1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * memfd GUP test-case
4 * This tests memfd interactions with get_user_pages(). We require the
5 * fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
6 * file-system delays _all_ reads by 1s and forces direct-IO. This means, any
7 * read() on files in that file-system will pin the receive-buffer pages for at
8 * least 1s via get_user_pages().
9 *
10 * We use this trick to race ADD_SEALS against a write on a memfd object. The
11 * ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
12 * the read() syscall with our memory-mapped memfd object as receive buffer to
13 * force the kernel to write into our memfd object.
14 */
15
16 #define _GNU_SOURCE
17 #define __EXPORTED_HEADERS__
18
19 #include <errno.h>
20 #include <inttypes.h>
21 #include <limits.h>
22 #include <linux/falloc.h>
23 #include <fcntl.h>
24 #include <linux/memfd.h>
25 #include <linux/types.h>
26 #include <sched.h>
27 #include <stdio.h>
28 #include <stdlib.h>
29 #include <signal.h>
30 #include <string.h>
31 #include <sys/mman.h>
32 #include <sys/stat.h>
33 #include <sys/syscall.h>
34 #include <sys/wait.h>
35 #include <unistd.h>
36
37 #include "common.h"
38
39 #define MFD_DEF_SIZE 8192
40 #define STACK_SIZE 65536
41
42 static size_t mfd_def_size = MFD_DEF_SIZE;
43
44 static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
45 {
46 int r, fd;
47
48 fd = sys_memfd_create(name, flags);
49 if (fd < 0) {
50 printf("memfd_create(\"%s\", %u) failed: %m\n",
51 name, flags);
52 abort();
53 }
54
55 r = ftruncate(fd, sz);
56 if (r < 0) {
57 printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
58 abort();
59 }
60
61 return fd;
62 }
63
64 static __u64 mfd_assert_get_seals(int fd)
65 {
66 long r;
67
68 r = fcntl(fd, F_GET_SEALS);
69 if (r < 0) {
70 printf("GET_SEALS(%d) failed: %m\n", fd);
71 abort();
72 }
73
74 return r;
75 }
76
77 static void mfd_assert_has_seals(int fd, __u64 seals)
78 {
79 __u64 s;
80
81 s = mfd_assert_get_seals(fd);
82 if (s != seals) {
83 printf("%llu != %llu = GET_SEALS(%d)\n",
84 (unsigned long long)seals, (unsigned long long)s, fd);
85 abort();
86 }
87 }
88
89 static void mfd_assert_add_seals(int fd, __u64 seals)
90 {
91 long r;
92 __u64 s;
93
94 s = mfd_assert_get_seals(fd);
95 r = fcntl(fd, F_ADD_SEALS, seals);
96 if (r < 0) {
97 printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
98 fd, (unsigned long long)s, (unsigned long long)seals);
99 abort();
100 }
101 }
102
103 static int mfd_busy_add_seals(int fd, __u64 seals)
104 {
105 long r;
106 __u64 s;
107
108 r = fcntl(fd, F_GET_SEALS);
109 if (r < 0)
110 s = 0;
111 else
112 s = r;
113
114 r = fcntl(fd, F_ADD_SEALS, seals);
115 if (r < 0 && errno != EBUSY) {
116 printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected with EBUSY: %m\n",
117 fd, (unsigned long long)s, (unsigned long long)seals);
118 abort();
119 }
120
121 return r;
122 }
123
124 static void *mfd_assert_mmap_shared(int fd)
125 {
126 void *p;
127
128 p = mmap(NULL,
129 mfd_def_size,
130 PROT_READ | PROT_WRITE,
131 MAP_SHARED,
132 fd,
133 0);
134 if (p == MAP_FAILED) {
135 printf("mmap() failed: %m\n");
136 abort();
137 }
138
139 return p;
140 }
141
142 static void *mfd_assert_mmap_private(int fd)
143 {
144 void *p;
145
146 p = mmap(NULL,
147 mfd_def_size,
148 PROT_READ | PROT_WRITE,
149 MAP_PRIVATE,
150 fd,
151 0);
152 if (p == MAP_FAILED) {
153 printf("mmap() failed: %m\n");
154 abort();
155 }
156
157 return p;
158 }
159
160 static int global_mfd = -1;
161 static void *global_p = NULL;
162
163 static int sealing_thread_fn(void *arg)
164 {
165 int sig, r;
166
167 /*
168 * This thread first waits 200ms so any pending operation in the parent
169 * is correctly started. After that, it tries to seal @global_mfd as
170 * SEAL_WRITE. This _must_ fail as the parent thread has a read() into
171 * that memory mapped object still ongoing.
172 * We then wait one more second and try sealing again. This time it
173 * must succeed as there shouldn't be anyone else pinning the pages.
