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

[Thomas Huth <thuth@redhat.com>]

On 08.09.2017 13:15, Eduardo Otubo wrote:
> 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;

Right, the default set should be excluded by default of course, too! :-)

 Thomas