Re: [Qemu-devel] [PATCH] Add syscalls for -runas and -chroot tothe seccomp sandbox

["Namsun Ch'o" <namnamc@Safe-mail.net>]

> Our intention since the beginning was to protect the host from the
> illegal guest operations. But you do have an interesting point about
> flaws on qemu itself. Perhaps this might be something I could work on to
> improve (start a bigger whitelist and get it tighter before guest
> launches).

The seccomp filters are always passed on through execve(), so it would not be
possible to have the parent have chroot() whitelisted to chroot, then spawn a
child without it. As far as I know, even a root process cannot chroot another
process, even its child, so if the process is to chroot at all, it must have
the chroot syscall whitelisted. What can be done, however, is using the
argument passed to -chroot as the filter. The same could be done with setuid,
by having it only whitelist the uid which is given at -runas.

An example, using chdir (I presume QEMU uses chdir(dir) then chroot(".")):

  sh# mkdir /tmp/chroot
  sh# cat | gcc -lseccomp -x c -
  #include <stdio.h>
  #include <fcntl.h>
  #include <seccomp.h>

  void main(void)
  {
        const char *dir = "/tmp/chroot";

        scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_TRAP);

        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mkdir), 0);
        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchdir), 0);
        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open), 0);
        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(brk), 0);
        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chdir), 1,
                SCMP_A0(SCMP_CMP_EQ, dir));
        seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(chroot), 1,
                SCMP_A0(SCMP_CMP_EQ, "."));

        seccomp_load(ctx);

        chdir(dir);
        chroot(".");

        /* evil code starts here */
        const int fd = open(".", O_DIRECTORY);
        mkdir("foo");
        chroot("foo");
        fchdir(fd);
        chdir("..");
        chdir("..");
        chdir("..");
        chroot(".");
  }^D^D
  sh# strace -qq -e open,mkdir,chdir,chroot ./a.out 2>&1 | fold -s -w 80
  chdir("/tmp/chroot")                    = 0
  chroot(".")                             = 0
  open(".", O_RDONLY|O_DIRECTORY)         = 3
  mkdir("foo", 0200000)                   = 0
  --- SIGSYS {si_signo=SIGSYS, si_code=SYS_SECCOMP, si_call_addr=0x34400a7d397,
  si_syscall=161, si_arch=3221225534} ---
  +++ killed by SIGSYS +++
  Bad system call
  sh# grep 161 /usr/include/asm/unistd_64.h
  #define __NR_chroot 161

So there's really no need to disable chroot() or setuid(), just filter the
arguments based on command line input to make them impossible to abuse.