diff mbox series

[17/24] btrfs/ioctl: allow idmapped BTRFS_IOC_SNAP_DESTROY{_V2} ioctl

Message ID 20210713111344.1149376-18-brauner@kernel.org (mailing list archive)
State New, archived
Headers show
Series btrfs: support idmapped mounts | expand

Commit Message

Christian Brauner July 13, 2021, 11:13 a.m. UTC 
From: Christian Brauner <christian.brauner@ubuntu.com>

Destroying subvolumes and snapshots are important features of btrfs. Both
operations are available to unprivileged users if the filesystem has been
mounted with the "user_subvol_rm_allowed" mount option. Allow subvolume and
snapshot deletion on idmapped mounts. This is a fairly straightforward
operation since all the permission checking helpers are already capable of
handling idmapped mounts. So we just need to pass down the mount's userns.

In addition to regular subvolume or snapshot deletion by specifying the name of
the subvolume or snapshot the BTRFS_IOC_SNAP_DESTROY_V2 ioctl allows the
deletion of subvolumes and snapshots via subvolume and snapshot ids when the
BTRFS_SUBVOL_SPEC_BY_ID flag is raised.

This feature is blocked on idmapped mounts as this allows filesystem wide
subvolume deletions and thus can escape the scope of what's exposed under the
mount identified by the fd passed with the ioctl.

Here is an example where a btrfs subvolume is deleted through a subvolume mount
that does not expose the subvolume to be delete but it can still be deleted by
using the subvolume id:

 /* Compile the following program as "delete_by_spec". */

 #define _GNU_SOURCE
 #include <fcntl.h>
 #include <inttypes.h>
 #include <linux/btrfs.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/ioctl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>

 static int rm_subvolume_by_id(int fd, uint64_t subvolid)
 {
 	struct btrfs_ioctl_vol_args_v2 args = {};
 	int ret;

 	args.flags = BTRFS_SUBVOL_SPEC_BY_ID;
 	args.subvolid = subvolid;

 	ret = ioctl(fd, BTRFS_IOC_SNAP_DESTROY_V2, &args);
 	if (ret < 0)
 		return -1;

 	return 0;
 }

 int main(int argc, char *argv[])
 {
 	int subvolid = 0;

 	if (argc < 3)
 		exit(1);

 	fprintf(stderr, "Opening %s\n", argv[1]);
 	int fd = open(argv[1], O_CLOEXEC | O_DIRECTORY);
 	if (fd < 0)
 		exit(2);

 	subvolid = atoi(argv[2]);

 	fprintf(stderr, "Deleting subvolume with subvolid %d\n", subvolid);
 	int ret = rm_subvolume_by_id(fd, subvolid);
 	if (ret < 0)
 		exit(3);

 	exit(0);
 }
 #include <stdio.h>"
 #include <stdlib.h>"
 #include <linux/btrfs.h"

 truncate -s 10G btrfs.img
 mkfs.btrfs btrfs.img
 export LOOPDEV=$(sudo losetup -f --show btrfs.img)
 mount ${LOOPDEV} /mnt
 sudo chown $(id -u):$(id -g) /mnt
 btrfs subvolume create /mnt/A
 btrfs subvolume create /mnt/B/C
 # Get subvolume id via:
 sudo btrfs subvolume show /mnt/A
 # Save subvolid
 SUBVOLID=<nr>
 sudo umount /mnt
 sudo mount ${LOOPDEV} -o subvol=B/C,user_subvol_rm_allowed /mnt
 ./delete_by_spec /mnt ${SUBVOLID}

With idmapped mounts this can potentially be used by users to delete
subvolumes/snapshots they would otherwise not have access to as the idmapping
would be applied to an inode that is not exposed in the mount of the subvolume.

The fact that this is a filesystem wide operation suggests it might be a good
idea to expose this under a separate ioctl that clearly indicates this. In
essence, the file descriptor passed with the ioctl is merely used to identify
the filesystem on which to operate when BTRFS_SUBVOL_SPEC_BY_ID is used.

