├── LICENSE.txt ├── README.md ├── kernel ├── .gitignore ├── Makefile └── nosecmem.c └── user ├── .gitignore ├── Makefile └── testapp.c /LICENSE.txt: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 2, June 1991 3 | 4 | Copyright (C) 1989, 1991 Free Software Foundation, Inc., 5 | 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA 6 | Everyone is permitted to copy and distribute verbatim copies 7 | of this license document, but changing it is not allowed. 8 | 9 | Preamble 10 | 11 | The licenses for most software are designed to take away your 12 | freedom to share and change it. By contrast, the GNU General Public 13 | License is intended to guarantee your freedom to share and change free 14 | software--to make sure the software is free for all its users. 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If this is what you want to do, use the GNU Lesser General 339 | Public License instead of this License. 340 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | nosecmem 2 | ======== 3 | 4 | This project demonstrates the 5 | [newly-added](https://github.com/torvalds/linux/commit/1507f51255c9) 6 | `memfd_secret` Linux system call, and a kernel module which can dump the 7 | contents of these so-called "secret" memory areas. 8 | 9 | # Background 10 | Certain media outlets have [over-hyped the feature](https://lwn.net/Articles/865256/), 11 | claiming that it provides for a secret memory area which is 12 | 13 | > inaccessible to the kernel (and to anybody who might be able to compromise 14 | > the kernel), 15 | 16 | While the pages are removed from the kernel direct map (a global 1:1 mapping of 17 | physical memory to virtual memory), it's a bit of a stretch to say that the 18 | memory is completely inaccessible to the kernel — the very thing that 19 | implements the mechanism in question. 20 | 21 | The man page is a bit more pragmatic, indicating that `memfd_secret()` mappings 22 | provide 23 | 24 | > ...stronger protection than usual RAM-based files and anonymous memory 25 | > mappings. 26 | > The memory areas backing the file created with `memfd_create(2)` are visible 27 | > only to the contexts that have access to the file descriptor These areas are 28 | > removed from the kernel page tables and only the page tables of the processes 29 | > holding the file descriptor map the corresponding physical memory. 30 | 31 | David Hildenbrand (a RedHat engineer who 32 | [reviewed](https://lkml.org/lkml/2021/5/14/225) the patchset) correctly 33 | [speculates](https://lwn.net/Articles/865545/): 34 | 35 | > I‘d like to note that secretmem does not protect against kernel exploits or 36 | > against root in most setups getting hold of that data. Once you‘re already in 37 | > the kernel, **you might just be able to remap the pages**. 38 | 39 | And that is exactly what this project does. 40 | 41 | ### Disclaimers 42 | Don't use this code on a production machine: 43 | - It delibarately circumvents a kernel security measure! 44 | - It probably has bugs which can crash your machine and/or corrupt your data. 45 | 46 | By no means am I discrediting Mike Rapoport or his work — the feature seems to 47 | do exactly what it is intended to do: Provide stronger guarantees about the 48 | protection of the memory. The intent of this project is to ensure that its 49 | limitations are well-understood. 50 | 51 | Tested on `v5.14-rc5-221-gdfa377c35d70`. 52 | 53 | # Usage 54 | 55 | ### Prerequisites 56 | - You need to be running a v5.14-rc1 or newer kernel. 