mdw@git.distorted.org.uk Git - checkpath/blob - checkpath.c

   1 /* -*-c-*-
   2  *
   3  * $Id: checkpath.c,v 1.6 2004/04/08 01:36:22 mdw Exp $
   4  *
   5  * Check a path for safety
   6  *
   7  * (c) 1999 Mark Wooding
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of chkpath.
  13  *
  14  * chkpath is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU General Public License as published by
  16  * the Free Software Foundation; either version 2 of the License, or
  17  * (at your option) any later version.
  18  *
  19  * chkpath is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU General Public License
  25  * along with chkpath; if not, write to the Free Software Foundation,
  26  * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  27  */
  28
  29 /*----- Header files ------------------------------------------------------*/
  30
  31 #include "config.h"
  32
  33 #include <errno.h>
  34 #include <stdarg.h>
  35 #include <stdio.h>
  36 #include <stdlib.h>
  37 #include <string.h>
  38
  39 #include <sys/types.h>
  40 #include <sys/stat.h>
  41 #include <unistd.h>
  42
  43 #include <pwd.h>
  44 #include <grp.h>
  45
  46 #include <mLib/alloc.h>
  47 #include <mLib/dstr.h>
  48
  49 #include "checkpath.h"
  50
  51 /*----- Data structures ---------------------------------------------------*/
  52
  53 /* --- An item in the directory list --- *
  54  *
  55  * Each directory becomes an element on a list which is manipulated in a
  56  * stack-like way.
  57  */
  58
  59 struct elt {
  60   struct elt *e_link;                   /* Pointer to the next one along */
  61   size_t e_offset;                      /* Offset of name in path string */
  62   unsigned e_flags;                     /* Various useful flags */
  63   char e_name[1];                       /* Name of the directory */
  64 };
  65
  66 #define f_sticky 1u                     /* Directory has sticky bit set */
  67
  68 #define f_last 1u                       /* This is the final item to check */
  69
  70 /*----- Static variables --------------------------------------------------*/
  71
  72 static struct elt rootnode = { 0, 0, 0 }; /* Root of the list */
  73 static struct elt *sp;                  /* Stack pointer for list */
  74 static dstr d = DSTR_INIT;              /* Current path string */
  75
  76 /*----- Main code ---------------------------------------------------------*/
  77
  78 /* --- @splitpath@ --- *
  79  *
  80  * Arguments:   @const char *path@ = path string to break apart
  81  *              @struct elt *tail@ = tail block to attach to end of list
  82  *
  83  * Returns:     Pointer to the new list head.
  84  *
  85  * Use:         Breaks a path string into directories and adds each one
  86  *              as a node on the list, in the right order.  These can then
  87  *              be pushed onto the directory stack as required.
  88  */
  89
  90 static struct elt *splitpath(const char *path, struct elt *tail)
  91 {
  92   struct elt *head, **ee = &head, *e;
  93
  94   while (*path) {
  95     size_t n;
  96
  97     /* --- Either a leading `/', or a doubled one --- *
  98      *
  99      * Either way, ignore it.
 100      */
 101
 102     if (*path == '/') {
 103       path++;
 104       continue;
 105     }
 106
 107     /* --- Skip to the next directory separator --- *
 108      *
 109      * Build a list element for it, and link it on.
 110      */
 111
 112     n = strcspn(path, "/");
 113     e = xmalloc(sizeof(struct elt) + n + 1);
 114     memcpy(e->e_name, path, n);
 115     e->e_name[n] = 0;
 116     e->e_flags = 0;
 117     *ee = e;
 118     ee = &e->e_link;
 119     path += n;
 120   }
 121
 122   /* --- Done --- */
 123
 124   *ee = tail;
 125   return (head);
 126 }
 127
 128 /* --- @pop@ --- *
 129  *
 130  * Arguments:   ---
 131  *
 132  * Returns:     ---
 133  *
 134  * Use:         Removes the top item from the directory stack.
 135  */
 136
 137 static void pop(void)
 138 {
 139   if (sp->e_link) {
 140     struct elt *e = sp->e_link;
 141     d.len = sp->e_offset;
 142     DPUTZ(&d);
 143     sp = e;
 144   }
 145 }
 146
 147 /* --- @popall@ --- *
 148  *
 149  * Arguments:   ---
 150  *
 151  * Returns:     ---
 152  *
 153  * Use:         Removes all the items from the directory stack.