Cc: Chris Mason <clm@fb.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/btrfs/ioctl.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

Comments

Qu Wenruo July 14, 2021, 1 a.m. UTC | #1 
On 2021/7/13 下午7:13, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@ubuntu.com>
>
> Destroying subvolumes and snapshots are important features of btrfs. Both
> operations are available to unprivileged users if the filesystem has been
> mounted with the "user_subvol_rm_allowed" mount option. Allow subvolume and
> snapshot deletion on idmapped mounts. This is a fairly straightforward
> operation since all the permission checking helpers are already capable of
> handling idmapped mounts. So we just need to pass down the mount's userns.
>
> In addition to regular subvolume or snapshot deletion by specifying the name of
> the subvolume or snapshot the BTRFS_IOC_SNAP_DESTROY_V2 ioctl allows the
> deletion of subvolumes and snapshots via subvolume and snapshot ids when the
> BTRFS_SUBVOL_SPEC_BY_ID flag is raised.
>
> This feature is blocked on idmapped mounts as this allows filesystem wide
> subvolume deletions and thus can escape the scope of what's exposed under the
> mount identified by the fd passed with the ioctl.
>
> Here is an example where a btrfs subvolume is deleted through a subvolume mount
> that does not expose the subvolume to be delete but it can still be deleted by
> using the subvolume id:
>
>   /* Compile the following program as "delete_by_spec". */
>
>   #define _GNU_SOURCE
>   #include <fcntl.h>
>   #include <inttypes.h>
>   #include <linux/btrfs.h>
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <sys/ioctl.h>
>   #include <sys/stat.h>
>   #include <sys/types.h>
>   #include <unistd.h>
>
>   static int rm_subvolume_by_id(int fd, uint64_t subvolid)
>   {
>   	struct btrfs_ioctl_vol_args_v2 args = {};
>   	int ret;
>
>   	args.flags = BTRFS_SUBVOL_SPEC_BY_ID;
>   	args.subvolid = subvolid;
>
>   	ret = ioctl(fd, BTRFS_IOC_SNAP_DESTROY_V2, &args);
>   	if (ret < 0)
>   		return -1;
>
>   	return 0;
>   }
>
>   int main(int argc, char *argv[])
>   {
>   	int subvolid = 0;
>
>   	if (argc < 3)
>   		exit(1);
>
>   	fprintf(stderr, "Opening %s\n", argv[1]);
>   	int fd = open(argv[1], O_CLOEXEC | O_DIRECTORY);
>   	if (fd < 0)
>   		exit(2);
>
>   	subvolid = atoi(argv[2]);
>
>   	fprintf(stderr, "Deleting subvolume with subvolid %d\n", subvolid);
>   	int ret = rm_subvolume_by_id(fd, subvolid);
>   	if (ret < 0)
>   		exit(3);
>
>   	exit(0);
>   }
>   #include <stdio.h>"
>   #include <stdlib.h>"
>   #include <linux/btrfs.h"
>
>   truncate -s 10G btrfs.img
>   mkfs.btrfs btrfs.img
>   export LOOPDEV=$(sudo losetup -f --show btrfs.img)
>   mount ${LOOPDEV} /mnt
>   sudo chown $(id -u):$(id -g) /mnt
>   btrfs subvolume create /mnt/A
>   btrfs subvolume create /mnt/B/C
>   # Get subvolume id via:
>   sudo btrfs subvolume show /mnt/A
>   # Save subvolid
>   SUBVOLID=<nr>
>   sudo umount /mnt
>   sudo mount ${LOOPDEV} -o subvol=B/C,user_subvol_rm_allowed /mnt
>   ./delete_by_spec /mnt ${SUBVOLID}
>
> With idmapped mounts this can potentially be used by users to delete
> subvolumes/snapshots they would otherwise not have access to as the idmapping
> would be applied to an inode that is not exposed in the mount of the subvolume.
>
> The fact that this is a filesystem wide operation suggests it might be a good
> idea to expose this under a separate ioctl that clearly indicates this. In
> essence, the file descriptor passed with the ioctl is merely used to identify
> the filesystem on which to operate when BTRFS_SUBVOL_SPEC_BY_ID is used.
>
> Cc: Chris Mason <clm@fb.com>
> Cc: Josef Bacik <josef@toxicpanda.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: David Sterba <dsterba@suse.com>
> Cc: linux-btrfs@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> ---
>   fs/btrfs/ioctl.c | 26 ++++++++++++++++++++------
>   1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 31115083f382..dd0fabdbeeeb 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -830,7 +830,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
>    *     nfs_async_unlink().
>    */
>
> -static int btrfs_may_delete(struct inode *dir, struct dentry *victim, int isdir)
> +static int btrfs_may_delete(struct user_namespace *mnt_userns,
> +			    struct inode *dir, struct dentry *victim, int isdir)
>   {
>   	int error;
>
> @@ -840,12 +841,12 @@ static int btrfs_may_delete(struct inode *dir, struct dentry *victim, int isdir)
>   	BUG_ON(d_inode(victim->d_parent) != dir);
>   	audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
>
> -	error = inode_permission(&init_user_ns, dir, MAY_WRITE | MAY_EXEC);
> +	error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
>   	if (error)
>   		return error;
>   	if (IS_APPEND(dir))
>   		return -EPERM;
> -	if (check_sticky(&init_user_ns, dir, d_inode(victim)) ||
> +	if (check_sticky(mnt_userns, dir, d_inode(victim)) ||
>   	    IS_APPEND(d_inode(victim)) || IS_IMMUTABLE(d_inode(victim)) ||
>   	    IS_SWAPFILE(d_inode(victim)))
>   		return -EPERM;
> @@ -2914,6 +2915,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
>   	struct btrfs_root *dest = NULL;
>   	struct btrfs_ioctl_vol_args *vol_args = NULL;
>   	struct btrfs_ioctl_vol_args_v2 *vol_args2 = NULL;
> +	struct user_namespace *mnt_userns = file_mnt_user_ns(file);
>   	char *subvol_name, *subvol_name_ptr = NULL;
>   	int subvol_namelen;
>   	int err = 0;
> @@ -2941,6 +2943,18 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
>   			if (err)
>   				goto out;
>   		} else {
> +			/*
> +			 * Deleting by subvolume id can be used to delete
> +			 * subvolumes/snapshots anywhere in the filesystem.
> +			 * Ensure that users can't abuse idmapped mounts of
> +			 * btrfs subvolumes/snapshots to perform operations in
> +			 * the whole filesystem.
> +			 */
> +			if (mnt_userns != &init_user_ns) {
> +				err = -EINVAL;
> +				goto out;
> +			}

I'm OK to disable subvolume deletion by subvolid for now as a workaround.

In fact, for idmapped environment, if using btrfs-progs to delete
subvolume, it will do the root backref lookup, and if it can't do that,
it will abort the deletion.

Although the ultimate solution would be to make root backref lookup to
idmap compatible.

For a valid subvolume deletion, it needs to find the dentry of the root,
just like below you can see the related code:

                         dentry = btrfs_get_dentry(fs_info->sb,
                                         BTRFS_FIRST_FREE_OBJECTID,
                                         vol_args2->subvolid, 0, 0);
                         if (IS_ERR(dentry)) {
                                 err = PTR_ERR(dentry);
                                 goto out_drop_write;
                         }

Not sure how hard it would be, but I guess it may be a big project.

Thanks,
Qu

> +
>   			if (vol_args2->subvolid < BTRFS_FIRST_FREE_OBJECTID) {
>   				err = -EINVAL;
>   				goto out;
> @@ -3025,7 +3039,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
>   	err = down_write_killable_nested(&dir->i_rwsem, I_MUTEX_PARENT);
>   	if (err == -EINTR)
>   		goto free_subvol_name;
> -	dentry = lookup_one_len(&init_user_ns, subvol_name, parent, subvol_namelen);
> +	dentry = lookup_one_len(mnt_userns, subvol_name, parent, subvol_namelen);
>   	if (IS_ERR(dentry)) {
>   		err = PTR_ERR(dentry);
>   		goto out_unlock_dir;
> @@ -3067,14 +3081,14 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
>   		if (root == dest)
>   			goto out_dput;
>
> -		err = inode_permission(&init_user_ns, inode,
> +		err = inode_permission(mnt_userns, inode,
>   				       MAY_WRITE | MAY_EXEC);
>   		if (err)
>   			goto out_dput;
>   	}
>
>   	/* check if subvolume may be deleted by a user */
> -	err = btrfs_may_delete(dir, dentry, 1);
> +	err = btrfs_may_delete(mnt_userns, dir, dentry, 1);
>   	if (err)
>   		goto out_dput;
>
>
diff mbox series

