glibc syslog() Heap-Based Buffer Overflow

Qualys Security AdvisoryCVE-2023-6246: Heap-based buffer overflow in the glibc's syslog()========================================================================Contents========================================================================SummaryAnalysisProof of conceptExploitationAcknowledgmentsTimeline========================================================================Summary========================================================================We discovered a heap-based buffer overflow in the GNU C Library's__vsyslog_internal() function, which is called by both syslog() andvsyslog(). This vulnerability was introduced in glibc 2.37 (in August2022) by the following commit:  https://sourceware.org/git?p=glibc.git;a=commit;h=52a5be0df411ef3ff45c10c7c308cb92993d15b1and was also backported to glibc 2.36 because this commit was a fix foranother, minor vulnerability in __vsyslog_internal() (CVE-2022-39046, an"uninitialized memory [read] from the heap"):  https://sourceware.org/bugzilla/show_bug.cgi?id=29536For example, we confirmed that Debian 12 and 13, Ubuntu 23.04 and 23.10,and Fedora 37 to 39 are vulnerable to this buffer overflow. Furthermore,we successfully exploited an up-to-date, default installation of Fedora38 (on amd64): a Local Privilege Escalation, from any unprivileged userto full root. Other distributions are probably also exploitable.To the best of our knowledge, this vulnerability cannot be triggeredremotely in any likely scenario (because it requires an argv[0], or anopenlog() ident argument, longer than 1024 bytes to be triggered).Last-minute note: in December 1997 Solar Designer published informationabout a very similar vulnerability in the vsyslog() of the old Linuxlibc (https://insecure.org/sploits/linux.libc.5.4.38.vsyslog.html).========================================================================Analysis========================================================================In the glibc, both syslog() and vsyslog() call the vulnerable function__vsyslog_internal():------------------------------------------------------------------------122 __vsyslog_internal (int pri, const char *fmt, va_list ap,123                     unsigned int mode_flags)124 {125   /* Try to use a static buffer as an optimization.  */126   char bufs[1024];127   char *buf = NULL;128   size_t bufsize = 0;...171 #define SYSLOG_HEADER(__pri, __timestamp, __msgoff, pid) \172   "<%d>%s%n%s%s%.0d%s: ",                                \173   __pri, __timestamp, __msgoff,                          \174   LogTag == NULL ? __progname : LogTag,                  \175   "[" + (pid == 0), pid, "]" + (pid == 0)...182     l = __snprintf (bufs, sizeof bufs,183                     SYSLOG_HEADER (pri, timestamp, &msgoff, pid));...187   if (0 <= l && l < sizeof bufs)188     {...202     }203 204   if (buf == NULL)205     {206       buf = malloc ((bufsize + 1) * sizeof (char));...213             __snprintf (buf, l + 1,214                         SYSLOG_HEADER (pri, timestamp, &msgoff, pid));...221           __vsnprintf_internal (buf + l, bufsize - l + 1, fmt, apc,222                                 mode_flags);------------------------------------------------------------------------- at lines 182-183, SYSLOG_HEADER() includes __progname (the basename()  of argv[0]) if LogTag is NULL (e.g., if openlog() was not called, or  called with a NULL ident argument);- because a local attacker fully controls argv[0] and hence __progname  (even when executing a SUID-root program such as su), at line 187 l  (the return value of __snprintf()) can be larger than sizeof bufs  (1024), in which case the code block at lines 188-202 is skipped;- consequently, at line 203 buf is still NULL and bufsize is still 0,  and at line 206 a very small 1-byte buf is malloc()ated (because  bufsize is 0);- at lines 213-214 this small buf is overflowed with the attacker-  controlled __progname (because l is larger than 1024), and at lines  221-222 this small buf is further overflowed (because bufsize - l + 1  is 0 - l + 1, a very large size_t).========================================================================Proof of concept========================================================================$ (exec -a "`printf '%0128000x' 1`" /usr/bin/su < /dev/null)Password: Segmentation fault (core dumped)========================================================================Exploitation========================================================================We decided to exploit this vulnerability through su (the most commonSUID-root program) on Fedora 38. To authenticate a user, su calls thePAM library, and if the password provided by the user is incorrect, thenPAM calls the glibc's syslog() function without calling openlog() first,thus allowing us to trigger the buffer overflow in __vsyslog_internal():------------------------------------------------------------------------782                         pam_syslog(pamh, LOG_NOTICE,783                                  "authentication failure; "784                                  "logname=%s uid=%d euid=%d "785                                  "tty=%s ruser=%s rhost=%s "786                                  "%s%s",787                                  new->name, new->uid, new->euid,788                                  tty ? (const char *)tty : "",789                                  ruser ? (const char *)ruser : "",790                                  rhost ? (const char *)rhost : "",791                                  (new->user && new->user[0] != '\0')792                                   ? " user=" : "",793                                  new->user794                         );------------------------------------------------------------------------107 pam_syslog (const pam_handle_t *pamh, int priority,108             const char *fmt, ...)109 {...113   pam_vsyslog (pamh, priority, fmt, args);------------------------------------------------------------------------ 73 pam_vsyslog (const pam_handle_t *pamh, int priority, 74              const char *fmt, va_list args) 75 { .. 81       if (asprintf (&msgbuf1, "%s(%s:%s):", pamh->mod_name, 82                     pamh->service_name?pamh->service_name:"<unknown>", 83                     _pam_choice2str (pamh->choice)) < 0) .. 91   if (vasprintf (&msgbuf2, fmt, args) < 0) .. 99   syslog (LOG_AUTHPRIV|priority, "%s %s",100           (msgbuf1 ? msgbuf1 : _PAM_SYSTEM_LOG_PREFIX), msgbuf2);------------------------------------------------------------------------But what should we overwrite in the heap to successfully exploit thisbuffer overflow? Initially, because su calls setlocale(LC_ALL, ""); atthe very beginning of its su_main() function, we tried to reuse the keyidea from our Baron Samedit exploits (CVE-2021-3156 in Sudo): we wrote arudimentary fuzzer to execute su with a random argv[0] and random localeenvironment variables and automatically inspect the resulting crashes ingdb. Unfortunately this fuzzer failed to produce interesting results: weonly obtained a handful of unique crashes, and they did not look verypromising.However, we did not investigate the reasons for this failure, becausewhile browsing through su's source code we noticed that su_main() callsenv_whitelist_from_string() to parse the argument of the -w command-lineoption:------------------------------------------------------------------------1118                 case 'w':1119                         env_whitelist_from_string(su, optarg);1120                         break;------------------------------------------------------------------------ 692 static int env_whitelist_from_string(struct su_context *su, const char *str) 693 { 694         char **all = strv_split(str, ","); ... 703         STRV_FOREACH(one, all) 704                 env_whitelist_add(su, *one); 705         strv_free(all); 706         return 0; 707 }------------------------------------------------------------------------ 662 static int env_whitelist_add(struct su_context *su, const char *name) 663 { 664         const char *env = getenv(name); 665  666         if (!env) 667                 return 1; 668         if (strv_extend(&su->env_whitelist_names, name)) 669                 err_oom(); 670         if (strv_extend(&su->env_whitelist_vals, env)) 671                 err_oom(); 672         return 0; 673 }------------------------------------------------------------------------Conveniently, env_whitelist_from_string() allows us (attackers) tomalloc()ate and free() an arbitrary number of arbitrary strings at thevery beginning of su's execution: an almost perfect heap feng shui. Wetherefore rewrote our fuzzer to execute su with a random argv[0] and arandom whitelist option (instead of random locale environment variables)and immediately observed numerous unique crashes; among these, three inparticular caught our attention.========================================================================1/ Corruption of PAM structures========================================================================Surprisingly, our fuzzer directly overwrote two PAM function pointers(in struct pam_data and struct handler):------------------------------------------------------------------------Thread 2.1 "su" received signal SIGSEGV, Segmentation fault.0x00007fa7d3b0e3ac in _pam_free_data (status=7, pamh=0x56211242ec10) at /usr/src/debug/pam-1.5.2-16.fc38.x86_64/libpam/pam_data.c:161161                 last->cleanup(pamh, last->data, status);...