Add --size option to control size of a --tmpfs

Signed-off-by: Tom Smeding <tom@tomsmeding.com>

1 files changed, 93 insertions, 18 deletions

diff --git a/bubblewrap.c b/bubblewrap.c
index 24a402c..9e67eb5 100644
--- a/bubblewrap.c
+++ b/bubblewrap.c
@@ -23,6 +23,7 @@
 #include <sched.h>
 #include <pwd.h>
 #include <grp.h>
+#include <ctype.h>
 #include <sys/mount.h>
 #include <sys/socket.h>
 #include <sys/wait.h>
@@ -52,6 +53,12 @@
        __result; }))
 #endif
 
+/* We limit the size of a tmpfs to half the architecture's address space,
+ * to avoid hitting arbitrary limits in the kernel.
+ * For example, on at least one x86_64 machine, the actual limit seems to be
+ * 2^64 - 2^12. */
+#define MAX_TMPFS_BYTES ((size_t) (SIZE_MAX >> 1))
+
 /* Globals to avoid having to use getuid(), since the uid/gid changes during runtime */
 static uid_t real_uid;
 static gid_t real_gid;
@@ -91,6 +98,7 @@ int opt_userns_fd = -1;
 int opt_userns2_fd = -1;
 int opt_pidns_fd = -1;
 int next_perms = -1;
+size_t next_size_arg = 0;
 
 #define CAP_TO_MASK_0(x) (1L << ((x) & 31))
 #define CAP_TO_MASK_1(x) CAP_TO_MASK_0(x - 32)
@@ -149,6 +157,7 @@ struct _SetupOp
   int         fd;
   SetupOpFlag flags;
   int         perms;
+  size_t      size;  /* number of bytes, zero means unset/default */
   SetupOp    *next;
 };
 
@@ -177,6 +186,7 @@ typedef struct
   uint32_t op;
   uint32_t flags;
   uint32_t perms;
+  size_t   size_arg;
   uint32_t arg1_offset;
   uint32_t arg2_offset;
 } PrivSepOp;
@@ -341,6 +351,7 @@ usage (int ecode, FILE *out)
            "    --cap-add CAP                Add cap CAP when running as privileged user\n"
            "    --cap-drop CAP               Drop cap CAP when running as privileged user\n"
            "    --perms OCTAL                Set permissions of next argument (--bind-data, --file, etc.)\n"
+           "    --size BYTES                 Set size of next argument (only for --tmpfs)\n"
            "    --chmod OCTAL PATH           Change permissions of PATH (must already exist)\n"
           );
   exit (ecode);
@@ -1001,6 +1012,7 @@ privileged_op (int         privileged_op_socket,
                uint32_t    op,
                uint32_t    flags,
                uint32_t    perms,
+               size_t      size_arg,
                const char *arg1,
                const char *arg2)
 {
@@ -1032,6 +1044,7 @@ privileged_op (int         privileged_op_socket,
       op_buffer->op = op;
       op_buffer->flags = flags;
       op_buffer->perms = perms;
+      op_buffer->size_arg = size_arg;
       op_buffer->arg1_offset = arg1_offset;
       op_buffer->arg2_offset = arg2_offset;
       if (arg1 != NULL)
@@ -1096,7 +1109,18 @@ privileged_op (int         privileged_op_socket,
 
     case PRIV_SEP_OP_TMPFS_MOUNT:
       {
-        cleanup_free char *mode = xasprintf ("mode=%#o", perms);
+        cleanup_free char *mode = NULL;
+
+        /* This check should be unnecessary since we checked this when parsing
+         * the --size option as well. However, better be safe than sorry. */
+        if (size_arg > MAX_TMPFS_BYTES)
+          die_with_error ("Specified tmpfs size too large (%zu > %zu)", size_arg, MAX_TMPFS_BYTES);
+
+        if (size_arg != 0)
+          mode = xasprintf ("mode=%#o,size=%zu", perms, size_arg);
+        else
+          mode = xasprintf ("mode=%#o", perms);
+
         cleanup_free char *opt = label_mount (mode, opt_file_label);
         if (mount ("tmpfs", arg1, "tmpfs", MS_NOSUID | MS_NODEV, opt) != 0)
           die_with_error ("Can't mount tmpfs on %s", arg1);
@@ -1197,12 +1221,12 @@ setup_newroot (bool unshare_pid,
                          PRIV_SEP_OP_BIND_MOUNT,
                          (op->type == SETUP_RO_BIND_MOUNT ? BIND_READONLY : 0) |
                          (op->type == SETUP_DEV_BIND_MOUNT ? BIND_DEVICES : 0),
-                         0, source, dest);
+                         0, 0, source, dest);
           break;
 
