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// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (C) 2026 Christian Brauner <brauner@kernel.org>
*
* Test that completing a filesystem context from another user namespace
* doesn't warn.
*
* fsopen() records the caller's user namespace in fc->user_ns and hands
* back an ordinary file descriptor. The task that issues
* FSCONFIG_CMD_CREATE need not be the one that created the context: the fd
* is inherited across fork() and exec() and it can be passed over a unix
* socket. vfs_cmd_create() authorizes the create with mount_capable(),
* which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN),
* and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of
* fc->user_ns.
*
* binfmt_misc and overlayfs used to WARN_ON() that mismatch, which let an
* unprivileged user taint the kernel, flood the log and panic a kernel
* booted with panic_on_warn. The mount must still be refused, but it must
* not warn.
*/
#define _GNU_SOURCE
#include <errno.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>
#include "../wrappers.h"
#include "../utils.h"
#include "../../kselftest_harness.h"
#ifndef FSCONFIG_CMD_CREATE
#define FSCONFIG_CMD_CREATE 6
#endif
/* TAINT_WARN, i.e. bit 9 of /proc/sys/kernel/tainted. */
#define TAINT_WARN_BIT 9
static bool taint_warn_set(void)
{
unsigned long taint = 0;
FILE *f;
f = fopen("/proc/sys/kernel/tainted", "r");
if (!f)
return false;
if (fscanf(f, "%lu", &taint) != 1)
taint = 0;
fclose(f);
return taint & (1UL << TAINT_WARN_BIT);
}
static int send_fd(int sock, int fd)
{
char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
char b[1] = { 'x' };
struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
struct msghdr msg = {
.msg_iov = &iov,
.msg_iovlen = 1,
.msg_control = cmsgbuf,
.msg_controllen = sizeof(cmsgbuf),
};
struct cmsghdr *cmsg;
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
return sendmsg(sock, &msg, 0) < 0 ? -1 : 0;
}
static int recv_fd(int sock)
{
char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
char b[1];
struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
struct msghdr msg = {
.msg_iov = &iov,
.msg_iovlen = 1,
.msg_control = cmsgbuf,
.msg_controllen = sizeof(cmsgbuf),
};
struct cmsghdr *cmsg;
int fd = -1;
if (recvmsg(sock, &msg, 0) <= 0)
return -1;
cmsg = CMSG_FIRSTHDR(&msg);
if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS)
return -1;
memcpy(&fd, CMSG_DATA(cmsg), sizeof(int));
return fd;
}
/*
* Create a context for @fsname in a child and complete it here. With @nest
* the child first creates its own user namespace, so that the context is
* created in a descendant of the namespace completing it. The child needs a
* mount namespace of its own as well: fsopen() gates on may_mount(), which
* asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount
* namespace.
*
* Returns the result of FSCONFIG_CMD_CREATE with errno set, or -ENODATA if
* the child could not create the context at all.
*/
static int create_from_child(const char *fsname, bool nest)
{
int sock[2], fd, ret, status;
pid_t pid;
if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock))
return -ENODATA;
pid = fork();
if (pid < 0) {
close(sock[0]);
close(sock[1]);
return -ENODATA;
}
if (pid == 0) {
close(sock[0]);
if (nest && unshare(CLONE_NEWUSER | CLONE_NEWNS))
_exit(1);
fd = sys_fsopen(fsname, 0);
if (fd < 0)
_exit(1);
if (send_fd(sock[1], fd))
_exit(1);
_exit(0);
}
close(sock[1]);
fd = recv_fd(sock[0]);
close(sock[0]);
wait_for_pid(pid);
waitpid(pid, &status, WNOHANG);
if (fd < 0)
return -ENODATA;
errno = 0;
ret = sys_fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
status = errno;
close(fd);
errno = status;
return ret;
}
FIXTURE(fscontext_ns) {
bool warn_before;
};
FIXTURE_SETUP(fscontext_ns)
{
self->warn_before = taint_warn_set();
if (setup_userns() != 0)
SKIP(return, "setup_userns failed");
}
FIXTURE_TEARDOWN(fscontext_ns)
{
}
/*
* The condition the kernel used to WARN about. It has to be refused, and it
* has to be refused quietly: an unprivileged task reaches this.
*/
FIXTURE_VARIANT(fscontext_ns) {
const char *fsname;
int expected_errno;
};
FIXTURE_VARIANT_ADD(fscontext_ns, binfmt_misc) {
.fsname = "binfmt_misc",
.expected_errno = EINVAL,
};
FIXTURE_VARIANT_ADD(fscontext_ns, overlay) {
.fsname = "overlay",
.expected_errno = EIO,
};
TEST_F(fscontext_ns, create_from_descendant_userns)
{
int ret;
ret = create_from_child(variant->fsname, true);
if (ret == -ENODATA)
SKIP(return, "%s unavailable", variant->fsname);
ASSERT_EQ(-1, ret);
ASSERT_EQ(variant->expected_errno, errno);
/*
* Only meaningful if nothing had warned before us. Note that an
* unrelated warning racing this test would look like a failure.
*/
if (self->warn_before)
TH_LOG("TAINT_WARN already set, not checking for a new warning");
else
ASSERT_FALSE(taint_warn_set());
}
/*
* The same handover within one user namespace is a supported thing to do and
* has to keep working. binfmt_misc takes no options, so the create succeeds
* outright and this also shows the test really drives the create path.
*/
TEST(create_from_same_userns)
{
int ret;
if (setup_userns() != 0)
SKIP(return, "setup_userns failed");
ret = create_from_child("binfmt_misc", false);
if (ret == -ENODATA)
SKIP(return, "binfmt_misc unavailable");
ASSERT_EQ(0, ret);
}
TEST_HARNESS_MAIN
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