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From: Daniel Borkmann <>
To: "Kees Cook" <>, "Mickaël Salaün" <>
Cc: linux-security-module <>,
	Andreas Gruenbacher <>,
	Andy Lutomirski <>,
	Andy Lutomirski <>, Arnd Bergmann <>,
	Casey Schaufler <>,
	David Drysdale <>,
	Eric Paris <>,
	James Morris <>,
	Jeff Dike <>, Julien Tinnes <>,
	Michael Kerrisk <>, Paul Moore <>,
	Richard Weinberger <>,
	"Serge E . Hallyn" <>,
	Stephen Smalley <>,
	Tetsuo Handa <>,
	Will Drewry <>,
	Linux API <>,
Subject: [kernel-hardening] Re: [RFC v1 00/17] seccomp-object: From attack surface reduction to sandboxing
Date: Sat, 21 May 2016 17:19:52 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Mickaël,

[ sorry for commenting so late ... ]

On 04/28/2016 04:36 AM, Kees Cook wrote:
> On Wed, Mar 23, 2016 at 6:46 PM, Mickaël Salaün <> wrote:
>> Hi,
>> This series is a proof of concept (not ready for production) to extend seccomp
>> with the ability to check argument pointers of syscalls as kernel object (e.g.
>> file path). This add a needed feature to create a full sandbox managed by
>> userland like the Seatbelt/XNU Sandbox or the OpenBSD Pledge. It was initially
>> inspired from a partial seccomp-LSM prototype [1] but has evolved a lot since :)
>> The audience for this RFC is limited to security-related actors to discuss
>> about this new feature before enlarging the scope to a wider audience. This
>> aims to focus on the security goal, usability and architecture before entering
>> into the gory details of each subsystem. I also wish to get constructive
>> criticisms about the userland API and intrusiveness of the code (and what could
>> be the other ways to do it better) before going further (and addressing the
>> TODO and FIXME in the code).
>> The approach taken is to add the minimum amount of code while still allowing
>> the userland to create access rules via seccomp. The current limitation of
>> seccomp is to get raw syscall arguments value but there is no way to
>> dereference a pointer to check its content (e.g. the first argument of the open
>> syscall). This seccomp evolution brings a generic way to check against argument
>> pointer regardless from the syscall unlike current LSMs.
> Okay, I've read through this whole series now (sorry for the huge
> delay). I think that it is overly complex for what it results in
> providing. Here are some background thoughts I had:
> 1) People have asked for "dereferenced argument inspection" (I will
> call this DAI...), in that they would like to be able to process
> arguments like how BPF traditionally processes packets. This series
> doesn't provide that. Rather, it provides static checks against
> specific arguments types (currently just path checks).
> 2) When I dig into the requirements people have around DAI, it's
> mostly about checking path names. There is some interest in some of
> the network structures, but mostly it's path names. This series
> certainly underscores this since your first example is path names. :)

Out of curiosity, did you have a look whether adding some very basic
eBPF support for seccomp-BPF could also enable you for the option of
inspecting arguments eventually?

With basic, I mean adding new eBPF program type BPF_PROG_TYPE_SECCOMP
and the only things allowed would be to use a very limited set of
helpers. No maps, etc allowed for this type. If needed for extracting
args, you could extend struct seccomp_data for eBPF use, and add new
set of helper functions that would allow you to extract/walk arguments,
and, for example, pass the extracted buffer back to the eBPF prog for
further inspection.

Have a look at samples in [1,2], which are for tracing though, but possibly
it could be designed in a more or less similar way, where clang compiles
this policy down into eBPF bytecode. Did you have a look at this direction
or any thoughts on it?


> 3) Solving ToCToU should also solve performance problems. For example,
> this series, on a successful syscall, will look up a pathname twice
> (once in seccomp, then again in the syscall, and then compares the
> results in the LSM as a ToCToU back-stop). This seems like a waste of
> effort, since this reimplements the work the kernel is already doing
> to pass the resulting structure to the LSM hooks. As such, since this
> series is doing static checks and not allowing byte processing for
> DAI, I'm convinced that it should entirely happen in the LSM hooks.
> 4) Performing the checks in the LSM hooks carries a risk of exposing
> the syscall's argument processing code to an attacker, but I think
> that is okay since very similar code would already need to be written
> to do the same thing before entering the syscall. The only way out of
> this, I think, would be to standardize syscall argument processing.
> 5) If we can standardize syscall argument processing, we could also
> change when it happens, and retain the results for the syscall,
> allowing for full byte processing style of DAI. e.g. copy userspace to
> kernel space, do BPF on the argument, if okay, pass the kernel copy to
> the syscall where it continues the processing. If the kernel copy
> wasn't already created by seccomp, the syscall would just make that
> copy itself, etc.
> So, I see DAI as going one of two ways:
> a) rewrite all syscall entry to use a common cacheable argument parser
> and offering true BPF processing of the argument bytes.
> b) use the existing LSM hooks and define a policy language that can be
> loaded ahead of time.
> Doing "a" has many problems, I think. Not the least of which is that I
> can't imagine a way for such an architectural change to not have
> negative performance impacts for the regular case.
> Doing "b" means writing a policy engine. I would expect it to look a
> lot like either AppArmor or TOMOYO. TOMOYO has network structure
> processing, so probably it would look more like TOMOYO if you wanted
> more than just file paths. Maybe a seccomp LSM could share logic from
> one of the existing path-based LSMs.
> Another note I had for this series was that because the checker tries
> to keep a cached struct path, it allows unprivileged users to check
> for path names existing or not, regardless of the user's permissions.
> Instead, you have to check the path against the policy each time.
> AppArmor does this efficiently with a pre-built deterministic finite
> automatons (built from regular expressions), and TOMOYO just does
> string compares and limited glob parsing every time.
> So, I can't take this as-is, but I'll take the one fix near the start.
> :) I hope this isn't too discouraging, since I'd love to see this
> solved. Hopefully you can keep chipping away at it!
> Thanks!
> -Kees

