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Date:   Tue, 13 Feb 2018 12:35:46 -0800
From:   Kees Cook <>
To:     Tom Hromatka <>
Cc:     Network Development <>,
        Sargun Dhillon <>,
        Will Drewry <>,
        Daniel Borkmann <>,
        Linux Containers <>,
        Alexei Starovoitov <>,
        Andy Lutomirski <>
Subject: Re: [PATCH net-next 0/3] eBPF Seccomp filters

On Tue, Feb 13, 2018 at 12:33 PM, Tom Hromatka <> wrote:
> On Tue, Feb 13, 2018 at 7:42 AM, Sargun Dhillon <> wrote:
>> This patchset enables seccomp filters to be written in eBPF. Although,
>> this patchset doesn't introduce much of the functionality enabled by
>> eBPF, it lays the ground work for it.
>> It also introduces the capability to dump eBPF filters via the PTRACE
>> API in order to make it so that CHECKPOINT_RESTORE will be satisifed.
>> In the attached samples, there's an example of this. One can then use
>> BPF_OBJ_GET_INFO_BY_FD in order to get the actual code of the program,
>> and use that at reload time.
>> The primary reason for not adding maps support in this patchset is
>> to avoid introducing new complexities around PR_SET_NO_NEW_PRIVS.
>> If we have a map that the BPF program can read, it can potentially
>> "change" privileges after running. It seems like doing writes only
>> is safe, because it can be pure, and side effect free, and therefore
>> not negatively effect PR_SET_NO_NEW_PRIVS. Nonetheless, if we come
>> to an agreement, this can be in a follow-up patchset.
> Coincidentally I also sent an RFC for adding eBPF hash maps to the seccomp
> userspace mailing list just last week:
> The kernel changes I proposed are in this email:
> In that email thread, Kees requested that I try out a binary tree in cBPF
> and evaluate its performance.  I just got a rough prototype working, and
> while not as fast as an eBPF hash map, the cBPF binary tree was a
> significant
> improvement over the linear list of ifs that are currently generated.  Also,
> it only required changing a single function within the libseccomp libary
> itself.
> Here are the results I am currently seeing using an in-house customer's
> seccomp filter and a simplistic test program that runs getppid() thousands
> of times.
> Test Case                      minimum TSC ticks to make syscall
> ----------------------------------------------------------------
> seccomp disabled                                             620
> getppid() at the front of 306-syscall seccomp filter         722
> getppid() in middle of 306-syscall seccomp filter           1392
> getppid() at the end of the 306-syscall filter              2452
> seccomp using a 306-syscall-sized EBPF hash map              800
> cBPF filter using a binary tree                              922

I still think that's a crazy filter. :) It should be inverted to just
check the 26 syscalls and a final "greater than" test. I would expect
it to be faster still. :)


Kees Cook
Pixel Security

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