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

[Thomas Huth <thuth@redhat.com>]

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).

 Thomas