1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Copyright (C) 2026 Christian Brauner <brauner@kernel.org> 4 * 5 * Test that completing a filesystem context from another user namespace 6 * doesn't warn. 7 * 8 * fsopen() records the caller's user namespace in fc->user_ns and hands 9 * back an ordinary file descriptor. The task that issues 10 * FSCONFIG_CMD_CREATE need not be the one that created the context: the fd 11 * is inherited across fork() and exec() and it can be passed over a unix 12 * socket. vfs_cmd_create() authorizes the create with mount_capable(), 13 * which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), 14 * and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of 15 * fc->user_ns. 16 * 17 * binfmt_misc and overlayfs used to WARN_ON() that mismatch, which let an 18 * unprivileged user taint the kernel, flood the log and panic a kernel 19 * booted with panic_on_warn. The mount must still be refused, but it must 20 * not warn. 21 */ 22 #define _GNU_SOURCE 23 24 #include <errno.h> 25 #include <sched.h> 26 #include <stdio.h> 27 #include <stdlib.h> 28 #include <string.h> 29 #include <sys/socket.h> 30 #include <sys/wait.h> 31 #include <unistd.h> 32 33 #include "../wrappers.h" 34 #include "../utils.h" 35 #include "../../kselftest_harness.h" 36 37 #ifndef FSCONFIG_CMD_CREATE 38 #define FSCONFIG_CMD_CREATE 6 39 #endif 40 41 /* TAINT_WARN, i.e. bit 9 of /proc/sys/kernel/tainted. */ 42 #define TAINT_WARN_BIT 9 43 44 static bool taint_warn_set(void) 45 { 46 unsigned long taint = 0; 47 FILE *f; 48 49 f = fopen("/proc/sys/kernel/tainted", "r"); 50 if (!f) 51 return false; 52 if (fscanf(f, "%lu", &taint) != 1) 53 taint = 0; 54 fclose(f); 55 56 return taint & (1UL << TAINT_WARN_BIT); 57 } 58 59 static int send_fd(int sock, int fd) 60 { 61 char cmsgbuf[CMSG_SPACE(sizeof(int))] = {}; 62 char b[1] = { 'x' }; 63 struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) }; 64 struct msghdr msg = { 65 .msg_iov = &iov, 66 .msg_iovlen = 1, 67 .msg_control = cmsgbuf, 68 .msg_controllen = sizeof(cmsgbuf), 69 }; 70 struct cmsghdr *cmsg; 71 72 cmsg = CMSG_FIRSTHDR(&msg); 73 cmsg->cmsg_level = SOL_SOCKET; 74 cmsg->cmsg_type = SCM_RIGHTS; 75 cmsg->cmsg_len = CMSG_LEN(sizeof(int)); 76 memcpy(CMSG_DATA(cmsg), &fd, sizeof(int)); 77 78 return sendmsg(sock, &msg, 0) < 0 ? -1 : 0; 79 } 80 81 static int recv_fd(int sock) 82 { 83 char cmsgbuf[CMSG_SPACE(sizeof(int))] = {}; 84 char b[1]; 85 struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) }; 86 struct msghdr msg = { 87 .msg_iov = &iov, 88 .msg_iovlen = 1, 89 .msg_control = cmsgbuf, 90 .msg_controllen = sizeof(cmsgbuf), 91 }; 92 struct cmsghdr *cmsg; 93 int fd = -1; 94 95 if (recvmsg(sock, &msg, 0) <= 0) 96 return -1; 97 98 cmsg = CMSG_FIRSTHDR(&msg); 99 if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS) 100 return -1; 101 memcpy(&fd, CMSG_DATA(cmsg), sizeof(int)); 102 103 return fd; 104 } 105 106 /* 107 * Create a context for @fsname in a child and complete it here. With @nest 108 * the child first creates its own user namespace, so that the context is 109 * created in a descendant of the namespace completing it. The child needs a 110 * mount namespace of its own as well: fsopen() gates on may_mount(), which 111 * asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount 112 * namespace. 113 * 114 * Returns the result of FSCONFIG_CMD_CREATE with errno set, or -ENODATA if 115 * the child could not create the context at all. 116 */ 117 static int create_from_child(const char *fsname, bool nest) 118 { 119 int sock[2], fd, ret, status; 120 pid_t pid; 121 122 if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock)) 123 return -ENODATA; 124 125 pid = fork(); 126 if (pid < 0) { 127 close(sock[0]); 128 close(sock[1]); 129 return -ENODATA; 130 } 131 132 if (pid == 0) { 133 close(sock[0]); 134 135 if (nest && unshare(CLONE_NEWUSER | CLONE_NEWNS)) 136 _exit(1); 137 138 fd = sys_fsopen(fsname, 0); 139 if (fd < 0) 140 _exit(1); 141 if (send_fd(sock[1], fd)) 142 _exit(1); 143 _exit(0); 144 } 145 146 close(sock[1]); 147 fd = recv_fd(sock[0]); 148 close(sock[0]); 149 wait_for_pid(pid); 150 waitpid(pid, &status, WNOHANG); 151 152 if (fd < 0) 153 return -ENODATA; 154 155 errno = 0; 156 ret = sys_fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0); 157 status = errno; 158 close(fd); 159 errno = status; 160 161 return ret; 162 } 163 164 FIXTURE(fscontext_ns) { 165 bool warn_before; 166 }; 167 168 FIXTURE_SETUP(fscontext_ns) 169 { 170 self->warn_before = taint_warn_set(); 171 172 if (setup_userns() != 0) 173 SKIP(return, "setup_userns failed"); 174 } 175 176 FIXTURE_TEARDOWN(fscontext_ns) 177 { 178 } 179 180 /* 181 * The condition the kernel used to WARN about. It has to be refused, and it 182 * has to be refused quietly: an unprivileged task reaches this. 183 */ 184 FIXTURE_VARIANT(fscontext_ns) { 185 const char *fsname; 186 int expected_errno; 187 }; 188 189 FIXTURE_VARIANT_ADD(fscontext_ns, binfmt_misc) { 190 .fsname = "binfmt_misc", 191 .expected_errno = EINVAL, 192 }; 193 194 FIXTURE_VARIANT_ADD(fscontext_ns, overlay) { 195 .fsname = "overlay", 196 .expected_errno = EIO, 197 }; 198 199 TEST_F(fscontext_ns, create_from_descendant_userns) 200 { 201 int ret; 202 203 ret = create_from_child(variant->fsname, true); 204 if (ret == -ENODATA) 205 SKIP(return, "%s unavailable", variant->fsname); 206 207 ASSERT_EQ(-1, ret); 208 ASSERT_EQ(variant->expected_errno, errno); 209 210 /* 211 * Only meaningful if nothing had warned before us. Note that an 212 * unrelated warning racing this test would look like a failure. 213 */ 214 if (self->warn_before) 215 TH_LOG("TAINT_WARN already set, not checking for a new warning"); 216 else 217 ASSERT_FALSE(taint_warn_set()); 218 } 219 220 /* 221 * The same handover within one user namespace is a supported thing to do and 222 * has to keep working. binfmt_misc takes no options, so the create succeeds 223 * outright and this also shows the test really drives the create path. 224 */ 225 TEST(create_from_same_userns) 226 { 227 int ret; 228 229 if (setup_userns() != 0) 230 SKIP(return, "setup_userns failed"); 231 232 ret = create_from_child("binfmt_misc", false); 233 if (ret == -ENODATA) 234 SKIP(return, "binfmt_misc unavailable"); 235 236 ASSERT_EQ(0, ret); 237 } 238 239 TEST_HARNESS_MAIN 240