信息来源：安全小组
Issue:
======
Locally exploitable flaws have been found in the Linux binary format
loaders\' uselib() functions that allow local users to gain root
privileges.
Details:
========
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 basically a simplified mmap()
coupled with some header parsing code.
An analyze 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 modifying 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.
Discussion:
=============
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(¤t->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(¤t->mm->mmap_sem);
if (error != ELF_PAGESTART(elf_phdata->p_vaddr))
goto out_free_ph;
elf_bss = elf_phdata->p_vaddr + elf_phdata->p_filesz;
padzero(elf_bss);
len = ELF_PAGESTART(elf_phdata->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 mmap 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->vm_mm = mm;
vma->vm_start = addr;
vma->vm_end = addr + len;
vma->vm_flags = flags;
vma->vm_page_prot = protection_map[flags & 0x0f];
vma->vm_ops = NULL;
vma->vm_pgoff = 0;
vma->vm_file = NULL;
vma->vm_private_data = NULL;
vma_link(mm, vma, prev, rb_link, rb_parent);
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. The code attached uses a similar technique like the
do_brk exploit and uses a LDT call gate to gain CPL0 privileges. However
another exploitation vectors exist: through page reference counters and
\'ghost PTEs\'.
Impact:
=======
Unprivileged local users can gain elevated (root) privileges.
Credits:
========
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.
Disclaimer:
===========
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.
Appendix:
=========
http://www.eviloctal.com/forum/read.php?tid=6636
