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