         case SETUP_REMOUNT_RO_NO_RECURSIVE:
           privileged_op (privileged_op_socket,
-                         PRIV_SEP_OP_REMOUNT_RO_NO_RECURSIVE, 0, 0, NULL, dest);
+                         PRIV_SEP_OP_REMOUNT_RO_NO_RECURSIVE, 0, 0, 0, NULL, dest);
           break;
 
         case SETUP_MOUNT_PROC:
@@ -1213,14 +1237,14 @@ setup_newroot (bool unshare_pid,
             {
               /* Our own procfs */
               privileged_op (privileged_op_socket,
-                             PRIV_SEP_OP_PROC_MOUNT, 0, 0,
+                             PRIV_SEP_OP_PROC_MOUNT, 0, 0, 0,
                              dest, NULL);
             }
           else
             {
               /* Use system procfs, as we share pid namespace anyway */
               privileged_op (privileged_op_socket,
-                             PRIV_SEP_OP_BIND_MOUNT, 0, 0,
+                             PRIV_SEP_OP_BIND_MOUNT, 0, 0, 0,
                              "oldroot/proc", dest);
             }
 
@@ -1242,7 +1266,7 @@ setup_newroot (bool unshare_pid,
                 }
 
               privileged_op (privileged_op_socket,
-                             PRIV_SEP_OP_BIND_MOUNT, BIND_READONLY, 0,
+                             PRIV_SEP_OP_BIND_MOUNT, BIND_READONLY, 0, 0,
                              subdir, subdir);
             }
 
@@ -1253,7 +1277,7 @@ setup_newroot (bool unshare_pid,
             die_with_error ("Can't mkdir %s", op->dest);
 
           privileged_op (privileged_op_socket,
-                         PRIV_SEP_OP_TMPFS_MOUNT, 0, 0755,
+                         PRIV_SEP_OP_TMPFS_MOUNT, 0, 0755, 0,
                          dest, NULL);
 
           static const char *const devnodes[] = { "null", "zero", "full", "random", "urandom", "tty" };
@@ -1264,7 +1288,7 @@ setup_newroot (bool unshare_pid,
               if (create_file (node_dest, 0444, NULL) != 0)
                 die_with_error ("Can't create file %s/%s", op->dest, devnodes[i]);
               privileged_op (privileged_op_socket,
-                             PRIV_SEP_OP_BIND_MOUNT, BIND_DEVICES, 0,
+                             PRIV_SEP_OP_BIND_MOUNT, BIND_DEVICES, 0, 0,
                              node_src, node_dest);
             }
 
@@ -1298,7 +1322,7 @@ setup_newroot (bool unshare_pid,
             if (mkdir (pts, 0755) == -1)
               die_with_error ("Can't create %s/devpts", op->dest);
             privileged_op (privileged_op_socket,
-                           PRIV_SEP_OP_DEVPTS_MOUNT, 0, 0, pts, NULL);
+                           PRIV_SEP_OP_DEVPTS_MOUNT, 0, 0, 0, pts, NULL);
 
             if (symlink ("pts/ptmx", ptmx) != 0)
               die_with_error ("Can't make symlink at %s/ptmx", op->dest);
@@ -1318,7 +1342,7 @@ setup_newroot (bool unshare_pid,
                 die_with_error ("creating %s/console", op->dest);
 
               privileged_op (privileged_op_socket,
-                             PRIV_SEP_OP_BIND_MOUNT, BIND_DEVICES, 0,
+                             PRIV_SEP_OP_BIND_MOUNT, BIND_DEVICES, 0, 0,
                              src_tty_dev, dest_console);
             }
 
@@ -1333,7 +1357,7 @@ setup_newroot (bool unshare_pid,
             die_with_error ("Can't mkdir %s", op->dest);
 
           privileged_op (privileged_op_socket,
-                         PRIV_SEP_OP_TMPFS_MOUNT, 0, op->perms,
+                         PRIV_SEP_OP_TMPFS_MOUNT, 0, op->perms, op->size,
                          dest, NULL);
           break;
 
@@ -1342,7 +1366,7 @@ setup_newroot (bool unshare_pid,
             die_with_error ("Can't mkdir %s", op->dest);
 
           privileged_op (privileged_op_socket,
-                         PRIV_SEP_OP_MQUEUE_MOUNT, 0, 0,
+                         PRIV_SEP_OP_MQUEUE_MOUNT, 0, 0, 0,
                          dest, NULL);
           break;
 
@@ -1423,7 +1447,7 @@ setup_newroot (bool unshare_pid,
             privileged_op (privileged_op_socket,
                            PRIV_SEP_OP_BIND_MOUNT,
                            (op->type == SETUP_MAKE_RO_BIND_FILE ? BIND_READONLY : 0),
-                           0, tempfile, dest);
+                           0, 0, tempfile, dest);
 
