_ns, (handle), (dir), (mode), (qstr),       \
+			 (goal), (owner), i_flags, 0, 0, 0)
+#define ext4_new_inode_start_handle(mnt_userns, dir, mode, qstr, goal, owner, \
 				    type, nblocks)		    \
-	__ext4_new_inode(NULL, (dir), (mode), (qstr), (goal), (owner), \
+	__ext4_new_inode((mnt_userns), NULL, (dir), (mode), (qstr), (goal), (owner), \
 			 0, (type), __LINE__, (nblocks))
 
 
diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
index 00c1ec6eee16..bf9028950a51 100644
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -919,7 +919,8 @@ static int ext4_xattr_credits_for_new_inode(struct inode *dir, mode_t mode,
  * For other inodes, search forward from the parent directory's block
  * group to find a free inode.
  */
-struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
+struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
+			       handle_t *handle, struct inode *dir,
 			       umode_t mode, const struct qstr *qstr,
 			       __u32 goal, uid_t *owner, __u32 i_flags,
 			       int handle_type, unsigned int line_no,
@@ -969,10 +970,10 @@ struct inode *__ext4_new_inode(handle_t *handle, struct inode *dir,
 		i_gid_write(inode, owner[1]);
 	} else if (test_opt(sb, GRPID)) {
 		inode->i_mode = mode;
-		inode->i_uid = current_fsuid();
+		inode->i_uid = fsuid_into_mnt(mnt_userns);
 		inode->i_gid = dir->i_gid;
 	} else
-		inode_init_owner(&init_user_ns, inode, dir, mode);
+		inode_init_owner(mnt_userns, inode, dir, mode);
 
 	if (ext4_has_feature_project(sb) &&
 	    ext4_test_inode_flag(dir, EXT4_INODE_PROJINHERIT))
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ce45535336fa..8fbf85b3547e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -20,6 +20,7 @@
  */
 
 #include <linux/fs.h>
+#include <linux/mount.h>
 #include <linux/time.h>
 #include <linux/highuid.h>
 #include <linux/pagemap.h>
@@ -5338,7 +5339,7 @@ int ext4_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 				  ATTR_GID | ATTR_TIMES_SET))))
 		return -EPERM;
 
-	error = setattr_prepare(&init_user_ns, dentry, attr);
+	error = setattr_prepare(mnt_userns, dentry, attr);
 	if (error)
 		return error;
 
@@ -5513,7 +5514,7 @@ out_mmap_sem:
 	}
 
 	if (!error) {
-		setattr_copy(&init_user_ns, inode, attr);
+		setattr_copy(mnt_userns, inode, attr);
 		mark_inode_dirty(inode);
 	}
 
@@ -5525,7 +5526,7 @@ out_mmap_sem:
 		ext4_orphan_del(NULL, inode);
 
 	if (!error && (ia_valid & ATTR_MODE))
-		rc = posix_acl_chmod(&init_user_ns, inode, inode->i_mode);
+		rc = posix_acl_chmod(mnt_userns, inode, inode->i_mode);
 
 err_out:
 	if  (error)
@@ -5572,7 +5573,7 @@ int ext4_getattr(struct user_namespace *mnt_userns, const struct path *path,
 				  STATX_ATTR_NODUMP |
 				  STATX_ATTR_VERITY);
 
-	generic_fillattr(&init_user_ns, inode, stat);
+	generic_fillattr(mnt_userns, inode, stat);
 	return 0;
 }
 
@@ -5583,7 +5584,7 @@ int ext4_file_getattr(struct user_namespace *mnt_userns,
 	struct inode *inode = d_inode(path->dentry);
 	u64 delalloc_blocks;
 
-	ext4_getattr(&init_user_ns, path, stat, request_mask, query_flags);
+	ext4_getattr(mnt_userns, path, stat, request_mask, query_flags);
 
