linux-kernel - Re: [PATCH v2 1/4] seccomp: Add sysctl to display available actions

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Message-ID: <CAGXu5jLDipe7tc7+Zf1_uEbvGYMFQYw_z_oB1BuACyT7iYEGCg@mail.gmail.com>
Date:   Tue, 7 Feb 2017 16:43:43 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Tyler Hicks <tyhicks@...onical.com>
Cc:     Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-audit@...hat.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] seccomp: Add sysctl to display available actions

On Tue, Feb 7, 2017 at 4:25 PM, Tyler Hicks <tyhicks@...onical.com> wrote:
> On 02/07/2017 06:03 PM, Kees Cook wrote:
>> On Thu, Feb 2, 2017 at 9:37 PM, Tyler Hicks <tyhicks@...onical.com> wrote:
>>> This patch creates a read-only sysctl containing an ordered list of
>>> seccomp actions that the kernel supports. The ordering, from left to
>>> right, is the lowest action value (kill) to the highest action value
>>> (allow). Currently, a read of the sysctl file would return "kill trap
>>> errno trace allow". The contents of this sysctl file can be useful for
>>> userspace code as well as the system administrator.
>>>
>>> The path to the sysctl is:
>>>
>>>   /proc/sys/kernel/seccomp/actions_avail
>>>
>>> libseccomp and other userspace code can easily determine which actions
>>> the current kernel supports. The set of actions supported by the current
>>> kernel may be different than the set of action macros found in kernel
>>> headers that were installed where the userspace code was built.
>>
>> This is certainly good: having a discoverable way to detect filter
>> capabilities. I do wonder if it'd still be easier to just expose the
>> max_log sysctl as a numeric value, since the SECCOMP_RET_* values are
>> all part of uapi, so we can't escape their values...
>
> I was very torn on whether to use a numeric or string representation
> here. The reason I decided on string representation is because I think
> these sysctls are mostly aimed for admins and numeric representations
> wouldn't be easy to use. I considered added a utility to libseccomp but,
> since the kernel code to do a string representation was so simple, I
> went with doing it in the kernel.

Yeah, I think I like it just because it gives a way to discover the
UAPI "level"... I will think more about this. For v3, let's keep the
string stuff.

> Another possibility is exposing the SECCOMP_RET_*_NAME macros as part of
> the uapi.

I like keeping the UAPI minimal. ;)

>>> +static int __init seccomp_sysctl_init(void)
>>> +{
>>> +       struct ctl_table_header *hdr;
>>> +
>>> +       hdr = register_sysctl_paths(seccomp_sysctl_path, seccomp_sysctl_table);
>>> +       kmemleak_not_leak(hdr);
>>
>> Will kmemleak complain about this if hdr is saved to a global (or not
>> saved at all)? Also, something should be reported in the failure
>> case...
>
> I have to admit to blindly following the example set by sysctl_init() in
> kernel/sysctl.c. I can test what kmemleak will/won't complain about and
> report back (tomorrow at the earliest).

Cool, no rush. I'm backlogged on reviews anyway. :)

-Kees

-- 
Kees Cook
Pixel Security