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1/*
2 * memfd GUP test-case
3 * This tests memfd interactions with get_user_pages(). We require the
4 * fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
5 * file-system delays _all_ reads by 1s and forces direct-IO. This means, any
6 * read() on files in that file-system will pin the receive-buffer pages for at
7 * least 1s via get_user_pages().
8 *
9 * We use this trick to race ADD_SEALS against a write on a memfd object. The
10 * ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
11 * the read() syscall with our memory-mapped memfd object as receive buffer to
12 * force the kernel to write into our memfd object.
13 */
14
15#define _GNU_SOURCE
16#define __EXPORTED_HEADERS__
17
18#include <errno.h>
19#include <inttypes.h>
20#include <limits.h>
21#include <linux/falloc.h>
22#include <linux/fcntl.h>
23#include <linux/memfd.h>
24#include <sched.h>
25#include <stdio.h>
26#include <stdlib.h>
27#include <signal.h>
28#include <string.h>
29#include <sys/mman.h>
30#include <sys/stat.h>
31#include <sys/syscall.h>
32#include <sys/wait.h>
33#include <unistd.h>
34
35#define MFD_DEF_SIZE 8192
36#define STACK_SIZE 65535
37
38static int sys_memfd_create(const char *name,
39 unsigned int flags)
40{
41 return syscall(__NR_memfd_create, name, flags);
42}
43
44static 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
64static __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
77static 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
89static 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
103static 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
124static 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
142static 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
160static int global_mfd = -1;
161static void *global_p = NULL;
162
163static 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
200static 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
223static void join_sealing_thread(pid_t pid)
224{
225 waitpid(pid, NULL, 0);
226}
227
228int main(int argc, char **argv)
229{
230 static const char zero[MFD_DEF_SIZE];
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 /* open FUSE memfd file for GUP testing */
242 printf("opening: %s\n", argv[1]);
243 fd = open(argv[1], O_RDONLY | O_CLOEXEC);
244 if (fd < 0) {
245 printf("cannot open(\"%s\"): %m\n", argv[1]);
246 abort();
247 }
248
249 /* create new memfd-object */
250 mfd = mfd_assert_new("kern_memfd_fuse",
251 MFD_DEF_SIZE,
252 MFD_CLOEXEC | MFD_ALLOW_SEALING);
253
254 /* mmap memfd-object for writing */
255 p = mfd_assert_mmap_shared(mfd);
256
257 /* pass mfd+mapping to a separate sealing-thread which tries to seal
258 * the memfd objects with SEAL_WRITE while we write into it */
259 global_mfd = mfd;
260 global_p = p;
261 pid = spawn_sealing_thread();
262
263 /* Use read() on the FUSE file to read into our memory-mapped memfd
264 * object. This races the other thread which tries to seal the
265 * memfd-object.
266 * If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
267 * This guarantees that the receive-buffer is pinned for 1s until the
268 * data is written into it. The racing ADD_SEALS should thus fail as
269 * the pages are still pinned. */
270 r = read(fd, p, MFD_DEF_SIZE);
271 if (r < 0) {
272 printf("read() failed: %m\n");
273 abort();
274 } else if (!r) {
275 printf("unexpected EOF on read()\n");
276 abort();
277 }
278
279 was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
280
281 /* Wait for sealing-thread to finish and verify that it
282 * successfully sealed the file after the second try. */
283 join_sealing_thread(pid);
284 mfd_assert_has_seals(mfd, F_SEAL_WRITE);
285
286 /* *IF* the memfd-object was sealed at the time our read() returned,
287 * then the kernel did a page-replacement or canceled the read() (or
288 * whatever magic it did..). In that case, the memfd object is still
289 * all zero.
290 * In case the memfd-object was *not* sealed, the read() was successfull
291 * and the memfd object must *not* be all zero.
292 * Note that in real scenarios, there might be a mixture of both, but
293 * in this test-cases, we have explicit 200ms delays which should be
294 * enough to avoid any in-flight writes. */
295
296 p = mfd_assert_mmap_private(mfd);
297 if (was_sealed && memcmp(p, zero, MFD_DEF_SIZE)) {
298 printf("memfd sealed during read() but data not discarded\n");
299 abort();
300 } else if (!was_sealed && !memcmp(p, zero, MFD_DEF_SIZE)) {
301 printf("memfd sealed after read() but data discarded\n");
302 abort();
303 }
304
305 close(mfd);
306 close(fd);
307
308 printf("fuse: DONE\n");
309
310 return 0;
311}