[PATCH] fs: require CAP_SYS_ADMIN in target namespace for idmapped mounts

[Seth Forshee <sforshee@digitalocean.com>]

Idmapped mounts should not allow a user to map file ownsership into a
range of ids which is not under the control of that user. However, we
currently don't check whether the mounter is privileged wrt to the
target user namespace.

Currently no FS_USERNS_MOUNT filesystems support idmapped mounts, thus
this is not a problem as only CAP_SYS_ADMIN in init_user_ns is allowed
to set up idmapped mounts. But this could change in the future, so add a
check to refuse to create idmapped mounts when the mounter does not have
CAP_SYS_ADMIN in the target user namespace.

Fixes: bd303368b776 ("fs: support mapped mounts of mapped filesystems")
Signed-off-by: Seth Forshee <sforshee@digitalocean.com>
---
 fs/namespace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 68789f896f08..51416e6caf90 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4003,8 +4003,9 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
 		return -EINVAL;
 
-	/* We're not controlling the superblock. */
-	if (!ns_capable(fs_userns, CAP_SYS_ADMIN))
+	/* Mounter doesn't control the source or target namespaces. */
+	if (!ns_capable(fs_userns, CAP_SYS_ADMIN) ||
+	    !ns_capable(kattr->mnt_userns, CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Mount has already been visible in the filesystem hierarchy. */
-- 
2.37.1