cap_inode_getsecurity() implements a handful of policies for capability xattrs read by userspace:
- It returns EINVAL if the on-disk capability is in v1 format. - It masks off all bits in magic_etc except for the version and VFS_CAP_FLAGS_EFFECTIVE. - v3 capabilities are converted to v2 format if the rootid returned to userspace would be 0 or if the rootid corresponds to root in an ancestor user namespace. - It returns EOVERFLOW for a v3 capability whose rootid does not map to a valid id in current_user_ns() or to root in an ancestor namespace. These policies must be maintained when converting vfs_caps to an xattr for userspace. Provide a vfs_caps_to_user_xattr() helper which will enforce these policies. Signed-off-by: Seth Forshee (DigitalOcean) <[email protected]> --- include/linux/capability.h | 4 +++ security/commoncap.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 82 insertions(+) diff --git a/include/linux/capability.h b/include/linux/capability.h index a0893ac4664b..eb06d7c6224b 100644 --- a/include/linux/capability.h +++ b/include/linux/capability.h @@ -218,6 +218,10 @@ ssize_t vfs_caps_to_xattr(struct mnt_idmap *idmap, struct user_namespace *dest_userns, const struct vfs_caps *vfs_caps, void *data, size_t size); +ssize_t vfs_caps_to_user_xattr(struct mnt_idmap *idmap, + struct user_namespace *dest_userns, + const struct vfs_caps *vfs_caps, + void *data, size_t size); /* audit system wants to get cap info from files as well */ int get_vfs_caps_from_disk(struct mnt_idmap *idmap, diff --git a/security/commoncap.c b/security/commoncap.c index 7531c9634997..289530e58c37 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -791,6 +791,84 @@ ssize_t vfs_caps_to_xattr(struct mnt_idmap *idmap, return ret; } +/** + * vfs_caps_to_user_xattr - convert vfs_caps to caps xattr for userspace + * + * @idmap: idmap of the mount the inode was found from + * @dest_userns: user namespace for ids in xattr data + * @vfs_caps: source vfs_caps data + * @data: destination buffer for rax xattr caps data + * @size: size of the @data buffer + * + * Converts a kernel-internal capability into the raw security.capability + * xattr format. Implements the following policies required for fscaps + * returned to userspace: + * + * - Returns -EINVAL if the on-disk capability is in v1 format. + * - Masks off all bits in magic_etc except for the version and + * VFS_CAP_FLAGS_EFFECTIVE. + * - Converts v3 capabilities to v2 format if the rootid returned to + * userspace would be 0 or if the rootid corresponds to root in an + * ancestor user namespace. + * - Returns EOVERFLOW for a v3 capability whose rootid does not map to a + * valid id in current_user_ns() or to root in an ancestor namespace. + * + * If the xattr is being read or written through an idmapped mount the + * idmap of the vfsmount must be passed through @idmap. This function + * will then take care to map the rootid according to @idmap. + * + * Return: On success, return the size of the xattr data. On error, + * return < 0. + */ +ssize_t vfs_caps_to_user_xattr(struct mnt_idmap *idmap, + struct user_namespace *dest_userns, + const struct vfs_caps *vfs_caps, + void *data, size_t size) +{ + struct vfs_ns_cap_data *ns_caps = data; + bool is_v3; + u32 magic; + + /* Preserve previous behavior of returning EINVAL for v1 caps */ + if ((vfs_caps->magic_etc & VFS_CAP_REVISION_MASK) == VFS_CAP_REVISION_1) + return -EINVAL; + + size = __vfs_caps_to_xattr(idmap, dest_userns, vfs_caps, data, size); + if (size < 0) + return size; + + magic = vfs_caps->magic_etc & + (VFS_CAP_REVISION_MASK | VFS_CAP_FLAGS_EFFECTIVE); + ns_caps->magic_etc = cpu_to_le32(magic); + + /* + * If this is a v3 capability with a valid, non-zero rootid, return + * the v3 capability to userspace. A v3 capability with a rootid of + * 0 will be converted to a v2 capability below for compatibility + * with old userspace. + */ + is_v3 = (vfs_caps->magic_etc & VFS_CAP_REVISION_MASK) == VFS_CAP_REVISION_3; + if (is_v3) { + uid_t rootid = le32_to_cpu(ns_caps->rootid); + if (rootid != (uid_t)-1 && rootid != (uid_t)0) + return size; + } + + if (!rootid_owns_currentns(vfs_caps->rootid)) + return -EOVERFLOW; + + /* This comes from a parent namespace. Return as a v2 capability. */ + if (is_v3) { + magic = VFS_CAP_REVISION_2 | + (vfs_caps->magic_etc & VFS_CAP_FLAGS_EFFECTIVE); + ns_caps->magic_etc = cpu_to_le32(magic); + ns_caps->rootid = cpu_to_le32(0); + size = XATTR_CAPS_SZ_2; + } + + return size; +} + /** * get_vfs_caps_from_disk - retrieve vfs caps from disk * -- 2.43.0