Patch

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 31115083f382..dd0fabdbeeeb 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -830,7 +830,8 @@  static int create_snapshot(struct btrfs_root *root, struct inode *dir,
  *     nfs_async_unlink().
  */
 
-static int btrfs_may_delete(struct inode *dir, struct dentry *victim, int isdir)
+static int btrfs_may_delete(struct user_namespace *mnt_userns,
+			    struct inode *dir, struct dentry *victim, int isdir)
 {
 	int error;
 
@@ -840,12 +841,12 @@  static int btrfs_may_delete(struct inode *dir, struct dentry *victim, int isdir)
 	BUG_ON(d_inode(victim->d_parent) != dir);
 	audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
 
-	error = inode_permission(&init_user_ns, dir, MAY_WRITE | MAY_EXEC);
+	error = inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
 	if (error)
 		return error;
 	if (IS_APPEND(dir))
 		return -EPERM;
-	if (check_sticky(&init_user_ns, dir, d_inode(victim)) ||
+	if (check_sticky(mnt_userns, dir, d_inode(victim)) ||
 	    IS_APPEND(d_inode(victim)) || IS_IMMUTABLE(d_inode(victim)) ||
 	    IS_SWAPFILE(d_inode(victim)))
 		return -EPERM;
@@ -2914,6 +2915,7 @@  static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	struct btrfs_root *dest = NULL;
 	struct btrfs_ioctl_vol_args *vol_args = NULL;
 	struct btrfs_ioctl_vol_args_v2 *vol_args2 = NULL;
+	struct user_namespace *mnt_userns = file_mnt_user_ns(file);
 	char *subvol_name, *subvol_name_ptr = NULL;
 	int subvol_namelen;
 	int err = 0;
@@ -2941,6 +2943,18 @@  static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 			if (err)
 				goto out;
 		} else {
+			/*
+			 * Deleting by subvolume id can be used to delete
+			 * subvolumes/snapshots anywhere in the filesystem.
+			 * Ensure that users can't abuse idmapped mounts of
+			 * btrfs subvolumes/snapshots to perform operations in
+			 * the whole filesystem.
+			 */
+			if (mnt_userns != &init_user_ns) {
+				err = -EINVAL;
+				goto out;
+			}
+
 			if (vol_args2->subvolid < BTRFS_FIRST_FREE_OBJECTID) {
 				err = -EINVAL;
 				goto out;
@@ -3025,7 +3039,7 @@  static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 	err = down_write_killable_nested(&dir->i_rwsem, I_MUTEX_PARENT);
 	if (err == -EINTR)
 		goto free_subvol_name;
-	dentry = lookup_one_len(&init_user_ns, subvol_name, parent, subvol_namelen);
+	dentry = lookup_one_len(mnt_userns, subvol_name, parent, subvol_namelen);
 	if (IS_ERR(dentry)) {
 		err = PTR_ERR(dentry);
 		goto out_unlock_dir;
@@ -3067,14 +3081,14 @@  static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 		if (root == dest)
 			goto out_dput;
 
-		err = inode_permission(&init_user_ns, inode,
+		err = inode_permission(mnt_userns, inode,
 				       MAY_WRITE | MAY_EXEC);
 		if (err)
 			goto out_dput;
 	}
 
 	/* check if subvolume may be deleted by a user */
-	err = btrfs_may_delete(dir, dentry, 1);
+	err = btrfs_may_delete(mnt_userns, dir, dentry, 1);
 	if (err)
 		goto out_dput;