=> 0x7fa7d3b0e3ac <pam_end+92>: call   *%raxrax            0x4141414141414141  4702111234474983745------------------------------------------------------------------------Thread 2.1 "su" received signal SIGSEGV, Segmentation fault.0x00007f928b5e5781 in _pam_dispatch_aux (use_cached_chain=<optimized out>, resumed=<optimized out>, h=0x55f2e374aae0, flags=0, pamh=0x55f2e374aae0) at /usr/src/debug/pam-1.5.2-16.fc38.x86_64/libpam/pam_dispatch.c:110110                 retval = h->func(pamh, flags, h->argc, h->argv);...=> 0x7f928b5e5781 <_pam_dispatch+465>:  call   *%raxrax            0x4545454545454545  4991471925827290437------------------------------------------------------------------------Although this sounds exciting at first (a call to 0x4141414141414141!)we decided to not pursue this avenue of exploitation:- we cannot overwrite such a function pointer with null bytes (because  we overflow __vsyslog_internal()'s buffer with a null-terminated  string), but userland addresses contain at least two null bytes;- we could try to partially overwrite such a function pointer, but we do  not control the end of the string that overflows __vsyslog_internal()'s  buffer (the end of the aforementioned pam_syslog() format string), and  such an uncontrolled, partially overwritten function pointer is very  unlikely to miraculously point to a useful ROP gadget.========================================================================2/ Corruption of heap metadata========================================================================Unsurprisingly, our fuzzer also overwrote various pieces of heapmetadata (chunk headers managed internally by the glibc's malloc), andtherefore triggered all kinds of assertion failures and security checks:------------------------------------------------------------------------$ grep -A1 __libc_message fuzzer.out | cut -d'"' -f2 | sort -u...chunk_main_arena (bck->bk)chunk_main_arena (fwd)corrupted double-linked listcorrupted double-linked list (not small)corrupted size vs. prev_sizecorrupted size vs. prev_size in fastbinsdouble free or corruption (out)free(): corrupted unsorted chunksfree(): invalid next size (fast)free(): invalid pointerfree(): invalid sizemalloc_consolidate(): invalid chunk sizemalloc(): corrupted top sizemalloc(): invalid size (unsorted)malloc(): smallbin double linked list corruptedmalloc(): unaligned tcache chunk detectedmalloc(): unsorted double linked list corruptedmunmap_chunk(): invalid pointer------------------------------------------------------------------------Although some of these corruptions might be exploitable, we decided tonot pursue this avenue of exploitation either:- we cannot overwrite a chunk header with a size field and an fd or bk  pointer that are both valid (they must both contain null bytes to be  valid), which severely limits our exploitation options;- in any case, we would probably need a specific heap, mmap, or stack  address to exploit such a corruption, but we do not have the luxury of  an information leak, and all these addresses are too heavily  randomized by ASLR to be brute forced.========================================================================3/ Corruption of nss structures========================================================================Our fuzzer also produced two crashes that immediately caught ourattention because they are directly related to one of the techniquesthat we used to exploit Baron Samedit:------------------------------------------------------------------------Thread 2.1 "su" received signal SIGSEGV, Segmentation fault.__GI___nss_lookup (ni=ni@entry=0x7ffe876a05e8, fct_name=fct_name@entry=0x7fbba214e4e7 "getpwnam_r", fct2_name=fct2_name@entry=0x0, fctp=fctp@entry=0x7ffe876a05f0) at nsswitch.c:6767        *fctp = __nss_lookup_function (*ni, fct_name);...=> 0x7fbba20ec50e <__GI___nss_lookup+30>:       mov    (%rax),%rdirax            0x4141414141414141  4702111234474983745------------------------------------------------------------------------Thread 2.1 "su" received signal SIGSEGV, Segmentation fault.__nss_module_get_function (module=0x4141414141414141, name=name@entry=0x7f0aed9034e7 "getpwnam_r") at nss_module.c:328328       if (!__nss_module_load (module))...=> 0x7f0aed8a34b7 <__nss_module_get_function+39>:       mov    (%rdi),%eaxrdi            0x4141414141414141  4702111234474983745------------------------------------------------------------------------As discussed in the "2/ struct service_user overwrite" subsection of ourBaron Samedit advisory, if we overwrite the name[] field of a heap-basedstruct nss_module with a string of characters that contains a slash (forexample "A/B/C"), then at lines 180-181 the name of a shared library isconstructed ("libnss_A/B/C.so.2"), and at line 187 this shared libraryis loaded from our current working directory (because its name containsa slash, but does not start with a slash) and executed as root (becausesu is a SUID-root program):------------------------------------------------------------------------170 module_load (struct nss_module *module)171 {...180     if (__asprintf (&shlib_name, "libnss_%s.so%s",181                     module->name, __nss_shlib_revision) < 0)...187     handle = __libc_dlopen (shlib_name);------------------------------------------------------------------------Unfortunately, the __progname part (which we control) of the string thatoverflows __vsyslog_internal()'s buffer cannot contain a slash (because__progname is the basename() of argv[0]). Luckily, however, the part ofthe overflowing string that we do not control (the pam_syslog() formatstring) includes the absolute path of our tty, which contains a slash.For example, if:- our tty is /dev/pts/23 (we use forkpty() in our exploit);- our unprivileged local user is nobody (uid 65534);- the argv[0] (and hence __progname) that we use to execute su is a long  string of 'A' characters (longer than 1024);then we can overwrite the name[] field of a heap-based struct nss_modulewith a string of the form:  "AAAAAAAAAA: pam_unix(su:auth): authentication failure; logname= uid=65534 euid=0 tty=/dev/pts/23 ruser=nobody rhost=  user=root"Consequently, if we first create the following three directories (in ourcurrent working directory):  "libnss_AAAAAAAAAA: pam_unix(su:auth): authentication failure; logname= uid=65534 euid=0 tty="  "libnss_AAAAAAAAAA: pam_unix(su:auth): authentication failure; logname= uid=65534 euid=0 tty=/dev"  "libnss_AAAAAAAAAA: pam_unix(su:auth): authentication failure; logname= uid=65534 euid=0 tty=/dev/pts"and also create the following shared library (in our current workingdirectory):  "libnss_AAAAAAAAAA: pam_unix(su:auth): authentication failure; logname= uid=65534 euid=0 tty=/dev/pts/23 ruser=nobody rhost=  user=root.so.2"then this shared library will eventually be loaded and executed withfull root privileges. In our tests, it takes a few 10,000s of tries tosuccessfully brute force the exploit parameters (the length of argv[0],and the whitelist option and its associated environment variables).Note: this exploit could certainly be made much more efficient; intheory, it could even be a one-shot exploit, because we do not need tobrute force the ASLR, only the heap layout.========================================================================Acknowledgments========================================================================We thank the glibc developers (Carlos O'Donell, Siddhesh Poyarekar,Arjun Shankar, Florian Weimer, and Adhemerval Zanella in particular),Red Hat Product Security (Guilherme Suckevicz in particular), and themembers of linux-distros@openwall (Salvatore Bonaccorso in particular).========================================================================Timeline========================================================================2023-11-07: We sent a preliminary draft of our advisory to Red HatProduct Security.2023-11-15: Red Hat Product Security acknowledged receipt of our email.2023-11-16: Red Hat Product Security asked us if we could share ourexploit with them.2023-11-17: We sent our exploit to Red Hat Product Security.2023-11-21: Red Hat Product Security confirmed that our exploit worked,and assigned CVE-2023-6246 to this heap-based buffer overflow in__vsyslog_internal().2023-12-05: Red Hat Product Security sent us a patch for CVE-2023-6246(written by the glibc developers), and asked us for our feedback.2023-12-07: While reviewing this patch, we discovered two more minorvulnerabilities in the same function (an off-by-one buffer overflow andan integer overflow). We immediately sent an analysis, proof of concept,and patch proposal to Red Hat Product Security, and suggested that wedirectly involve the glibc security team.2023-12-08: Red Hat Product Security acknowledged receipt of our email,and agreed that we should directly involve the glibc security team. Wecontacted them on the same day, and they immediately replied with veryconstructive comments.2023-12-11: The glibc security team suggested that we postpone thecoordinated disclosure of all three vulnerabilities until January 2024(because of the upcoming holiday season). We agreed.2023-12-13: Red Hat Product Security assigned CVE-2023-6779 to theoff-by-one buffer overflow and CVE-2023-6780 to the integer overflow in__vsyslog_internal().2024-01-04: We suggested either January 23 or January 30 for theCoordinated Release Date of these vulnerabilities. The glibc developersagreed on January 30.2024-01-12: The glibc developers sent us an updated version of thepatches for these vulnerabilities.2024-01-13: We reviewed these patches, and sent our feedback to theglibc developers.2024-01-15: The glibc developers sent us the final version of thepatches for these vulnerabilities.2024-01-16: We sent these patches and a draft of our advisory to thelinux-distros@openwall. They immediately acknowledged receipt of ouremail.2024-01-30: Coordinated Release Date (18:00 UTC).