57 | - You need to boot your kernel with the command-line option `secretmem.enable=1`. 58 | - You need GCC, Make, and kernel headers. 59 | 60 | ### Build Test App 61 | ``` 62 | $ cd user/ 63 | $ make 64 | ``` 65 | 66 | ### Build Kernel Module 67 | ``` 68 | $ cd kernel/ 69 | $ vim Makefile 70 | 71 | $ make 72 | ``` 73 | 74 | ### Testing 75 | 76 | In one terminal, load the module and watch kernel output: 77 | ``` 78 | $ sudo insmod kernel/nosecmem.ko 79 | $ sudo dmesg --human --follow 80 | ``` 81 | 82 | In another terminal, run the test app and trigger its inspection: 83 | ``` 84 | $ user/testapp & 85 | PID: 592 86 | Copied 13 bytes to secret area 0x7f2eab17f000 87 | Waiting... 88 | 89 | $ echo $(pidof testapp) > /proc/nosecmem 90 | ``` 91 | 92 | If all goes well you should see this in the `dmesg` output: 93 | ``` 94 | nosecmem: Write a PID to /proc/nosecmem to see all of the "secret memory" areas for that process. 95 | nosecmem: Inspecting task 592 [testapp] 96 | nosecmem: Found secretmem file size 13 at vma 0x7F2EAB17F000-0x7F2EAB180000 97 | nosecmem: 00000000: 53 65 63 72 65 74 20 64 61 74 61 21 00 Secret data!. 98 | ``` 99 | -------------------------------------------------------------------------------- /kernel/.gitignore: -------------------------------------------------------------------------------- 1 | *.cmd 2 | *.o 3 | *.ko 4 | *.mod.c 5 | *.mod 6 | modules.order 7 | Module.symvers 8 | -------------------------------------------------------------------------------- /kernel/Makefile: -------------------------------------------------------------------------------- 1 | # Build against running kernel 2 | #KDIR := /lib/modules/$(shell uname -r)/build 3 | 4 | # Build against a kernel repo 5 | #KDIR := $(shell realpath ~/gitrepos/linux) 6 | 7 | ifndef KDIR 8 | $(error KDIR is not set) 9 | endif 10 | 11 | obj-m := nosecmem.o 12 | 13 | modules: 14 | $(MAKE) -C $(KDIR) M=$(PWD) modules 15 | 16 | clean: 17 | $(MAKE) -C $(KDIR) M=$(PWD) clean 18 | -------------------------------------------------------------------------------- /kernel/nosecmem.c: -------------------------------------------------------------------------------- 1 | #define MODNAME "nosecmem" 2 | #define pr_fmt(fmt) "%s: " fmt, MODNAME 3 | 4 | #include 5 | #include 6 | #include 7 | #include 8 | #include 9 | #include 10 | #include 11 | #include 12 | 13 | MODULE_AUTHOR("Jonathon Reinhart "); 14 | MODULE_LICENSE("GPL"); 15 | MODULE_DESCRIPTION("Demonstrate reading memfd_secret() files from the kernel"); 16 | MODULE_VERSION("0.0.0"); 17 | 18 | static bool is_secretmem_file(struct file *file) 19 | { 20 | return file && file_inode(file)->i_sb->s_magic == SECRETMEM_MAGIC; 21 | } 22 | 23 | static size_t file_size(struct file *file) 24 | { 25 | return file_inode(file)->i_size; 26 | } 27 | 28 | static int handle_secretmem_file(struct file *file) 29 | { 30 | const unsigned long len = file_size(file); 31 | void *map = NULL; 32 | int ret = 0; 33 | 34 | map = (void *) vm_mmap(file, 0, len, PROT_READ, MAP_SHARED, 0); 35 | if (IS_ERR(map)) { 36 | ret = PTR_ERR(map); 37 | map = NULL; 38 | pr_err("vm_mmap() failed: %d\n", ret); 39 | goto out; 40 | } 41 | 42 | print_hex_dump(KERN_INFO, MODNAME ": ", DUMP_PREFIX_OFFSET, 43 | 16, 1, map, len, true); 44 | 45 | out: 46 | if (map) 47 | vm_munmap((unsigned long)map, len); 48 | return ret; 49 | } 50 | 51 | /* task_lock() and rcu_read_lock() must be held */ 52 | static struct file *get_file_by_fd(struct files_struct *files, unsigned int fd) 53 | { 54 | /** 55 | * fcheck_files() was renamed to files_lookup_fd_rcu() in commit 56 | * v5.10-rc1-10-gf36c29432741 57 | * 58 | * fcheck_files() is present in Debian Bullseye kernel 5.10.0-8 headers. 