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1// SPDX-License-Identifier: GPL-2.0
2#define _GNU_SOURCE
3
4#include <errno.h>
5#include <fcntl.h>
6#include <limits.h>
7#include <sched.h>
8#include <stdio.h>
9#include <unistd.h>
10#include <sys/fsuid.h>
11#include <sys/stat.h>
12#include <sys/syscall.h>
13
14#include <linux/mount.h>
15#include <linux/types.h>
16
17#include "kselftest_harness.h"
18#include "wrappers.h"
19#include "utils.h"
20
21/*
22 * The test mount maps caller-visible ids [0, MAP_RANGE) onto the on-disk range
23 * [MAP_HOST, MAP_HOST + MAP_RANGE). An id outside [0, MAP_RANGE) therefore has
24 * no mapping in the mount and is not representable in the filesystem.
25 */
26#define MAP_HOST 10000
27#define MAP_RANGE 10000
28#define UNMAPPED 50000
29
30#ifndef MOUNT_ATTR_IDMAP
31#define MOUNT_ATTR_IDMAP 0x00100000
32#endif
33
34#ifndef __NR_mount_setattr
35#define __NR_mount_setattr 442
36#endif
37
38static inline int sys_mount_setattr(int dfd, const char *path,
39 unsigned int flags,
40 struct mount_attr *attr, size_t size)
41{
42 return syscall(__NR_mount_setattr, dfd, path, flags, attr, size);
43}
44
45/*
46 * Clone @path into a detached mount idmapped so that caller-visible ids
47 * [0, MAP_RANGE) map onto the on-disk ids [MAP_HOST, MAP_HOST + MAP_RANGE).
48 * Returns the mount fd, or -1 if idmapped mounts are not available.
49 */
50static int idmapped_clone(const char *path)
51{
52 struct mount_attr attr = {
53 .attr_set = MOUNT_ATTR_IDMAP,
54 };
55 int fd_tree, userns_fd, ret;
56
57 fd_tree = sys_open_tree(AT_FDCWD, path,
58 OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC);
59 if (fd_tree < 0)
60 return -1;
61
62 userns_fd = get_userns_fd(MAP_HOST, 0, MAP_RANGE);
63 if (userns_fd < 0) {
64 close(fd_tree);
65 return -1;
66 }
67
68 attr.userns_fd = userns_fd;
69 ret = sys_mount_setattr(fd_tree, "", AT_EMPTY_PATH, &attr, sizeof(attr));
70 close(userns_fd);
71 if (ret) {
72 close(fd_tree);
73 return -1;
74 }
75
76 return fd_tree;
77}
78
79FIXTURE(idmapped_tmpfile) {
80 char dir[64]; /* non-idmapped path to the layer directory */
81};
82
83FIXTURE_SETUP(idmapped_tmpfile)
84{
85 /* Private mount namespace so test mounts need no cleanup. */
86 ASSERT_EQ(unshare(CLONE_NEWNS), 0);
87 ASSERT_EQ(sys_mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL), 0);
88 ASSERT_EQ(sys_mount("tmpfs", "/tmp", "tmpfs", 0, NULL), 0);
89
90 snprintf(self->dir, sizeof(self->dir), "/tmp/d");
91 ASSERT_EQ(mkdir(self->dir, 0777), 0);
92 /* World-writable so an unmapped caller still passes permission(). */
93 ASSERT_EQ(chmod(self->dir, 0777), 0);
94 /*
95 * The directory itself must be inside the range the mount maps.
96 * Left owned by 0 it has no mapping in the mount, and
97 * inode_permission() then fails MAY_WRITE via HAS_UNMAPPED_ID()
98 * before any of the tmpfile logic is reached, regardless of mode.
99 */
100 ASSERT_EQ(chown(self->dir, MAP_HOST, MAP_HOST), 0);
101}
102
103FIXTURE_TEARDOWN(idmapped_tmpfile)
104{
105}
106
107/*
108 * A caller whose fsuid/fsgid have no mapping in the idmapped mount must not be
109 * able to create an O_TMPFILE. Without the check in vfs_tmpfile() the inode
110 * would be created owned by (uid_t)-1 and could then be linked into the
111 * namespace.
112 */
113TEST_F(idmapped_tmpfile, unmapped_caller_is_refused)
114{
115 int mfd, fd;
116
117 mfd = idmapped_clone(self->dir);
118 if (mfd < 0)
119 SKIP(return, "idmapped mounts not supported");
120
121 /* Become a caller outside the mount's [0, MAP_RANGE) range. */
122 setfsgid(UNMAPPED);
123 setfsuid(UNMAPPED);
124 ASSERT_EQ(setfsuid(-1), UNMAPPED);
125
126 fd = openat(mfd, ".", O_TMPFILE | O_WRONLY, 0644);
127 ASSERT_LT(fd, 0);
128 EXPECT_EQ(errno, EOVERFLOW);
129 if (fd >= 0)
130 close(fd);
131
132 EXPECT_EQ(close(mfd), 0);
133}
134
135/*
136 * A mapped caller can create an O_TMPFILE and link it into the namespace; the
137 * ownership round-trips through the mount idmap. This is what makes refusing
138 * the unmapped case above necessary in the first place.
139 */
140TEST_F(idmapped_tmpfile, mapped_caller_creates_and_links)
141{
142 char path[PATH_MAX];
143 struct stat st;
144 int mfd, fd;
145
146 mfd = idmapped_clone(self->dir);
147 if (mfd < 0)
148 SKIP(return, "idmapped mounts not supported");
149
150 /* Caller is uid/gid 0, which maps to MAP_HOST through the mount. */
151 fd = openat(mfd, ".", O_TMPFILE | O_RDWR, 0600);
152 ASSERT_GE(fd, 0);
153
154 ASSERT_EQ(fstat(fd, &st), 0);
155 EXPECT_EQ(st.st_uid, 0);
156 EXPECT_EQ(st.st_gid, 0);
157
158 /* The tmpfile is linkable: splice it into the directory. */
159 ASSERT_EQ(linkat(fd, "", mfd, "linked", AT_EMPTY_PATH), 0);
160 EXPECT_EQ(close(fd), 0);
161
162 ASSERT_EQ(fstatat(mfd, "linked", &st, 0), 0);
163 EXPECT_EQ(st.st_uid, 0);
164 EXPECT_EQ(st.st_gid, 0);
165
166 /* On the underlying, non-idmapped tmpfs it is stored as MAP_HOST. */
167 snprintf(path, sizeof(path), "%s/linked", self->dir);
168 ASSERT_EQ(stat(path, &st), 0);
169 EXPECT_EQ(st.st_uid, MAP_HOST);
170 EXPECT_EQ(st.st_gid, MAP_HOST);
171
172 EXPECT_EQ(close(mfd), 0);
173}
174
175TEST_HARNESS_MAIN