  parent reply	other threads:[~2016-05-21 15:19 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-24  1:46 [kernel-hardening] [RFC v1 00/17] seccomp-object: From attack surface reduction to sandboxing Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 01/17] um: Export the sys_call_table Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 02/17] seccomp: Fix typo Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 03/17] selftest/seccomp: Fix the flag name SECCOMP_FILTER_FLAG_TSYNC Mickaël Salaün
2016-03-24  4:35   ` [kernel-hardening] " Kees Cook
2016-03-29 15:35     ` Shuah Khan
2016-03-29 18:46       ` [kernel-hardening] [PATCH 1/2] " Mickaël Salaün
2016-03-29 19:06         ` [kernel-hardening] " Shuah Khan
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 04/17] selftest/seccomp: Fix the seccomp(2) signature Mickaël Salaün
2016-03-24  4:36   ` [kernel-hardening] " Kees Cook
2016-03-29 15:38     ` Shuah Khan
2016-03-29 18:51       ` [kernel-hardening] [PATCH 2/2] " Mickaël Salaün
2016-03-29 19:07         ` [kernel-hardening] " Shuah Khan
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 05/17] security/seccomp: Add LSM and create arrays of syscall metadata Mickaël Salaün
2016-03-24 15:47   ` [kernel-hardening] " Casey Schaufler
2016-03-24 16:01   ` Casey Schaufler
2016-03-24 21:31     ` Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 06/17] seccomp: Add the SECCOMP_ADD_CHECKER_GROUP command Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 07/17] seccomp: Add seccomp object checker evaluation Mickaël Salaün
2016-03-24  1:46 ` [kernel-hardening] [RFC v1 08/17] selftest/seccomp: Remove unknown_ret_is_kill_above_allow test Mickaël Salaün
2016-03-24  2:53 ` [kernel-hardening] [RFC v1 09/17] selftest/seccomp: Extend seccomp_data until matches[6] Mickaël Salaün
2016-03-24  2:53   ` [kernel-hardening] [RFC v1 10/17] selftest/seccomp: Add field_is_valid_syscall test Mickaël Salaün
2016-03-24  2:53   ` [kernel-hardening] [RFC v1 11/17] selftest/seccomp: Add argeval_open_whitelist test Mickaël Salaün
2016-03-24  2:53   ` [kernel-hardening] [RFC v1 12/17] audit,seccomp: Extend audit with seccomp state Mickaël Salaün
2016-03-24  2:53   ` [kernel-hardening] [RFC v1 13/17] selftest/seccomp: Rename TRACE_poke to TRACE_poke_sys_read Mickaël Salaün
2016-03-24  2:53   ` [kernel-hardening] [RFC v1 14/17] selftest/seccomp: Make tracer_poke() more generic Mickaël Salaün
2016-03-24  2:54   ` [kernel-hardening] [RFC v1 15/17] selftest/seccomp: Add argeval_toctou_argument test Mickaël Salaün
2016-03-24  2:54   ` [kernel-hardening] [RFC v1 16/17] security/seccomp: Protect against filesystem TOCTOU Mickaël Salaün
2016-03-24  2:54   ` [kernel-hardening] [RFC v1 17/17] selftest/seccomp: Add argeval_toctou_filesystem test Mickaël Salaün
2016-03-24 16:24 ` [kernel-hardening] Re: [RFC v1 00/17] seccomp-object: From attack surface reduction to sandboxing Kees Cook
2016-03-27  5:03   ` Loganaden Velvindron
2016-04-20 18:21 ` Mickaël Salaün
2016-04-26 22:46   ` Kees Cook
2016-04-28  2:36 ` Kees Cook
2016-04-28 23:45   ` Mickaël Salaün
2016-05-21 12:58     ` Mickaël Salaün
2016-05-02 22:19   ` James Morris
2016-05-21 15:19   ` Daniel Borkmann [this message]
2016-05-22 21:30     ` Mickaël Salaün

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