Re: [PATCH v6 2/3] seccomp_filters: system call filtering using BPF

From: Will Drewry
Date: Fri Feb 03 2012 - 18:14:54 EST

On Thu, Feb 2, 2012 at 7:32 AM, Serge E. Hallyn
<serge.hallyn@xxxxxxxxxxxxx> wrote:
> Quoting Will Drewry (wad@xxxxxxxxxxxx):
>> [This patch depends on luto@xxxxxxx's no_new_privs patch:
>> ]
>> This patch adds support for seccomp mode 2.  This mode enables dynamic
>> enforcement of system call filtering policy in the kernel as specified
>> by a userland task.  The policy is expressed in terms of a Berkeley
>> Packet Filter program, as is used for userland-exposed socket filtering.
>> Instead of network data, the BPF program is evaluated over struct
>> seccomp_filter_data at the time of the system call.
>> A filter program may be installed by a userland task by calling
>>   prctl(PR_ATTACH_SECCOMP_FILTER, &fprog);
>> where fprog is of type struct sock_fprog.
>> If the first filter program allows subsequent prctl(2) calls, then
>> additional filter programs may be attached.  All attached programs
>> must be evaluated before a system call will be allowed to proceed.
>> To avoid CONFIG_COMPAT related landmines, once a filter program is
>> installed using specific is_compat_task() value, it is not allowed to
>> make system calls using the alternate entry point.
>> Filter programs will be inherited across fork/clone and execve, however
>> the installation of filters must be preceded by setting 'no_new_privs'
>> to ensure that unprivileged tasks cannot attach filters that affect
>> privileged tasks (e.g., setuid binary).  Tasks with CAP_SYS_ADMIN
>> in their namespace may install inheritable filters without setting
>> the no_new_privs bit.
>> There are a number of benefits to this approach. A few of which are
>> as follows:
>> - BPF has been exposed to userland for a long time.
>> - Userland already knows its ABI: system call numbers and desired
>>   arguments
>> - No time-of-check-time-of-use vulnerable data accesses are possible.
>> - system call arguments are loaded on demand only to minimize copying
>>   required for system call number-only policy decisions.
>> This patch includes its own BPF evaluator, but relies on the
>> net/core/filter.c BPF checking code.  It is possible to share
>> evaluators, but the performance sensitive nature of the network
>> filtering path makes it an iterative optimization which (I think :) can
>> be tackled separately via separate patchsets. (And at some point sharing
>> BPF JIT code!)
>>  v6: - fix memory leak on attach compat check failure
>>      - require no_new_privs || CAP_SYS_ADMIN prior to filter
>>        installation. (luto@xxxxxxx)
>>      - s/seccomp_struct_/seccomp_/ for macros/functions
>>        (amwang@xxxxxxxxxx)
>>      - cleaned up Kconfig (amwang@xxxxxxxxxx)
>>      - on block, note if the call was compat (so the # means something)
>>  v5: - uses syscall_get_arguments
>>        (indan@xxxxxx,oleg@xxxxxxxxxx, mcgrathr@xxxxxxxxxxxx)
>>      - uses union-based arg storage with hi/lo struct to
>>        handle endianness.  Compromises between the two alternate
>>        proposals to minimize extra arg shuffling and account for
>>        endianness assuming userspace uses offsetof().
>>        (mcgrathr@xxxxxxxxxxxx, indan@xxxxxx)
>>      - update Kconfig description
>>      - add include/seccomp_filter.h and add its installation
>>      - (naive) on-demand syscall argument loading
>>      - drop seccomp_t (eparis@xxxxxxxxxx)
>>  v4: - adjusted prctl to make room for PR_[SG]ET_NO_NEW_PRIVS
>>      - now uses current->no_new_privs
>>          (luto@xxxxxxx,torvalds@xxxxxxxxxxxxxxxxxxxx)
>>      - assign names to seccomp modes (rdunlap@xxxxxxxxxxxx)
>>      - fix style issues (rdunlap@xxxxxxxxxxxx)
>>      - reworded Kconfig entry (rdunlap@xxxxxxxxxxxx)
>>  v3: - macros to inline (oleg@xxxxxxxxxx)
>>      - init_task behavior fixed (oleg@xxxxxxxxxx)
>>      - drop creator entry and extra NULL check (oleg@xxxxxxxxxx)
>>      - alloc returns -EINVAL on bad sizing (serge.hallyn@xxxxxxxxxxxxx)
>>      - adds tentative use of "always_unprivileged" as per
>>        torvalds@xxxxxxxxxxxxxxxxxxxx and luto@xxxxxxx
>>  v2: - (patch 2 only)
>> Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
> Hi Will,
> as far as I can tell based on changelog I suspect you could have
> kept my Acked-by (from v3?).  However, I'll wait until your next
> submission (as I see there were a few change requests), and do a
> final complete new review of that.

Thanks, Serge! I just failed at the proper protocol and didn't mean
to not include your Acked-by. However, I am changing a fair amount
of the internals this time around, so I'll be happy to have another
full review.

> Thanks for continuing to push on this.

Definitely! I've been traveling this week, so it's been a bit slow
going, but I hope to have the next rev up early next week if not

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