Resolving kernel symbols

Posted by snare on 25 February 2012. Tags: mac os x, kernel, kexts, rootkits, mach-o

KXLD doesn’t like us much. He has KPIs to meet and doesn’t have time to help out shifty rootkit developers. KPIs are Kernel Programming Interfaces - lists of symbols in the kernel that KXLD (the kernel extension linker) will allow kexts to be linked against. The KPIs on which your kext depends are specified in the Info.plist file like this:

<key>OSBundleLibraries</key>
<dict>
	<key>com.apple.kpi.bsd</key>
	<string>11.0</string>
	<key>com.apple.kpi.libkern</key>
	<string>11.0</string>
	<key>com.apple.kpi.mach</key>
	<string>11.0</string>
	<key>com.apple.kpi.unsupported</key>
	<string>11.0</string>
	<key>com.apple.kpi.iokit</key>
	<string>11.0</string>
	<key>com.apple.kpi.dsep</key>
	<string>11.0</string>
</dict>

Those bundle identifiers correspond to the CFBundleIdentifier key specified in the Info.plist files for “plug-ins” to the System.kext kernel extension. Each KPI has its own plug-in kext - for example, the com.apple.kpi.bsd symbol table lives in BSDKernel.kext. These aren’t exactly complete kexts, they’re just Mach-O binaries with symbol tables full of undefined symbols (they really reside within the kernel image), which you can see if we dump the load commands:

$ otool -l /System/Library/Extensions/System.kext/PlugIns/BSDKernel.kext/BSDKernel 
/System/Library/Extensions/System.kext/PlugIns/BSDKernel.kext/BSDKernel:
Load command 0
     cmd LC_SYMTAB
 cmdsize 24
  symoff 80
   nsyms 830
  stroff 13360
 strsize 13324
Load command 1
     cmd LC_UUID
 cmdsize 24
    uuid B171D4B0-AC45-47FC-8098-5B2F89B474E6

That’s it - just the LC_SYMTAB (symbol table). So, how many symbols are there in the kernel image?

$ nm /mach_kernel|wc -l
   16122

Surely all the symbols in all the KPI symbol tables add up to the same number, right?

$ find /System/Library/Extensions/System.kext/PlugIns -type f|grep -v plist|xargs nm|sort|uniq|wc -l
    7677

Nope. Apple doesn’t want us to play with a whole bunch of their toys. 8445 of them. Some of them are pretty fun too :( Like allproc:

$ nm /mach_kernel|grep allproc
ffffff80008d9e40 S _allproc
$ find /System/Library/Extensions/System.kext/PlugIns -type f|grep -v plist|xargs nm|sort|uniq|grep allproc
$ 

Damn. The allproc symbol is the head of the kernel’s list (the queue(3) kind of list) of running processes. It’s what gets queried when you run ps(1) or top(1). Why do we want to find allproc? If we want to hide processes in a kernel rootkit that’s the best place to start. So, what happens if we build a kernel extension that imports allproc and try to load it?

bash-3.2# kextload AllProcRocks.kext
/Users/admin/AllProcRocks.kext failed to load - (libkern/kext) link error; check the system/kernel logs for errors or try kextutil(8).

Console says:

25/02/12 6:30:47.000 PM kernel: kxld[ax.ho.kext.AllProcRocks]: The following symbols are unresolved for this kext:
25/02/12 6:30:47.000 PM kernel: kxld[ax.ho.kext.AllProcRocks]: 	_allproc

OK, whatever.

What do we do?

There are a few steps that we need to take in order to resolve symbols in the kernel (or any other Mach-O binary):

• Find the __LINKEDIT segment - this contains an array of struct nlist_64’s which represent all the symbols in the symbol table, and an array of symbol name strings.
• Find the LC_SYMTAB load command - this contains the offsets within the file of the symbol and string tables.
• Calculate the position of the string table within __LINKEDIT based on the offsets in the LC_SYMTAB load command.
• Iterate through the struct nlist_64’s in __LINKEDIT, comparing the corresponding string in the string table to the name of the symbol we’re looking for until we find it (or reach the end of the symbol table).
• Grab the address of the symbol from the struct nlist_64 we’ve found.

