Merge pull request #4590 from smitsohu/mountid

Read mount id also on legacy kernels

3 files changed, 63 insertions, 48 deletions

diff --git a/src/firejail/firejail.h b/src/firejail/firejail.h
index 2a7d88575..e1ba8f48a 100644
--- a/src/firejail/firejail.h
+++ b/src/firejail/firejail.h
@@ -563,7 +563,7 @@ typedef struct {
 
 // mountinfo.c
 MountData *get_last_mount(void);
-int get_mount_id(const char *path);
+int get_mount_id(int fd);
 char **build_mount_array(const int mount_id, const char *path);
 
 // fs_var.c
diff --git a/src/firejail/fs.c b/src/firejail/fs.c
index 6d01b5e5d..3144156a3 100644
--- a/src/firejail/fs.c
+++ b/src/firejail/fs.c
@@ -633,34 +633,30 @@ out:
 }
 
 // remount recursively; requires a resolved path
-static void fs_remount_rec(const char *dir, OPERATION op) {
+static void fs_remount_rec(const char *path, OPERATION op) {
         EUID_ASSERT();
-        assert(dir);
+        assert(op < OPERATION_MAX);
+        assert(path);
 
-        struct stat s;
-        if (stat(dir, &s) != 0)
-                return;
-        if (!S_ISDIR(s.st_mode)) {
-                // no need to search in /proc/self/mountinfo for submounts if not a directory
-                fs_remount_simple(dir, op);
+        // no need to search /proc/self/mountinfo for submounts if not a directory
+        int fd = open(path, O_PATH|O_DIRECTORY|O_NOFOLLOW|O_CLOEXEC);
+        if (fd < 0) {
+                fs_remount_simple(path, op);
                 return;
         }
-        // get mount point of the directory
-        int mountid = get_mount_id(dir);
-        if (mountid == -1)
-                return;
-        if (mountid == -2) {
-                // falling back to a simple remount on old kernels
-                static int mount_warning = 0;
-                if (!mount_warning) {
-                        fwarning("read-only, read-write and noexec options are not applied recursively\n");
-                        mount_warning = 1;
-                }
-                fs_remount_simple(dir, op);
+
+        // get mount id of the directory
+        int mountid = get_mount_id(fd);
+        close(fd);
+        if (mountid < 0) {
+                // falling back to a simple remount
+                fwarning("%s %s not applied recursively\n", opstr[op], path);
+                fs_remount_simple(path, op);
                 return;
         }
+
         // build array with all mount points that need to get remounted
-        char **arr = build_mount_array(mountid, dir);
+        char **arr = build_mount_array(mountid, path);
         assert(arr);
         // remount
         char **tmp = arr;
diff --git a/src/firejail/mountinfo.c b/src/firejail/mountinfo.c
index 64a94bd84..304f80eee 100644
--- a/src/firejail/mountinfo.c
+++ b/src/firejail/mountinfo.c
@@ -19,6 +19,7 @@
 */
 
 #include "firejail.h"
+#include <errno.h>
 
 #include <fcntl.h>
 #ifndef O_PATH
@@ -151,53 +152,71 @@ MountData *get_last_mount(void) {
         return &mdata;
 }
 
-// Extract the mount id from /proc/self/fdinfo and return it.
-int get_mount_id(const char *path) {
+// Returns mount id, or -1 if fd refers to a procfs or sysfs file
+static int get_mount_id_from_handle(int fd) {
         EUID_ASSERT();
-        assert(path);
 
-        int fd = open(path, O_PATH|O_CLOEXEC);
-        if (fd == -1)
-                return -1;
+        char *proc;
+        if (asprintf(&proc, "/proc/self/fd/%d", fd) == -1)
+                errExit("asprintf");
+        struct file_handle *fh = malloc(sizeof *fh);
+        if (!fh)
+                errExit("malloc");
+        fh->handle_bytes = 0;
+
+        int rv = -1;
+        int tmp;
+        if (name_to_handle_at(-1, proc, fh, &tmp, AT_SYMLINK_FOLLOW) != -1) {
+                fprintf(stderr, "Error: unexpected result from name_to_handle_at\n");
+                exit(1);
+        }
+        if (errno == EOVERFLOW && fh->handle_bytes)
+                rv = tmp;
+
+        free(proc);
+        free(fh);
+        return rv;
+}
+
+// Returns mount id, or -1 on kernels < 3.15
+static int get_mount_id_from_fdinfo(int fd) {
+        EUID_ASSERT();
+        int rv = -1;
 
-        char *fdinfo;
-        if (asprintf(&fdinfo, "/proc/self/fdinfo/%d", fd) == -1)
+        char *proc;
+        if (asprintf(&proc, "/proc/self/fdinfo/%d", fd) == -1)
                 errExit("asprintf");
         EUID_ROOT();
-        FILE *fp = fopen(fdinfo, "re");
+        FILE *fp = fopen(proc, "re");
         EUID_USER();
-        free(fdinfo);
         if (!fp)
                 goto errexit;
 
-        // read the file
         char buf[MAX_BUF];
-        if (fgets(buf, MAX_BUF, fp) == NULL)
-                goto errexit;
-        do {
+        while (fgets(buf, MAX_BUF, fp)) {
                 if (strncmp(buf, "mnt_id:", 7) == 0) {
-                        char *ptr = buf + 7;
-                        while (*ptr != '\0' && (*ptr == ' ' || *ptr == '\t')) {
-                                ptr++;
-                        }
-                        if (*ptr == '\0')
+                        if (sscanf(buf + 7, "%d", &rv) != 1)
                                 goto errexit;
-                        fclose(fp);
-                        close(fd);
-                        return atoi(ptr);
+                        break;
                 }
-        } while (fgets(buf, MAX_BUF, fp));
+        }
 
-        // fallback, kernels older than 3.15 don't expose the mount id in this place
+        free(proc);
         fclose(fp);
-        close(fd);
-        return -2;
+        return rv;
 
 errexit:
         fprintf(stderr, "Error: cannot read proc file\n");
         exit(1);
 }
 
+int get_mount_id(int fd) {
+        int rv = get_mount_id_from_fdinfo(fd);
+        if (rv < 0)
+                rv = get_mount_id_from_handle(fd);
+        return rv;
+}
+
 // Check /proc/self/mountinfo if path contains any mounts points.
 // Returns an array that can be iterated over for recursive remounting.
 char **build_mount_array(const int mount_id, const char *path) {