MacOSX 10.9.2/XNU HFS Hard Linking

`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 `\0'); 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] == '\0') { <===== 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/  
  
  
`