             /* Remove the file so we're sure the app can't get to it in any other way.
                Its outside the container chroot, so it shouldn't be possible, but lets
@@ -1441,7 +1465,7 @@ setup_newroot (bool unshare_pid,
         case SETUP_SET_HOSTNAME:
           assert (op->dest != NULL);  /* guaranteed by the constructor */
           privileged_op (privileged_op_socket,
-                         PRIV_SEP_OP_SET_HOSTNAME, 0, 0,
+                         PRIV_SEP_OP_SET_HOSTNAME, 0, 0, 0,
                          op->dest, NULL);
           break;
 
@@ -1450,7 +1474,7 @@ setup_newroot (bool unshare_pid,
         }
     }
   privileged_op (privileged_op_socket,
-                 PRIV_SEP_OP_DONE, 0, 0, NULL, NULL);
+                 PRIV_SEP_OP_DONE, 0, 0, 0, NULL, NULL);
 }
 
 /* Do not leak file descriptors already used by setup_newroot () */
@@ -1537,6 +1561,7 @@ read_priv_sec_op (int          read_socket,
                   size_t       buffer_size,
                   uint32_t    *flags,
                   uint32_t    *perms,
+                  size_t      *size_arg,
                   const char **arg1,
                   const char **arg2)
 {
@@ -1561,6 +1586,7 @@ read_priv_sec_op (int          read_socket,
 
   *flags = op->flags;
   *perms = op->perms;
+  *size_arg = op->size_arg;
   *arg1 = resolve_string_offset (buffer, rec_len, op->arg1_offset);
   *arg2 = resolve_string_offset (buffer, rec_len, op->arg2_offset);
 
@@ -1577,7 +1603,8 @@ print_version_and_exit (void)
 static int
 is_modifier_option (const char *option)
 {
-  return strcmp (option, "--perms") == 0;
+  return strcmp (option, "--perms") == 0
+         || strcmp(option, "--size") == 0;
 }
 
 static void
@@ -1871,6 +1898,13 @@ parse_args_recurse (int          *argcp,
             op->perms = 0755;
 
           next_perms = -1;
+
+          /* If the option is unset, next_size_arg is zero, which results in
+           * the default tmpfs size. This is exactly what we want. */
+          op->size = next_size_arg;
+
+          next_size_arg = 0;
+
           argv += 1;
           argc -= 1;
         }
@@ -2380,6 +2414,42 @@ parse_args_recurse (int          *argcp,
           argv += 1;
           argc -= 1;
         }
+      else if (strcmp (arg, "--size") == 0)
+        {
+          unsigned long long size;
+          char *endptr = NULL;
+
+          if (is_privileged)
+            die ("The --size option is not permitted in setuid mode");
+
+          if (argc < 2)
+            die ("--size takes an argument");
+
+          if (next_size_arg != 0)
+            die ("--size given twice for the same action");
+
+          errno = 0;  /* reset errno so we can detect ERANGE from strtoull */
+
+          size = strtoull (argv[1], &endptr, 0);
+
+          /* isdigit: Not only check that the first digit is not '\0', but
+           * simultaneously guard against negative numbers or preceding
+           * spaces. */
+          if (errno != 0  /* from strtoull */
+              || !isdigit(argv[1][0])
+              || endptr == NULL
+              || *endptr != '\0'
+              || size == 0)
+            die ("--size takes a non-zero number of bytes");
+
+          if (size > MAX_TMPFS_BYTES)
+            die ("--size (for tmpfs) is limited to %zu", MAX_TMPFS_BYTES);
+
+          next_size_arg = (size_t) size;
+
+          argv += 1;
+          argc -= 1;
+        }
       else if (strcmp (arg, "--chmod") == 0)
         {
           unsigned long perms;
@@ -2425,6 +2495,10 @@ parse_args_recurse (int          *argcp,
       if (!is_modifier_option(arg) && next_perms >= 0)
         die ("--perms must be followed by an option that creates a file");
 
+      /* Similarly for --size. */
+      if (!is_modifier_option(arg) && next_size_arg != 0)
+        die ("--size must be followed by --tmpfs");
+
       argv++;
       argc--;
     }
@@ -3006,6 +3080,7 @@ main (int    argc,
           int status;
           uint32_t buffer[2048];  /* 8k, but is int32 to guarantee nice alignment */
           uint32_t op, flags, perms;
+          size_t size_arg;
           const char *arg1, *arg2;
           cleanup_fd int unpriv_socket = -1;
 
@@ -3015,8 +3090,8 @@ main (int    argc,
           do
             {
               op = read_priv_sec_op (unpriv_socket, buffer, sizeof (buffer),
-                                     &flags, &perms, &arg1, &arg2);
-              privileged_op (-1, op, flags, perms, arg1, arg2);
+                                     &flags, &perms, &size_arg, &arg1, &arg2);
+              privileged_op (-1, op, flags, perms, size_arg, arg1, arg2);
               if (write (unpriv_socket, buffer, 1) != 1)
                 die ("Can't write to op_socket");
             }