 154  */
 155
 156 static void popall(void)
 157 {
 158   while (sp->e_link)
 159     pop();
 160 }
 161
 162 /* --- @push@ --- *
 163  *
 164  * Arguments:   @struct elt *e@ = pointer to directory element
 165  *
 166  * Returns:     ---
 167  *
 168  * Use:         Pushes a new subdirectory onto the stack.
 169  */
 170
 171 static void push(struct elt *e)
 172 {
 173   e->e_link = sp;
 174   e->e_offset = d.len;
 175   DPUTC(&d, '/');
 176   DPUTS(&d, e->e_name);
 177   sp = e;
 178 }
 179
 180 /* --- @report@ --- *
 181  *
 182  * Arguments:   @const struct checkpath *cp@ = pointer to context
 183  *              @unsigned what@ = what sort of report is this?
 184  *              @int verbose@ = how verbose is this?
 185  *              @const char *p@ = what path does it refer to?
 186  *              @const char *msg@ = the message to give to the user
 187  *
 188  * Returns:     ---
 189  *
 190  * Use:         Formats and presents messages to the client.
 191  */
 192
 193 static void report(const struct checkpath *cp, unsigned what, int verbose,
 194                    const char *p, const char *msg, ...)
 195 {
 196   /* --- Decide whether to bin this message --- */
 197
 198   if (!cp->cp_report || verbose > cp->cp_verbose || !(cp->cp_what & what))
 199     return;
 200
 201   /* --- Format the message nicely --- */
 202
 203   if (cp->cp_what & CP_REPORT) {
 204     dstr d = DSTR_INIT;
 205     va_list ap;
 206     const char *q = msg;
 207     size_t n;
 208     int e = errno;
 209
 210     va_start(ap, msg);
 211     if (verbose > 1)
 212       dstr_puts(&d, "[ ");
 213     if (p)
 214       dstr_putf(&d, "Path: %s: ", p);
 215     while (*q) {
 216       if (*q == '%') {
 217         q++;
 218         switch (*q) {
 219           case 'e':
 220             dstr_puts(&d, strerror(e));
 221             break;
 222           case 'u': {
 223             uid_t u = (uid_t)va_arg(ap, int);
 224             struct passwd *pw = getpwuid(u);
 225             if (pw)
 226               dstr_putf(&d, "`%s'", pw->pw_name);
 227             else
 228               dstr_putf(&d, "%i", (int)u);
 229           } break;
 230           case 'g': {
 231             gid_t g = (gid_t)va_arg(ap, int);
 232             struct group *gr = getgrgid(g);
 233             if (gr)
 234               dstr_putf(&d, "`%s'", gr->gr_name);
 235             else
 236               dstr_putf(&d, "%i", (int)g);
 237           } break;
 238           case 's': {
 239             const char *s = va_arg(ap, const char *);
 240             dstr_puts(&d, s);
 241           } break;
 242           case '%':
 243             dstr_putc(&d, '%');
 244             break;
 245           default:
 246             dstr_putc(&d, '%');
 247             dstr_putc(&d, *q);
 248             break;
 249         }
 250         q++;
 251       } else {
 252         n = strcspn(q, "%");
 253         DPUTM(&d, q, n);
 254         q += n;
 255       }
 256     }
 257     if (verbose > 1)
 258       dstr_puts(&d, " ]");
 259     DPUTZ(&d);
 260     cp->cp_report(what, verbose, p, d.buf, cp->cp_arg);
 261     dstr_destroy(&d);
 262     va_end(ap);
 263   } else
 264     cp->cp_report(what, verbose, p, 0, cp->cp_arg);
 265 }
 266
 267 /* --- @sanity@ --- *
 268  *
 269  * Arguments:   @const char *p@ = name of directory to check
 270  *              @struct stat *st@ = pointer to @stat@(2) block for it
 271  *              @const struct checkpath *cp@ = pointer to caller parameters
 272  *              @unsigned f@ = various flags
 273  *
 274  * Returns:     Zero if everything's OK, else bitmask of problems.
