Re: [Qemu-devel] [PATCHv5 4/5] seccomp: add spawn argument to command line

[Eduardo Otubo <otubo@redhat.com>]

On Fri, Sep 08, 2017 at 11:50:12AM +0200, Thomas Huth wrote:
> On 08.09.2017 11:10, Eduardo Otubo wrote:
> > This patch adds [,spawn=deny] argument to `-sandbox on' option. It
> > blacklists fork and execve system calls, avoiding Qemu to spawn new
> > threads or processes.
> > 
> > Signed-off-by: Eduardo Otubo <otubo@redhat.com>
> > ---
> >  include/sysemu/seccomp.h |  1 +
> >  qemu-options.hx          |  9 +++++++--
> >  qemu-seccomp.c           | 12 ++++++++++++
> >  vl.c                     | 16 ++++++++++++++++
> >  4 files changed, 36 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/sysemu/seccomp.h b/include/sysemu/seccomp.h
> > index 4a9e63c7cd..3ab5fc4f61 100644
> > --- a/include/sysemu/seccomp.h
> > +++ b/include/sysemu/seccomp.h
> > @@ -18,6 +18,7 @@
> >  #define QEMU_SECCOMP_SET_DEFAULT     (1 << 0)
> >  #define QEMU_SECCOMP_SET_OBSOLETE    (1 << 1)
> >  #define QEMU_SECCOMP_SET_PRIVILEGED  (1 << 2)
> > +#define QEMU_SECCOMP_SET_SPAWN       (1 << 3)
> >  
> >  #include <seccomp.h>
> >  
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 5c1b163fb5..2b04b9f170 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -4018,6 +4018,7 @@ ETEXI
> >  
> >  DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
> >      "-sandbox on[,obsolete=allow|deny][,elevateprivileges=allow|deny|children]\n" \
> > +    "          [,spawn=allow|deny]\n" \
> >      "                Enable seccomp mode 2 system call filter (default 'off').\n" \
> >      "                use 'obsolete' to allow obsolete system calls that are provided\n" \
> >      "                    by the kernel, but typically no longer used by modern\n" \
> > @@ -4025,10 +4026,12 @@ DEF("sandbox", HAS_ARG, QEMU_OPTION_sandbox, \
> >      "                use 'elevateprivileges' to allow or deny QEMU process to elevate\n" \
> >      "                    its privileges by blacklisting all set*uid|gid system calls.\n" \
> >      "                    The value 'children' will deny set*uid|gid system calls for\n" \
> > -    "                    main QEMU process but will allow forks and execves to run unprivileged\n",
> > +    "                    main QEMU process but will allow forks and execves to run unprivileged\n" \
> > +    "                use 'spawn' to avoid QEMU to spawn new threads or processes by\n" \
> > +    "                     blacklisting *fork and execve\n",
> >      QEMU_ARCH_ALL)
> >  STEXI
> > -@item -sandbox @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}]
> > +@item -sandbox @var{arg}[,obsolete=@var{string}][,elevateprivileges=@var{string}][,spawn=@var{string}]
> >  @findex -sandbox
> >  Enable Seccomp mode 2 system call filter. 'on' will enable syscall filtering and 'off' will
> >  disable it.  The default is 'off'.
> > @@ -4037,6 +4040,8 @@ disable it.  The default is 'off'.
> >  Enable Obsolete system calls
> >  @item elevateprivileges=@var{string}
> >  Disable set*uid|gid system calls
> > +@item spawn=@var{string}
> > +Disable *fork and execve
> >  @end table
> >  ETEXI
> >  
> > diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> > index 2bad16cafb..4c169febf8 100644
> > --- a/qemu-seccomp.c
> > +++ b/qemu-seccomp.c
> > @@ -79,6 +79,10 @@ static const struct QemuSeccompSyscall blacklist[] = {
> >      { SCMP_SYS(setresgid),              4, QEMU_SECCOMP_SET_PRIVILEGED },
> >      { SCMP_SYS(setfsuid),               4, QEMU_SECCOMP_SET_PRIVILEGED },
> >      { SCMP_SYS(setfsgid),               4, QEMU_SECCOMP_SET_PRIVILEGED },
> > +    /* spawn */
> > +    { SCMP_SYS(fork),                   8, QEMU_SECCOMP_SET_SPAWN },
> > +    { SCMP_SYS(vfork),                  8, QEMU_SECCOMP_SET_SPAWN },
> > +    { SCMP_SYS(execve),                 8, QEMU_SECCOMP_SET_SPAWN },
> >  };
> >  
> >  
> > @@ -109,6 +113,14 @@ int seccomp_start(uint32_t seccomp_opts)
> >              }
> >  
> >              break;
> > +        case QEMU_SECCOMP_SET_SPAWN:
> > +            if (seccomp_opts & QEMU_SECCOMP_SET_SPAWN) {
> > +                break;
> > +            } else {
> > +                continue;
> > +            }
> > +
> 
> Remove the above empty line?
> 
> Anyway, it's somewhat ugly that you need a switch-case statement here at
> all. Couldn't you simply check it like this:
> 
> 	if (!(seccomp_opts & blacklist[i].set)) {
> 		continue;
> 	}
> ?
> 
> You then just have to invert the meaning of the
> QEMU_SECCOMP_SET_OBSOLETE bit in the second patch, so that this bit is
> treated in the same way as the others (i.e. use
>  uint32_t seccomp_opts = QEMU_SECCOMP_SET_OBSOLETE;
> instead of
>  uint32_t seccomp_opts = 0x00000;
> in vl.c in the second patch).

That's indeed much better, but perhaps:
 uint32_t seccomp_opts = QEMU_SECCOMP_SET_DEFAULT | QEMU_SECCOMP_SET_OBSOLETE;
?

-- 
Eduardo Otubo
Senior Software Engineer @ RedHat