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btrfs: check whether fsgid/fsuid are mapped during subvolume creation

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When a new subvolume is created btrfs currently doesn't check whether
the fsgid/fsuid of the caller actually have a mapping in the user
namespace attached to the filesystem. The VFS always checks this to make
sure that the caller's fsgid/fsuid can be represented on-disk. This is
most relevant for filesystems that can be mounted inside user namespaces
but it is in general a good hardening measure to prevent unrepresentable
gid/uid from being written to disk.

Since we want to support idmapped mounts for btrfs ioctls to create
subvolumes in follow-up patches this becomes important since we want to
make sure the fsgid/fsuid of the caller as mapped according to the
idmapped mount can be represented on-disk. Simply add the missing
fsuidgid_has_mapping() line from the VFS may_create() version to
btrfs_may_create().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Christian Brauner 2021-07-27 12:48:51 +02:00 committed by David Sterba
parent 3bc71ba02c
commit 5474bf400f
1 changed files with 2 additions and 0 deletions
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2
fs/btrfs/ioctl.c
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@ -877,6 +877,8 @@ static inline int btrfs_may_create(struct inode *dir, struct dentry *child)
return -EEXIST;
if (IS_DEADDIR(dir))
return -ENOENT;
if (!fsuidgid_has_mapping(dir->i_sb, &init_user_ns))
return -EOVERFLOW;
return inode_permission(&init_user_ns, dir, MAY_WRITE | MAY_EXEC);
}