Parse the load commands

One easy way to look at the symbol table would be to read the kernel file on disk at /mach_kernel, but we can do better than that if we’re already in the kernel - the kernel image is loaded into memory at a known address. If we have a look at the load commands for the kernel binary:

$ otool -l /mach_kernel
/mach_kernel:
Load command 0
      cmd LC_SEGMENT_64
  cmdsize 472
  segname __TEXT
   vmaddr 0xffffff8000200000
   vmsize 0x000000000052f000
  fileoff 0
 filesize 5435392
  maxprot 0x00000007
 initprot 0x00000005
   nsects 5
    flags 0x0
<snip>

We can see that the vmaddr field of the first segment is 0xffffff8000200000. If we fire up GDB and point it at a VM running Mac OS X (as per my previous posts here and here), we can see the start of the Mach-O header in memory at this address:

gdb$ x/xw 0xffffff8000200000
0xffffff8000200000:	0xfeedfacf

0xfeedfacf is the magic number denoting a 64-bit Mach-O image (the 32-bit version is 0xfeedface). We can actually display this as a struct if we’re using the DEBUG kernel with all the DWARF info:

gdb$ print *(struct mach_header_64 *)0xffffff8000200000
$1 = {
  magic = 0xfeedfacf, 
  cputype = 0x1000007, 
  cpusubtype = 0x3, 
  filetype = 0x2, 
  ncmds = 0x12, 
  sizeofcmds = 0x1010, 
  flags = 0x1, 
  reserved = 0x0
}

The mach_header and mach_header_64 structs (along with the other Mach-O-related structs mentioned in this post) are documented in the Mach-O File Format Reference, but we aren’t particularly interested in the header at the moment. I recommend having a look at the kernel image with MachOView to get the gist of where everything is and how it’s laid out.

Directly following the Mach-O header is the first load command:

gdb$ set $mh=(struct mach_header_64 *)0xffffff8000200000
gdb$ print *(struct load_command*)((void *)$mh + sizeof(struct mach_header_64))
$6 = {
  cmd = 0x19, 
  cmdsize = 0x1d8
}

This is the load command for the first __TEXT segment we saw with otool. We can cast it as a segment_command_64 in GDB and have a look:

gdb$ set $lc=((void *)$mh + sizeof(struct mach_header_64))
gdb$ print *(struct segment_command_64 *)$lc
$7 = {
  cmd = 0x19, 
  cmdsize = 0x1d8, 
  segname = "__TEXT\000\000\000\000\000\000\000\000\000", 
  vmaddr = 0xffffff8000200000, 
  vmsize = 0x8c8000, 
  fileoff = 0x0, 
  filesize = 0x8c8000, 
  maxprot = 0x7, 
  initprot = 0x5, 
  nsects = 0x5, 
  flags = 0x0
}

This isn’t the load command we are looking for, so we have to iterate through all of them until we come across a segment with cmd of 0x19 (LC_SEGMENT_64) and segname of __LINKEDIT. In the debug kernel, this happens to be located at 0xffffff8000200e68:

gdb$ set $lc=0xffffff8000200e68
gdb$ print *(struct load_command*)$lc   
$14 = {
  cmd = 0x19, 
  cmdsize = 0x48
}
gdb$ print *(struct segment_command_64*)$lc
$16 = {
  cmd = 0x19, 
  cmdsize = 0x48, 
  segname = "__LINKEDIT\000\000\000\000\000", 
  vmaddr = 0xffffff8000d08000, 
  vmsize = 0x109468, 
  fileoff = 0xaf4698, 
  filesize = 0x109468, 
  maxprot = 0x7, 
  initprot = 0x1, 
  nsects = 0x0, 
  flags = 0x0
}

Then we grab the vmaddr field from the load command, which specifies the address at which the __LINKEDIT segment’s data will be located:

gdb$ set $linkedit=((struct segment_command_64*)$lc)->vmaddr
gdb$ print $linkedit
$19 = 0xffffff8000d08000
gdb$ print *(struct nlist_64 *)$linkedit
$20 = {
  n_un = {
    n_strx = 0x68a29
  }, 
  n_type = 0xe, 
  n_sect = 0x1, 
  n_desc = 0x0, 
  n_value = 0xffffff800020a870
}

And there’s the first struct nlist_64.

