[Full-Disclosure] Linux kernel uselib() privilege elevation, corrected

Type securityvulns
Reporter Securityvulns
Modified 2005-01-07T00:00:00


Hi all,

first of all I must comply about the handling of this vulnerability that I reported to vendorsec. Obviously my code posted there has been stolen and plagiated in order to put the blame on Stefan Esser from Ematters and disturb the security community.

I really apologize to Stefan Esser for the inconvenience and thank him for his cool reaction - the plagiate worked.

Synopsis: Linux kernel uselib() privilege elevation Product: Linux kernel Version: 2.4 up to and including 2.4.29-rc2, 2.6 up to and including 2.6.10 Vendor: http://www.kernel.org/ URL: http://isec.pl/vulnerabilities/isec-0021-uselib.txt CVE: CAN-2004-1235 Author: Paul Starzetz <ihaquer@isec.pl> Date: Jan 07, 2005


Locally exploitable flaws have been found in the Linux binary format loaders' uselib() functions that allow local users to gain root privileges.


The Linux kernel provides a binary format loader layer to load (execute) programs of different binary formats like ELF or a.out and more. The kernel also provides a function named sys_uselib() to load a corresponding library. This function is dispatched to the current process's binary format handler and is basicaly a simplified mmap() code coupled with some header parsing code.

An analyse of the uselib function load_elf_library() from binfmt_elf.c revealed a flaw in the handling of the library's brk segment (VMA). That segment is created with the current->mm->mmap_sem semaphore NOT held while modyfying the memory layout of the calling process. This can be used to disturb the memory management and gain elevated privileges. Also the binfmt_aout binary format loader code is affected in the same way.


The vulnerable code resides for example in fs/binfmt_elf.c in your kernel source code tree:

static int load_elf_library(struct file *file) { [904] down_write(&current->mm->mmap_sem); error = do_mmap(file, ELF_PAGESTART(elf_phdata->p_vaddr), (elf_phdata->p_filesz + ELF_PAGEOFFSET(elf_phdata->p_vaddr)), PROT_READ | PROT_WRITE | PROT_EXEC, MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE, (elf_phdata->p_offset - ELF_PAGEOFFSET(elf_phdata->p_vaddr))); up_write(&current->mm->mmap_sem); if (error != ELF_PAGESTART(elf_phdata->p_vaddr)) goto out_free_ph;

   elf_bss = elf_phdata-&gt;p_vaddr + elf_phdata-&gt;p_filesz;

   len = ELF_PAGESTART&#40;elf_phdata-&gt;p_filesz +

elf_phdata->p_vaddr + ELF_MIN_ALIGN - 1); bss = elf_phdata->p_memsz + elf_phdata->p_vaddr; if (bss > len) do_brk(len, bss - len);

The line numbers are all valid for the 2.4.28 kernel version. As can be seen the mmap_sem is released prior to calling do_brk() in order to create the data section of the ELF library. On the other hand, looking into the code of sys_brk() from mm/mmap.c reveals that do_brk() must be called with the semaphore held.

A short look into the code of do_brk() shows that:

[1094] vma = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL); if (!vma) return -ENOMEM;

   vma-&gt;vm_mm = mm;
   vma-&gt;vm_start = addr;
   vma-&gt;vm_end = addr + len;
   vma-&gt;vm_flags = flags;
   vma-&gt;vm_page_prot = protection_map[flags & 0x0f];
   vma-&gt;vm_ops = NULL;
   vma-&gt;vm_pgoff = 0;
   vma-&gt;vm_file = NULL;
   vma-&gt;vm_private_data = NULL;

   vma_link&#40;mm, vma, prev, rb_link, rb_parent&#41;;

where rb_link and rb_parent were both found by
calling find_vma_prepare(). Obviously, if the kmem_cache_alloc() call sleeps, the newly created VMA descriptor may be inserted at wrong position because the process's VMA list and the VMA RB-tree may have been changed by another thread. This is absolutely enough to gain root privileges.

We have found at least three different ways to
exploit this vulnerability. The race condition can be easily won by consuming a big amount of memory. A proof of concept code exists but will not be released yet.


Unprivileged local users can gain elevated (root) privileges.


Paul Starzetz <ihaquer@isec.pl> has identified the vulnerability and performed further research. COPYING, DISTRIBUTION, AND MODIFICATION OF INFORMATION PRESENTED HERE IS ALLOWED ONLY WITH EXPRESS PERMISSION OF ONE OF THE AUTHORS.


This document and all the information it contains are provided "as is", for educational purposes only, without warranty of any kind, whether express or implied.

The authors reserve the right not to be responsible for the topicality, correctness, completeness or quality of the information provided in this document. Liability claims regarding damage caused by the use of any information provided, including any kind of information which is incomplete or incorrect, will therefore be rejected.


Code attached.

-- Paul Starzetz iSEC Security Research http://isec.pl/