 	/*
 	 * If there is inline data in the inode, the inode will normally not
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index ab80e2493fdc..56ad9c4b6350 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -107,10 +107,12 @@ void ext4_reset_inode_seed(struct inode *inode)
  * important fields of the inodes.
  *
  * @sb:         the super block of the filesystem
+ * @mnt_userns:	user namespace of the mount the inode was found from
  * @inode:      the inode to swap with EXT4_BOOT_LOADER_INO
  *
  */
 static long swap_inode_boot_loader(struct super_block *sb,
+				struct user_namespace *mnt_userns,
 				struct inode *inode)
 {
 	handle_t *handle;
@@ -139,7 +141,7 @@ static long swap_inode_boot_loader(struct super_block *sb,
 	}
 
 	if (IS_RDONLY(inode) || IS_APPEND(inode) || IS_IMMUTABLE(inode) ||
-	    !inode_owner_or_capable(&init_user_ns, inode) ||
+	    !inode_owner_or_capable(mnt_userns, inode) ||
 	    !capable(CAP_SYS_ADMIN)) {
 		err = -EPERM;
 		goto journal_err_out;
@@ -815,6 +817,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	struct inode *inode = file_inode(filp);
 	struct super_block *sb = inode->i_sb;
 	struct ext4_inode_info *ei = EXT4_I(inode);
+	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
 	unsigned int flags;
 
 	ext4_debug("cmd = %u, arg = %lu\n", cmd, arg);
@@ -830,7 +833,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	case FS_IOC_SETFLAGS: {
 		int err;
 
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EACCES;
 
 		if (get_user(flags, (int __user *) arg))
@@ -872,7 +875,7 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		__u32 generation;
 		int err;
 
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EPERM;
 
 		if (ext4_has_metadata_csum(inode->i_sb)) {
@@ -1011,7 +1014,7 @@ mext_out:
 	case EXT4_IOC_MIGRATE:
 	{
 		int err;
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EACCES;
 
 		err = mnt_want_write_file(filp);
@@ -1033,7 +1036,7 @@ mext_out:
 	case EXT4_IOC_ALLOC_DA_BLKS:
 	{
 		int err;
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EACCES;
 
 		err = mnt_want_write_file(filp);
@@ -1052,7 +1055,7 @@ mext_out:
 		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
-		err = swap_inode_boot_loader(sb, inode);
+		err = swap_inode_boot_loader(sb, mnt_userns, inode);
 		mnt_drop_write_file(filp);
 		return err;
 	}
@@ -1218,7 +1221,7 @@ resizefs_out:
 
 	case EXT4_IOC_CLEAR_ES_CACHE:
 	{
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EACCES;
 		ext4_clear_inode_es(inode);
 		return 0;
@@ -1264,7 +1267,7 @@ resizefs_out:
 			return -EFAULT;
 
 		/* Make sure caller has proper permission */
-		if (!inode_owner_or_capable(&init_user_ns, inode))
+		if (!inode_owner_or_capable(mnt_userns, inode))
 			return -EACCES;
 
 		if (fa.fsx_xflags & ~EXT4_SUPPORTED_FS_XFLAGS)
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 13dff80aedcb..877c602ae063 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2610,8 +2610,8 @@ static int ext4_create(struct user_namespace *mnt_userns, struct inode *dir,
 	credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
 		   EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
 retry:
-	inode = ext4_new_inode_start_handle(dir, mode, &dentry->d_name, 0,
-					    NULL, EXT4_HT_DIR, credits);
+	inode = ext4_new_inode_start_handle(mnt_userns, dir, mode, &dentry->d_name,
+					    0, NULL, EXT4_HT_DIR, credits);
 	handle = ext4_journal_current_handle();
 	err = PTR_ERR(inode);
 	if (!IS_ERR(inode)) {
@@ -2645,8 +2645,8 @@ static int ext4_mknod(struct user_namespace *mnt_userns, struct inode *dir,
 	credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
 		   EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
 retry:
-	inode = ext4_new_inode_start_handle(dir, mode, &dentry->d_name, 0,
-					    NULL, EXT4_HT_DIR, credits);
+	inode = ext4_new_inode_start_handle(mnt_userns, dir, mode, &dentry->d_name,
+					    0, NULL, EXT4_HT_DIR, credits);
 	handle = ext4_journal_current_handle();
 	err = PTR_ERR(inode);
 	if (!IS_ERR(inode)) {
@@ -2677,7 +2677,7 @@ static int ext4_tmpfile(struct user_namespace *mnt_userns, struct inode *dir,
 		return err;
 