As for the LC_SYMTAB load command, we just need to iterate through the load commands until we find one with the cmd field value of 0x02 (LC_SYMTAB). In this case, it’s located at 0xffffff8000200eb0:

gdb$ set $symtab=*(struct symtab_command*)0xffffff8000200eb0
gdb$ print $symtab
$23 = {
  cmd = 0x2, 
  cmdsize = 0x18, 
  symoff = 0xaf4698, 
  nsyms = 0x699d, 
  stroff = 0xb5e068, 
  strsize = 0x9fa98
}

The useful parts here are the symoff field, which specifies the offset in the file to the symbol table (start of the __LINKEDIT segment), and the stroff field, which specifies the offset in the file to the string table (somewhere in the middle of the __LINKEDIT segment). Why, you ask, did we need to find the __LINKEDIT segment as well, since we have the offset here in the LC_SYMTAB command? If we were looking at the file on disk we wouldn’t have needed to, but as the kernel image we’re inspecting has already been loaded into memory, the binary segments have been loaded at the virtual memory addresses specified in their load commands. This means that the symoff and stroff fields are not correct any more. However, they’re still useful, as the difference between the two helps us figure out the offset into the __LINKEDIT segment at which the string table exists:

gdb$ print $linkedit
$24 = 0xffffff8000d08000
gdb$ print $linkedit + ($symtab->stroff - $symtab->symoff)
$25 = 0xffffff8000d719d0
gdb$ set $strtab=$linkedit + ($symtab->stroff - $symtab->symoff)
gdb$ x/16s $strtab
0xffffff8000d719d0:	 ""
0xffffff8000d719d1:	 ""
0xffffff8000d719d2:	 ""
0xffffff8000d719d3:	 ""
0xffffff8000d719d4:	 ".constructors_used"
0xffffff8000d719e7:	 ".destructors_used"
0xffffff8000d719f9:	 "_AddFileExtent"
0xffffff8000d71a08:	 "_AllocateNode"
0xffffff8000d71a16:	 "_Assert"
0xffffff8000d71a1e:	 "_BF_decrypt"
0xffffff8000d71a2a:	 "_BF_encrypt"
0xffffff8000d71a36:	 "_BF_set_key"
0xffffff8000d71a42:	 "_BTClosePath"
0xffffff8000d71a4f:	 "_BTDeleteRecord"
0xffffff8000d71a5f:	 "_BTFlushPath"
0xffffff8000d71a6c:	 "_BTGetInformation"

Actually finding some symbols

Now that we know where the symbol table and string table live, we can get on to the srs bznz. So, let’s find that damn _allproc symbol we need. Have a look at that first struct nlist_64 again:

gdb$ print *(struct nlist_64 *)$linkedit
$28 = {
  n_un = {
    n_strx = 0x68a29
  }, 
  n_type = 0xe, 
  n_sect = 0x1, 
  n_desc = 0x0, 
  n_value = 0xffffff800020a870
}

The n_un.nstrx field there specifies the offset into the string table at which the string corresponding to this symbol exists. If we add that offset to the address at which the string table starts, we’ll see the symbol name:

gdb$ x/s $strtab + ((struct nlist_64 *)$linkedit)->n_un.n_strx
0xffffff8000dda3f9:	 "_ps_vnode_trim_init"

