Linux kernel uselib() privilege elevation, corrected
From: Paul Starzetz (ihaquer_at_isec.pl)
Date: Fri, 7 Jan 2005 22:19:04 +0100 (CET) To: email@example.com
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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
plagiarized 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 plagiarism did work.
Further steps must be taken to investigate the security leak on vendorsec.
Synopsis: Linux kernel uselib() privilege elevation
Product: Linux kernel
Version: 2.4 up to and including 2.4.29-pre3, 2.6 up to and including
Author: Paul Starzetz <firstname.lastname@example.org>
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
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)
error = do_mmap(file,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
if (error != ELF_PAGESTART(elf_phdata->p_vaddr))
elf_bss = elf_phdata->p_vaddr + elf_phdata->p_filesz;
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 semaphore held.
A short look into the code of do_brk() shows that:
 vma = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
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. A proof of concept code exists but will not be
Unprivileged local users can gain elevated (root) privileges.
Paul Starzetz <email@example.com> 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.
iSEC Security Research
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iSEC Security Research
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