174 */
175
176 /* wait 200ms for FUSE-request to be active */
177 usleep(200000);
178
179 /* unmount mapping before sealing to avoid i_mmap_writable failures */
180 munmap(global_p, mfd_def_size);
181
182 /* Try sealing the global file; expect EBUSY or success. Current
183 * kernels will never succeed, but in the future, kernels might
184 * implement page-replacements or other fancy ways to avoid racing
185 * writes. */
186 r = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);
187 if (r >= 0) {
188 printf("HURRAY! This kernel fixed GUP races!\n");
189 } else {
190 /* wait 1s more so the FUSE-request is done */
191 sleep(1);
192
193 /* try sealing the global file again */
194 mfd_assert_add_seals(global_mfd, F_SEAL_WRITE);
195 }
196
197 return 0;
198 }
199
200 static pid_t spawn_sealing_thread(void)
201 {
202 uint8_t *stack;
203 pid_t pid;
204
205 stack = malloc(STACK_SIZE);
206 if (!stack) {
207 printf("malloc(STACK_SIZE) failed: %m\n");
208 abort();
209 }
210
211 pid = clone(sealing_thread_fn,
212 stack + STACK_SIZE,
213 SIGCHLD | CLONE_FILES | CLONE_FS | CLONE_VM,
214 NULL);
215 if (pid < 0) {
216 printf("clone() failed: %m\n");
217 abort();
218 }
219
220 return pid;
221 }
222
223 static void join_sealing_thread(pid_t pid)
224 {
225 waitpid(pid, NULL, 0);
226 }
227
228 int main(int argc, char **argv)
229 {
230 char *zero;
231 int fd, mfd, r;
232 void *p;
233 int was_sealed;
234 pid_t pid;
235
236 if (argc < 2) {
237 printf("error: please pass path to file in fuse_mnt mount-point\n");
238 abort();
239 }
240
241 if (argc >= 3) {
242 if (!strcmp(argv[2], "hugetlbfs")) {
243 unsigned long hpage_size = default_huge_page_size();
244
245 if (!hpage_size) {
246 printf("Unable to determine huge page size\n");
247 abort();
248 }
249
250 hugetlbfs_test = 1;
251 mfd_def_size = hpage_size * 2;
252 } else {
253 printf("Unknown option: %s\n", argv[2]);
254 abort();
255 }
256 }
257
258 zero = calloc(sizeof(*zero), mfd_def_size);
259
260 /* open FUSE memfd file for GUP testing */
261 printf("opening: %s\n", argv[1]);
262 fd = open(argv[1], O_RDONLY | O_CLOEXEC);
263 if (fd < 0) {
264 printf("cannot open(\"%s\"): %m\n", argv[1]);
265 abort();
266 }
267
268 /* create new memfd-object */
269 mfd = mfd_assert_new("kern_memfd_fuse",
270 mfd_def_size,
271 MFD_CLOEXEC | MFD_ALLOW_SEALING);
272
273 /* mmap memfd-object for writing */
274 p = mfd_assert_mmap_shared(mfd);
275
276 /* pass mfd+mapping to a separate sealing-thread which tries to seal
277 * the memfd objects with SEAL_WRITE while we write into it */
278 global_mfd = mfd;
279 global_p = p;
280 pid = spawn_sealing_thread();
281
282 /* Use read() on the FUSE file to read into our memory-mapped memfd
283 * object. This races the other thread which tries to seal the
284 * memfd-object.
285 * If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
286 * This guarantees that the receive-buffer is pinned for 1s until the
287 * data is written into it. The racing ADD_SEALS should thus fail as
288 * the pages are still pinned. */
289 r = read(fd, p, mfd_def_size);
290 if (r < 0) {
291 printf("read() failed: %m\n");
292 abort();
293 } else if (!r) {
294 printf("unexpected EOF on read()\n");
295 abort();
296 }
297
298 was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
299
300 /* Wait for sealing-thread to finish and verify that it
301 * successfully sealed the file after the second try. */
302 join_sealing_thread(pid);
303 mfd_assert_has_seals(mfd, F_SEAL_WRITE);
304
305 /* *IF* the memfd-object was sealed at the time our read() returned,
306 * then the kernel did a page-replacement or canceled the read() (or
307 * whatever magic it did..). In that case, the memfd object is still
308 * all zero.
309 * In case the memfd-object was *not* sealed, the read() was successful
310 * and the memfd object must *not* be all zero.
311 * Note that in real scenarios, there might be a mixture of both, but
312 * in this test-cases, we have explicit 200ms delays which should be
313 * enough to avoid any in-flight writes. */
314
315 p = mfd_assert_mmap_private(mfd);
316 if (was_sealed && memcmp(p, zero, mfd_def_size)) {
317 printf("memfd sealed during read() but data not discarded\n");
318 abort();
319 } else if (!was_sealed && !memcmp(p, zero, mfd_def_size)) {
320 printf("memfd sealed after read() but data discarded\n");
321 abort();
322 }
323
324 close(mfd);
325 close(fd);
326
327 printf("fuse: DONE\n");
328 free(zero);
329
330 return 0;
331 }
332
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