59 | */ 60 | #if LINUX_VERSION_CODE > KERNEL_VERSION(5, 10, 0) 61 | return files_lookup_fd_rcu(files, fd); 62 | #else 63 | return fcheck_files(files, fd); 64 | #endif 65 | } 66 | 67 | static void handle_task_files(struct task_struct *task) 68 | { 69 | struct files_struct *files = task->files; 70 | unsigned int fd; 71 | 72 | /* See proc_readfd_common() */ 73 | /* requires task_lock() or atomic_inc(&files->count) */ 74 | if (!files) 75 | return; 76 | 77 | for (fd = 0; fd < files_fdtable(files)->max_fds; fd++) { 78 | struct file *file; 79 | 80 | file = get_file_by_fd(files, fd); 81 | 82 | if (!is_secretmem_file(file)) 83 | continue; 84 | 85 | pr_info("Found secretmem file size %ld at fd %d\n", 86 | file_size(file), fd); 87 | handle_secretmem_file(file); 88 | } 89 | } 90 | 91 | /** 92 | * Like the unexported mm_access(), except: 93 | * - Does not take task_lock() (assumes it is already held) 94 | */ 95 | static struct mm_struct *__mm_access(struct task_struct *task) 96 | { 97 | struct mm_struct *mm; 98 | int err; 99 | 100 | err = down_read_killable(&task->signal->exec_update_lock); 101 | if (err) 102 | return ERR_PTR(err); 103 | 104 | /* Don't call get_task_mm(); it takes task_lock() */ 105 | mm = task->mm; 106 | if (mm) { 107 | if (task->flags & PF_KTHREAD) 108 | mm = NULL; 109 | else 110 | mmget(mm); 111 | } 112 | 113 | up_read(&task->signal->exec_update_lock); 114 | 115 | return mm; 116 | } 117 | 118 | static struct vm_area_struct *first_vma(struct mm_struct *mm) 119 | { 120 | /* TODO: Another way to get first VMA? */ 121 | return find_vma(mm, 0); 122 | } 123 | 124 | static void handle_task_mm(struct task_struct *task) 125 | { 126 | /* See proc_pid_maps_op */ 127 | struct mm_struct *mm; 128 | struct vm_area_struct *vma; 129 | 130 | mm = __mm_access(task); 131 | if (IS_ERR(mm)) { 132 | pr_err("__mm_access() failed: %ld\n", PTR_ERR(mm)); 133 | goto out; 134 | } 135 | 136 | if (mmap_read_lock_killable(mm)) { 137 | pr_err("mmap_read_lock_killable() failed\n"); 138 | goto out_mmput; 139 | } 140 | 141 | for (vma = first_vma(mm); vma; vma = vma->vm_next) { 142 | struct file* file = vma->vm_file; 143 | 144 | if (!is_secretmem_file(file)) 145 | continue; 146 | 147 | pr_info("Found secretmem file size %ld at vma 0x%lX-0x%lX\n", 148 | file_size(file), vma->vm_start, vma->vm_end); 149 | handle_secretmem_file(file); 150 | } 151 | 152 | 153 | mmap_read_unlock(mm); 154 | out_mmput: 155 | mmput(mm); 156 | out: 157 | return; 158 | } 159 | 160 | static void handle_task(struct task_struct *task) 161 | { 162 | task_lock(task); 163 | 164 | pr_info("Inspecting task %d [%s]\n", task->pid, task->comm); 165 | 166 | /* TODO: get task files and mm and release task lock */ 167 | 168 | handle_task_files(task); 169 | handle_task_mm(task); 170 | 171 | task_unlock(task); 172 | } 173 | 174 | static int handle_pid(struct pid *pid) 175 | { 176 | struct task_struct *task; 177 | 178 | task = get_pid_task(pid, PIDTYPE_PID); 179 | if (!task) 180 | return -ESRCH; 181 | 182 | rcu_read_lock(); 183 | handle_task(task); 184 | rcu_read_unlock(); 185 | 186 | put_task_struct(task); 187 | return 0; 188 | } 189 | 190 | static int handle_pid_nr(u32 pid_nr) 191 | { 192 | struct pid *pid = NULL; 193 | int rc; 194 | 195 | pid = find_get_pid(pid_nr); 196 | if (!pid) 197 | return -ESRCH; 198 | 199 | rc = handle_pid(pid); 200 | 201 | put_pid(pid); 202 | return rc; 203 | } 204 | 205 | static ssize_t nosecmem_write(struct file *file, const char __user *buf, 206 | size_t count, loff_t *ppos) 207 | { 208 | u32 pid_nr; 209 | int rc; 210 | 211 | if (*ppos != 0) { 212 | /* No partial writes. */ 213 | return -EINVAL; 214 | } 215 | 216 | rc = kstrtou32_from_user(buf, count, 10, &pid_nr); 217 | if (rc < 0) 218 | return rc; 219 | 220 | rc = handle_pid_nr(pid_nr); 221 | if (rc < 0) 222 | return rc; 223 | 224 | return count; 225 | } 226 | 227 | static const struct proc_ops nosecmem_ops = { 228 | .proc_write = nosecmem_write, 229 | .proc_lseek = default_llseek, 230 | }; 231 | 232 | #define PROCNAME MODNAME 233 | 234 | static struct proc_dir_entry *procent; 235 | 236 | static int __init kmod_init(void) 237 | { 238 | procent = proc_create(PROCNAME, S_IWUGO, NULL, &nosecmem_ops); 239 | if (!procent) { 240 | pr_err("Failed to register proc interface\n"); 241 | return -ENOMEM; 242 | } 243 | 244 | pr_info("Write a PID to /proc/"PROCNAME" to see all of the " 245 | "\"secret memory\" areas for that process.\n"); 246 | return 0; 247 | } 248 | 249 | static void __exit kmod_exit(void) 250 | { 251 | proc_remove(procent); 252 | procent = NULL; 253 | } 254 | 255 | module_init(kmod_init); 256 | module_exit(kmod_exit); 257 | -------------------------------------------------------------------------------- /user/.gitignore: -------------------------------------------------------------------------------- 1 | *.o 2 | testapp 3 | -------------------------------------------------------------------------------- /user/Makefile: -------------------------------------------------------------------------------- 1 | CC = gcc 2 | CFLAGS = -Wall -Werror 3 | 4 | .PHONY: all 5 | all: testapp 6 | 7 | testapp: testapp.o 8 | 9 | .PHONY: clean 10 | clean: 11 | $(RM) testapp *.o 12 | -------------------------------------------------------------------------------- /user/testapp.c: -------------------------------------------------------------------------------- 1 | #include 2 | #include 3 | #include 4 | #include 5 | #include 6 | #include 7 | 8 | #define SYS_memfd_secret 447 9 | 10 | static int memfd_secret(unsigned int flags) 11 | { 12 | return syscall(SYS_memfd_secret, flags); 13 | } 14 | 15 | static void *secret_alloc(size_t size) 16 | { 17 | int fd = -1; 18 | void *m; 19 | void *result = NULL; 20 | 21 | fd = memfd_secret(0); 22 | if (fd < 0) 23 | goto out; 24 | 25 | if (ftruncate(fd, size) < 0) 26 | goto out; 27 | 28 | m = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); 29 | if (m == MAP_FAILED) 30 | goto out; 31 | 32 | result = m; 33 | 34 | out: 35 | if (fd >= 0) 36 | close(fd); 37 | return result; 38 | } 39 | 40 | static void secret_free(void *p, size_t size) 41 | { 42 | munmap(p, size); 43 | } 44 | 45 | int main(void) 46 | { 47 | static const char data[] = "Secret data!"; 48 | static const size_t size = sizeof(data); 49 | void *s; 50 | 51 | printf("PID: %d\n", getpid()); 52 | 53 | s = secret_alloc(size); 54 | if (!s) 55 | error(2, errno, "secret_alloc() failed"); 56 | 57 | strncpy(s, data, size); 58 | printf("Copied %zu bytes to secret area %p\n", size, s); 59 | 60 | printf("Waiting...\n"); 61 | pause(); 62 | 63 | secret_free(s, size); 64 | return 0; 65 | } 66 | --------------------------------------------------------------------------------