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Re: [Qemu-devel] [PATCHv5 4/5] seccomp: add spawn argument to command li
From: |
Thomas Huth |
Subject: |
Re: [Qemu-devel] [PATCHv5 4/5] seccomp: add spawn argument to command line |
Date: |
Fri, 8 Sep 2017 13:31:22 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
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 <address@hidden>
>>> ---
>>> 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
>>> address@hidden -sandbox @var{arg}[,address@hidden,address@hidden
>>> address@hidden -sandbox
>>> @var{arg}[,address@hidden,address@hidden,address@hidden
>>> @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 address@hidden
>>> Disable set*uid|gid system calls
>>> address@hidden address@hidden
>>> +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
- Re: [Qemu-devel] [PATCHv5 1/5] seccomp: changing from whitelist to blacklist, (continued)
[Qemu-devel] [PATCHv5 2/5] seccomp: add obsolete argument to command line, Eduardo Otubo, 2017/09/08
[Qemu-devel] [PATCHv5 4/5] seccomp: add spawn argument to command line, Eduardo Otubo, 2017/09/08
[Qemu-devel] [PATCHv5 3/5] seccomp: add elevateprivileges argument to command line, Eduardo Otubo, 2017/09/08
[Qemu-devel] [PATCHv5 5/5] seccomp: add resourcecontrol argument to command line, Eduardo Otubo, 2017/09/08