xattr: always invoke cap_convert_nscap() for security.capability writes

Commit 8db6c34f1d ("Introduce v3 namespaced file capabilities") changed
cap_inode_setxattr() to return 0 unconditionally for XATTR_NAME_CAPS
("security.capability") and moved capability validation into
cap_convert_nscap(), but placed the call inside the "if (size)" block in
setxattr(). Commit 7c03e2cda4 ("vfs: move cap_convert_nscap() call into
vfs_setxattr()") preserved that "size &&" condition in vfs_setxattr():

	if (size && strcmp(name, XATTR_NAME_CAPS) == 0) {
		error = cap_convert_nscap(idmap, dentry, &value, size);
		...
	}

For security.* extended attributes, xattr_permission() skips
inode_permission(MAY_WRITE) and leaves permission checks to the security
module, while cap_inode_setxattr() returns 0 for XATTR_NAME_CAPS assuming
cap_convert_nscap() has already validated the write. Because
__vfs_setxattr() treats size == 0 as an empty attribute write (setting
value = "") rather than a removal, calling setxattr(2) on
"security.capability" with size == 0 bypasses cap_convert_nscap() and
allows an unprivileged caller without CAP_SETFCAP or write permission on
the target inode to write a 0-byte security.capability xattr (or
overwrite an existing one).

When that binary is subsequently executed, get_vfs_caps_from_disk() reads
the 0-byte security.capability xattr and fails the
size < sizeof(magic_etc) check with -EINVAL, causing execve(2) to fail
with -EINVAL.

Always invoke cap_convert_nscap() when strcmp(name, XATTR_NAME_CAPS) == 0
regardless of size, so that 0-byte writes hit the existing !*ivalue /
!validheader() checks in cap_convert_nscap() and are rejected with
-EINVAL.

Fixes: 8db6c34f1d ("Introduce v3 namespaced file capabilities")
Signed-off-by: Dominik Maier <dmnk@google.com>
Link: https://patch.msgid.link/20261022232119.2537209-1-dmnk@google.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/xattr.c

@@ -332,7 +332,7 @@ vfs_setxattr(const struct mnt_idmap *idmap, struct dentry *dentry,
 	const void  *orig_value = value;
 	int error;
 
-	if (size && strcmp(name, XATTR_NAME_CAPS) == 0) {
+	if (strcmp(name, XATTR_NAME_CAPS) == 0) {
 		error = cap_convert_nscap(idmap, dentry, &value, size);
 		if (error < 0)
 			return error;