From: Seth Forshee sforshee@digitalocean.com
[ Upstream commit bf1ac16edf6770a92bc75cf2373f1f9feea398a4 ]
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 Reviewed-by: Christian Brauner (Microsoft) brauner@kernel.org Link: https://lore.kernel.org/r/20220816164752.2595240-1-sforshee@digitalocean.com Signed-off-by: Christian Brauner (Microsoft) brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/namespace.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/fs/namespace.c b/fs/namespace.c index e6a7e769d25dd..a59f8d645654a 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -4238,6 +4238,13 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize, err = -EPERM; goto out_fput; } + + /* We're not controlling the target namespace. */ + if (!ns_capable(mnt_userns, CAP_SYS_ADMIN)) { + err = -EPERM; + goto out_fput; + } + kattr->mnt_userns = get_user_ns(mnt_userns);
out_fput: