otubo/qemu — pull request #2 by otubo (2012-07-17 17:37:01+00:00)
Title: Final v4 review before sending to qemu-devel

Hello all,

This patch is an effort to sandbox Qemu guests using Libseccomp[0]. The patches that follows are pretty simple and straightforward. I added the correct options and checks to the configure script and the basic calls to libseccomp in the main loop at vl.c. Details of each one are in the emails of the patch set.

This support limits the system call footprint of the entire QEMU process to a limited set of syscalls, those that we know QEMU uses. The idea is to limit the allowable syscalls, therefore limiting the impact that an attacked guest could have on the host system.

It's important to note that the libseccomp itself needs the seccomp mode 2 feature in the kernel, which is only available in kernel versions older (or equal) than 3.5-rc1.

v2: The now is separated in qemu-seccomp.c and qemu-seccomp.h for a cleaner implementation. The development was tested with the 3.5-rc1 kernel.

v3: As we discussed in previous emails in this mailing list, this feature is not supposed to replace existing security feature, but add another layer to the whole. The whitelist should contain all the syscalls QEMU needs. And as stated by Will Drewry's commit message[1]: "Filter programs will be inherited across fork/clone and execve.", the same white list should be passed along from the father process to the child, then execve() shouldn't be a problem. Note that there's a feature PR_SET_NO_NEW_PRIVS in seccomp mode 2 in the kernel, this prevents processes from gaining privileges on execve. For example, this will prevent qemu (if running unprivileged) from executing setuid programs[2].

v4: Introducing "debug" mode on libseccomp support. The "debug" mode will set the flag SCMP_ACT_TRAP when calling seccomp_start(). It will verbosely print a message to the stderr in the form "seccomp: illegal system call execution trapped: XXX" and resume the execution. This is really just used as debug mode, it helps users and developers to full fill the whitelist.

As always, comments are more than welcome.

Regards,

[0] - http://sourceforge.net/projects/libseccomp/
[1] - http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git;a=commit;h=e2cfabdfd075648216f99c2c03821cf3f47c1727
[2] - https://lkml.org/lkml/2012/4/12/457