Now all we need to do is iterate through all the struct nlist_64’s until we find the one with the matching name. In this case it’s at 0xffffff8000d482a0:

gdb$ set $nlist=0xffffff8000d482a0
gdb$ print *(struct nlist_64*)$nlist
$31 = {
  n_un = {
    n_strx = 0x35a07
  }, 
  n_type = 0xf, 
  n_sect = 0xb, 
  n_desc = 0x0, 
  n_value = 0xffffff8000cb5ca0
}
gdb$ x/s $strtab + ((struct nlist_64 *)$nlist)->n_un.n_strx
0xffffff8000da73d7:	 "_allproc"

The n_value field there (0xffffff8000cb5ca0) is the virtual memory address at which the symbol’s data/code exists. _allproc is not a great example as it’s a piece of data, rather than a function, so let’s try it with a function:

gdb$ set $nlist=0xffffff8000d618f0
gdb$ print *(struct nlist_64*)$nlist
$32 = {
  n_un = {
    n_strx = 0x52ed3
  }, 
  n_type = 0xf, 
  n_sect = 0x1, 
  n_desc = 0x0, 
  n_value = 0xffffff80007cceb0
}
gdb$ x/s $strtab + ((struct nlist_64 *)$nlist)->n_un.n_strx
0xffffff8000dc48a3:	 "_proc_lock"

If we disassemble a few instructions at that address:

gdb$ x/12i 0xffffff80007cceb0
0xffffff80007cceb0 <proc_lock>:	push   rbp
0xffffff80007cceb1 <proc_lock+1>:	mov    rbp,rsp
0xffffff80007cceb4 <proc_lock+4>:	sub    rsp,0x10
0xffffff80007cceb8 <proc_lock+8>:	mov    QWORD PTR [rbp-0x8],rdi
0xffffff80007ccebc <proc_lock+12>:	mov    rax,QWORD PTR [rbp-0x8]
0xffffff80007ccec0 <proc_lock+16>:	mov    rcx,0x50
0xffffff80007cceca <proc_lock+26>:	add    rax,rcx
0xffffff80007ccecd <proc_lock+29>:	mov    rdi,rax
0xffffff80007cced0 <proc_lock+32>:	call   0xffffff800035d270 <lck_mtx_lock>
0xffffff80007cced5 <proc_lock+37>:	add    rsp,0x10
0xffffff80007cced9 <proc_lock+41>:	pop    rbp
0xffffff80007cceda <proc_lock+42>:	ret

We can see that GDB has resolved the symbol for us, and we’re right on the money.

Sample code

I’ve posted an example kernel extension on github to check out. When we load it with kextload KernelResolver.kext, we should see something like this on the console:

25/02/12 8:06:49.000 PM kernel: [+] _allproc @ 0xffffff8000cb5ca0
25/02/12 8:06:49.000 PM kernel: [+] _proc_lock @ 0xffffff80007cceb0
25/02/12 8:06:49.000 PM kernel: [+] _kauth_cred_setuidgid @ 0xffffff80007abbb0
25/02/12 8:06:49.000 PM kernel: [+] __ZN6OSKext13loadFromMkextEjPcjPS0_Pj @ 0xffffff80008f8606

Update: It was brought to my attention that I was using a debug kernel in these examples. Just to be clear - the method described in this post, as well as the sample code, works on a non-debug, default install >=10.7.0 (xnu-1699.22.73) kernel as well, but the GDB inspection probably won’t (unless you load up the struct definitions etc, as they are all stored in the DEBUG kernel). The debug kernel contains every symbol from the source, whereas many symbols are stripped from the distribution kernel (e.g. sLoadedKexts). Previously (before 10.7), the kernel would write out the symbol table to a file on disk and jettison it from memory altogether. I suppose when kernel extensions were loaded, kextd or kextload would resolve symbols from within that on-disk symbol table or from the on-disk kernel image. These days the symbol table memory is just marked as pageable, so it can potentially get paged out if the system is short of memory.

I hope somebody finds this useful. Shoot me an email or get at me on twitter if you have any questions. I’ll probably sort out comments for this blog at some point, but I cbf at the moment.

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