 275  *
 276  * Use:         Performs the main load of sanity-checking on a directory.
 277  */
 278
 279 static unsigned sanity(const char *p, struct stat *st,
 280                        const struct checkpath *cp, unsigned f)
 281 {
 282   unsigned bad = 0;
 283   int stickyok = 0;
 284
 285   if (S_ISDIR(st->st_mode) &&
 286       (!(f & f_last) || (cp->cp_what & CP_STICKYOK)))
 287     stickyok = 01000;
 288
 289   /* --- Check for world-writability --- */
 290
 291   if ((cp->cp_what & CP_WRWORLD) &&
 292       (st->st_mode & (0002 | stickyok)) == 0002) {
 293     bad |= CP_WRWORLD;
 294     report(cp, CP_WRWORLD, 1, p, "** world writable **");
 295   }
 296
 297   /* --- Check for group-writability --- */
 298
 299   if ((cp->cp_what & (CP_WRGRP | CP_WROTHGRP)) &&
 300       (st->st_mode & (0020 | stickyok)) == 0020) {
 301     int i;
 302     unsigned b = CP_WRGRP;
 303
 304     if (cp->cp_what & CP_WROTHGRP) {
 305       b = CP_WROTHGRP;
 306       for (i = 0; i < cp->cp_gids; i++) {
 307         if (st->st_gid == cp->cp_gid[i])
 308           b = cp->cp_what & CP_WRGRP;
 309       }
 310     }
 311     if (b) {
 312       bad |= b;
 313       report(cp, b, 1, p, "writable by %sgroup %g",
 314              (b == CP_WROTHGRP) ? "other " : "", st->st_gid);
 315     }
 316   }
 317
 318   /* --- Check for user-writability --- */
 319
 320   if ((cp->cp_what & CP_WROTHUSR) &&
 321       st->st_uid != cp->cp_uid &&
 322       st->st_uid != 0) {
 323     bad |= CP_WROTHUSR;
 324     report(cp, CP_WROTHUSR, 1, p, "owner is user %u", st->st_uid);
 325   }
 326
 327   /* --- Done sanity check --- */
 328
 329   return (bad);
 330 }
 331
 332 /* --- @checkpath@ --- *
 333  *
 334  * Arguments:   @const char *p@ = directory name which needs checking
 335  *              @const struct checkpath *cp@ = parameters for the check
 336  *
 337  * Returns:     Zero if all is well, otherwise bitmask of problems.
 338  *
 339  * Use:         Scrutinises a directory path to see what evil things other
 340  *              users could do to it.
 341  */
 342
 343 unsigned checkpath(const char *p, const struct checkpath *cp)
 344 {
 345   char cwd[PATH_MAX];
 346   struct elt *e, *ee;
 347   struct stat st;
 348   unsigned bad = 0;
 349
 350   /* --- Initialize stack pointer and path string --- */
 351
 352   sp = &rootnode;
 353   dstr_destroy(&d);
 354
 355   /* --- Try to find the current directory --- */
 356
 357   if (!getcwd(cwd, sizeof(cwd))) {
 358     report(cp, CP_ERROR, 0, 0, "can't find current directory: %e");
 359     return (CP_ERROR);
 360   }
 361
 362   /* --- Check that the root directory is OK --- */
 363
 364   if (stat("/", &st)) {
 365     report(cp, CP_ERROR, 0, 0, "can't stat root directory: %e");
 366     return (CP_ERROR);
 367   }
 368
 369   report(cp, CP_REPORT, 3, p, "begin scan");
 370   bad |= sanity("/", &st, cp, 0);
 371
 372   /* --- Get the initial list of things to process --- */
 373
 374   ee = splitpath(p, 0);
 375   if (*p != '/')
 376     ee = splitpath(cwd, ee);
 377
 378   /* --- While there are list items which still need doing --- */
 379
 380   while (ee) {
 381     e = ee->e_link;
 382
 383     /* --- Strip off simple `.' elements --- */
 384
 385     if (strcmp(ee->e_name, ".") == 0) {
 386       free(ee);
 387       ee = e;
 388       continue;
 389     }
 390
 391     /* --- Backtrack on `..' elements --- */
