Date: Tue, 25 Sep 2018 10:17:28 -0700
From: Qualys Security Advisory <qsa@...lys.com>
To: fulldisclosure@...lists.org
Subject: [FD] Integer overflow in Linux's create_elf_tables()
	(CVE-2018-14634)


Qualys Security Advisory

Mutagen Astronomy: Integer overflow in Linux's create_elf_tables()
(CVE-2018-14634)


========================================================================
Contents
========================================================================

Summary
Analysis
Exploitation
Acknowledgments
Timeline


========================================================================
Summary
========================================================================

We discovered an integer overflow in the Linux kernel's
create_elf_tables() function: on a 64-bit system, a local attacker can
exploit this vulnerability via a SUID-root binary and obtain full root
privileges.

Only kernels with commit b6a2fea39318 ("mm: variable length argument
support", from July 19, 2007) but without commit da029c11e6b1 ("exec:
Limit arg stack to at most 75% of _STK_LIM", from July 7, 2017) are
exploitable.

Most Linux distributions backported commit da029c11e6b1 to their
long-term-supported kernels, but Red Hat Enterprise Linux and CentOS
(and Debian 8, the current "oldstable" version) have not, and are
therefore vulnerable and exploitable.


========================================================================
Analysis
========================================================================

 150 #define STACK_ROUND(sp, items) \
 151         (((unsigned long) (sp - items)) &~ 15UL)
 ...
 165 create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
 ...
 169         int argc = bprm->argc;
 170         int envc = bprm->envc;
 171         elf_addr_t __user *sp;
 ...
 178         int items;
 ...
 190         p = arch_align_stack(p);
 ...
 287         items = (argc + 1) + (envc + 1) + 1;
 288         bprm->p = STACK_ROUND(sp, items);
 ...
 295         sp = (elf_addr_t __user *)bprm->p;

"argc", the number of command-line arguments passed to the execve()
system call, is limited to MAX_ARG_STRINGS (in fs/exec.c); "envc", the
number of environment variables passed to execve(), is also limited to
MAX_ARG_STRINGS; but because MAX_ARG_STRINGS is 0x7FFFFFFF, we can
overflow the integer "items" (at line 287) and make it negative.

As a result, we can increase the userland stack pointer instead of
decreasing it (at lines 288 and 295 -- the stack normally grows down on
x86_64), redirect the userland stack to the middle of our argument and
environment strings (which were copied to the top of the stack in
fs/exec.c), and hence overwrite these strings during the userland
execution of a SUID-root binary.


========================================================================
Exploitation
========================================================================

We execve() a SUID-root binary with exactly 0x80000000 "items" (i.e.,
INT_MIN "items"): roughly 0x80000000 * sizeof(char *) = 16GB of argument
pointers, 16GB of argument strings, and 16GB of environment strings. Our
exploit requires "only" 2 * 16GB = 32GB of memory, instead of 3 * 16GB =
48GB or more, because we use a few tricks to reduce its memory footprint
(for example, we replace the nearly 16GB of equal argument pointers with
equivalent file-backed mappings that consume practically no memory).

The following diagram represents the layout of our userland stack when
the execution of the SUID-root binary starts, in ld.so:

               | argument strings  |          environment strings          |
--|---|--------|---------+---------|---------+---------+---------+---------|--
  | A | sprand | protect | padding | protect | scratch | onebyte | padding |
--|---|--------|---------+---------|---------+---------+------^--+---------|--
  |     0-8192              ~16GB                1MB         rsp    ~16GB
  v                                               <-------+---|----------|
  |                                                 stack | B | pointers |
  \-------------->-------------->-------------->--------------/   16GB
             0x80000000 * sizeof(elf_addr_t) = 16GB

- "A" ("alpha") is the amount of stack space allocated by
  create_elf_tables() between lines 190 and 287 exclusive (for example,
  the platform and base-platform capability strings): it is
  approximately 512 bytes.

- "sprand" is a random amount of stack space allocated by
  create_elf_tables() at line 190: it varies from 0 to 8192 bytes.

- The "protect" argument strings are vital command-line arguments and
  options that must be safe from memory corruption (for example,
  argv[0], the filename of the SUID-root binary).

