[PATCH v2 07/25] capability: provide a helper for converting vfs_caps to xattr for userspace

Seth Forshee (DigitalOcean)
2024-02-21T21:25:06Z

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) <sforshee@kernel.org>
---
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