 392
 393     else if (strcmp(ee->e_name, "..") == 0) {
 394       pop();
 395       free(ee);
 396       ee = e;
 397       continue;
 398     }
 399
 400     /* --- Everything else gets pushed on the end --- */
 401
 402     push(ee);
 403     ee = e;
 404
 405     /* --- Find out what sort of a thing this is --- */
 406
 407     if (lstat(d.buf, &st)) {
 408       report(cp, CP_ERROR, 0, d.buf, "can't stat: %e");
 409       bad |= CP_ERROR;
 410       break;
 411     }
 412
 413     /* --- Handle symbolic links specially --- */
 414
 415     if (S_ISLNK(st.st_mode)) {
 416       dstr buf = DSTR_INIT;
 417       int i;
 418
 419       /* --- Resolve the link --- */
 420
 421       dstr_ensure(&buf, st.st_size + 1);
 422       if ((i = readlink(d.buf, buf.buf, buf.sz)) < 0) {
 423         report(cp, CP_ERROR, 0, d.buf, "can't readlink: %e");
 424         bad |= CP_ERROR;
 425         break;
 426       }
 427       buf.buf[i] = 0;
 428       report(cp, CP_SYMLINK, 2, d.buf, "symlink -> `%s'", buf.buf);
 429
 430       /* --- Handle sticky parents --- *
 431        *
 432        * If I make a symlink in a sticky directory, I can later modify it.
 433        * However, nobody else can (except the owner of the directory, and
 434        * we'll already have noticed that if we care).
 435        */
 436
 437       if ((cp->cp_what & CP_WROTHUSR) &&
 438           (sp->e_link->e_flags & f_sticky) &&
 439           st.st_uid != cp->cp_uid && st.st_uid != 0) {
 440         bad |= CP_WROTHUSR;
 441         report(cp, CP_WROTHUSR, 1, d.buf,
 442                "symlink modifiable by user %u", st.st_uid);
 443       }
 444
 445       /* --- Sort out what to do from here --- */
 446
 447       if (buf.buf[0] == '/')
 448         popall();
 449       else
 450         pop();
 451       ee = splitpath(buf.buf, ee);
 452       dstr_destroy(&buf);
 453       continue;
 454     }
 455
 456     /* --- Run the sanity check on this path element --- */
 457
 458     bad |= sanity(d.buf, &st, cp, ee ? 0 : f_last);
 459
 460     if (S_ISDIR(st.st_mode)) {
 461       if (st.st_mode & 01000)
 462         sp->e_flags |= f_sticky;
 463       report(cp, CP_REPORT, 4, d.buf, "directory");
 464       continue;
 465     }
 466
 467     /* --- Something else I don't understand --- */
 468
 469     break;
 470   }
 471
 472   /* --- Check for leftover junk --- */
 473
 474   if (ee) {
 475     if (!(bad & CP_ERROR))
 476       report(cp, CP_ERROR, 0, 0, "junk left over after reaching leaf");
 477     while (ee) {
 478       e = ee->e_link;
 479       free(ee);
 480       ee = e;
 481     }
 482   }
 483
 484   popall();
 485   return (bad);
 486 }
 487
 488 /* --- @checkpath_setids@ --- *
 489  *
 490  * Arguments:   @struct checkpath *cp@ = pointer to block to fill in
 491  *
 492  * Returns:     ---
 493  *
 494  * Use:         Fills in the user ids and things in the structure.
 495  */
 496
 497 void checkpath_setids(struct checkpath *cp)
 498 {
 499   int n, i;
 500   gid_t g = getgid();
 501
 502   cp->cp_uid = getuid();
 503   n = getgroups(sizeof(cp->cp_gid) / sizeof(cp->cp_gid[0]), cp->cp_gid);
 504
 505   for (i = 0; i < n; i++) {
 506     if (cp->cp_gid[i] == g)
 507       goto gid_ok;
 508   }
 509   cp->cp_gid[n++] = g;
 510 gid_ok:
 511   cp->cp_gids = n;
 512 }
 513
 514 /*----- That's all, folks -------------------------------------------------*/