- The "padding" argument strings occupy roughly 16GB of stack space.

- The "protect" environment strings are vital environment variables that
  must be safe from memory corruption (for example, our LD_PRELOAD
  environment variable, which will be processed by ld.so's
  handle_ld_preload() function).

- The "scratch" environment strings are 1MB of safe stack space for the
  userland execution of the SUID-root binary: the integer overflow of
  "items" redirects the userland stack pointer "rsp" to the middle of
  our argument and environment strings (to an offset of 0x80000000 *
  sizeof(elf_addr_t) = 16GB) -- more precisely, to the middle of our
  "onebyte" environment strings.

- The "onebyte" environment strings are 256KB of one-byte (empty)
  environment variables that will be partly overwritten by the 4KB
  fname[] buffer in ld.so's handle_ld_preload() function.

- The "padding" environment strings occupy roughly 16GB of stack space.

- The 16GB of argument and environment "pointers" (i.e., the argv[] and
  envp[] arrays) are written on top of our "padding" environment strings
  by create_elf_tables(), after the integer overflow of "items" and the
  redirection of the userland stack pointer "rsp".

- "B" ("beta") is the amount of stack space allocated by ld.so before
  the call to handle_ld_preload(): it is approximately 9KB and is
  allocated in the middle of our "onebyte" environment strings.

As a result, ld.so partly overwrites (i.e., rewrites) our "onebyte"
environment variables with the fname[] buffer in handle_ld_preload()
(whose contents we control through our LD_PRELOAD environment variable)
and thereby nullifies process_envvars()'s filtering of UNSECURE_ENVVARS
(LD_AUDIT, LD_LIBRARY_PATH, LD_PRELOAD, etc). The exploitation of this
lack of UNSECURE_ENVVARS filtering in ld.so (via a suitable SUID-root
binary) is left as an exercise for the interested reader.

Our proof-of-concept (poc-exploit.c) exploits the integer overflow in
create_elf_tables() and the resulting lack of UNSECURE_ENVVARS filtering
in ld.so: it executes the main() of a SUID-root binary (poc-suidbin.c)
while LD_LIBRARY_PATH remains set, even though it should have been
removed from the environment variables by ld.so. Demonstration:

# gcc -O0 -o poc-suidbin poc-suidbin.c
# chown root poc-suidbin
# chmod 4555 poc-suidbin

$ gcc -o poc-exploit poc-exploit.c
$ time ./poc-exploit
...
ERROR: ld.so: object 'LD_LIBRARY_PATH=.0LD_LIBRARY_PATH=.0LD_LIBRARY_PATH=.' from LD_PRELOAD cannot be preloaded: ignored.
ERROR: ld.so: object 'LD_LIBRARY_PATH=.0LD_LIBRARY_PATH=.' from LD_PRELOAD cannot be preloaded: ignored.
ERROR: ld.so: object 'LD_LIBRARY_PATH=.' from LD_PRELOAD cannot be preloaded: ignored.
argc 2147090419
stack 0x7ffbe115008f < 0x7ffbe1150188 < 0x7fffe0e50128 < 0x7ff7e11503ea < 0x7ffbe102cdea
getenv 0x7ffbe114d83b .
0x7ffbe114d82b LD_LIBRARY_PATH=.
0x7ffbe114df60 LD_LIBRARY_PATH=.
0x7ffbe114df72 LD_LIBRARY_PATH=.
...
0x7ffbe114e69e LD_LIBRARY_PATH=.
0x7ffbe114e6b0 LD_LIBRARY_PATH=.
0x7ffbe114e6c2 LD_LIBRARY_PATH=.

real    5m38.666s
user    0m0.049s
sys     1m57.828s


========================================================================
Acknowledgments
========================================================================

We thank Red Hat Product Security and the members of
linux-distros@...openwall.org and security@...nel.org.


========================================================================
Timeline
========================================================================

2018-08-31: Contacted secalert@...hat.com.

2018-09-18: Contacted linux-distros@...openwall.org and
security@...nel.org.

2018-09-25: Coordinated Release Date (Time: 5:00 PM UTC).


View attachment "poc-suidbin.c" of type "text/plain" (1458 bytes)

View attachment "poc-exploit.c" of type "text/plain" (7581 bytes)


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