 retry:
-	inode = ext4_new_inode_start_handle(dir, mode,
+	inode = ext4_new_inode_start_handle(mnt_userns, dir, mode,
 					    NULL, 0, NULL,
 					    EXT4_HT_DIR,
 			EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb) +
@@ -2792,7 +2792,7 @@ static int ext4_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	credits = (EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
 		   EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3);
 retry:
-	inode = ext4_new_inode_start_handle(dir, S_IFDIR | mode,
+	inode = ext4_new_inode_start_handle(mnt_userns, dir, S_IFDIR | mode,
 					    &dentry->d_name,
 					    0, NULL, EXT4_HT_DIR, credits);
 	handle = ext4_journal_current_handle();
@@ -3335,7 +3335,7 @@ static int ext4_symlink(struct user_namespace *mnt_userns, struct inode *dir,
 			  EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3;
 	}
 
-	inode = ext4_new_inode_start_handle(dir, S_IFLNK|S_IRWXUGO,
+	inode = ext4_new_inode_start_handle(mnt_userns, dir, S_IFLNK|S_IRWXUGO,
 					    &dentry->d_name, 0, NULL,
 					    EXT4_HT_DIR, credits);
 	handle = ext4_journal_current_handle();
@@ -3664,7 +3664,8 @@ static void ext4_update_dir_count(handle_t *handle, struct ext4_renament *ent)
 	}
 }
 
-static struct inode *ext4_whiteout_for_rename(struct ext4_renament *ent,
+static struct inode *ext4_whiteout_for_rename(struct user_namespace *mnt_userns,
+					      struct ext4_renament *ent,
 					      int credits, handle_t **h)
 {
 	struct inode *wh;
@@ -3678,7 +3679,8 @@ static struct inode *ext4_whiteout_for_rename(struct ext4_renament *ent,
 	credits += (EXT4_MAXQUOTAS_TRANS_BLOCKS(ent->dir->i_sb) +
 		    EXT4_XATTR_TRANS_BLOCKS + 4);
 retry:
-	wh = ext4_new_inode_start_handle(ent->dir, S_IFCHR | WHITEOUT_MODE,
+	wh = ext4_new_inode_start_handle(mnt_userns, ent->dir,
+					 S_IFCHR | WHITEOUT_MODE,
 					 &ent->dentry->d_name, 0, NULL,
 					 EXT4_HT_DIR, credits);
 
@@ -3705,9 +3707,9 @@ retry:
  * while new_{dentry,inode) refers to the destination dentry/inode
  * This comes from rename(const char *oldpath, const char *newpath)
  */
-static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
-		       struct inode *new_dir, struct dentry *new_dentry,
-		       unsigned int flags)
+static int ext4_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
+		       struct dentry *old_dentry, struct inode *new_dir,
+		       struct dentry *new_dentry, unsigned int flags)
 {
 	handle_t *handle = NULL;
 	struct ext4_renament old = {
@@ -3791,7 +3793,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 			goto end_rename;
 		}
 	} else {
-		whiteout = ext4_whiteout_for_rename(&old, credits, &handle);
+		whiteout = ext4_whiteout_for_rename(mnt_userns, &old, credits, &handle);
 		if (IS_ERR(whiteout)) {
 			retval = PTR_ERR(whiteout);
 			whiteout = NULL;
@@ -4110,7 +4112,7 @@ static int ext4_rename2(struct user_namespace *mnt_userns,
 					 new_dir, new_dentry);
 	}
 
-	return ext4_rename(old_dir, old_dentry, new_dir, new_dentry, flags);
+	return ext4_rename(mnt_userns, old_dir, old_dentry, new_dir, new_dentry, flags);
 }
 
 /*
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 9a6f9875aa34..a77fbb79e813 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -6654,7 +6654,7 @@ static struct file_system_type ext4_fs_type = {
 	.name		= "ext4",
 	.mount		= ext4_mount,
 	.kill_sb	= kill_block_super,
-	.fs_flags	= FS_REQUIRES_DEV,
+	.fs_flags	= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("ext4");