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From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
To:     Daniel Vetter <daniel@ffwll.ch>, Kees Cook <keescook@chromium.org>,
        "airlied@gmail.com" <airlied@gmail.com>
Cc:     Will Drewry <wad@chromium.org>, Jann Horn <jannh@google.com>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chris Wilson <chris@chris-wilson.co.uk>
References: <20210205163752.11932-1-chris@chris-wilson.co.uk>
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Subject: Re: [PATCH] kernel: Expose SYS_kcmp by default
Message-ID: <9e5de17c-9912-18b3-1e2e-8d6672818504@daenzer.net>
Date:   Mon, 8 Feb 2021 14:11:47 +0100
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On 2021-02-08 12:49 p.m., Michel Dänzer wrote:
> On 2021-02-05 9:53 p.m., Daniel Vetter wrote:
>> On Fri, Feb 5, 2021 at 7:37 PM Kees Cook <keescook@chromium.org> wrote:
>>>
>>> On Fri, Feb 05, 2021 at 04:37:52PM +0000, Chris Wilson wrote:
>>>> Userspace has discovered the functionality offered by SYS_kcmp and has
>>>> started to depend upon it. In particular, Mesa uses SYS_kcmp for
>>>> os_same_file_description() in order to identify when two fd (e.g. 
>>>> device
>>>> or dmabuf) point to the same struct file. Since they depend on it for
>>>> core functionality, lift SYS_kcmp out of the non-default
>>>> CONFIG_CHECKPOINT_RESTORE into the selectable syscall category.
>>>>
>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>> Cc: Kees Cook <keescook@chromium.org>
>>>> Cc: Andy Lutomirski <luto@amacapital.net>
>>>> Cc: Will Drewry <wad@chromium.org>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Dave Airlie <airlied@gmail.com>
>>>> Cc: Daniel Vetter <daniel@ffwll.ch>
>>>> Cc: Lucas Stach <l.stach@pengutronix.de>
>>>> ---
>>>>   init/Kconfig                                  | 11 +++++++++++
>>>>   kernel/Makefile                               |  2 +-
>>>>   tools/testing/selftests/seccomp/seccomp_bpf.c |  2 +-
>>>>   3 files changed, 13 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/init/Kconfig b/init/Kconfig
>>>> index b77c60f8b963..f62fca13ac5b 100644
>>>> --- a/init/Kconfig
>>>> +++ b/init/Kconfig
>>>> @@ -1194,6 +1194,7 @@ endif # NAMESPACES
>>>>   config CHECKPOINT_RESTORE
>>>>        bool "Checkpoint/restore support"
>>>>        select PROC_CHILDREN
>>>> +     select KCMP
>>>>        default n
>>>>        help
>>>>          Enables additional kernel features in a sake of 
>>>> checkpoint/restore.
>>>> @@ -1737,6 +1738,16 @@ config ARCH_HAS_MEMBARRIER_CALLBACKS
>>>>   config ARCH_HAS_MEMBARRIER_SYNC_CORE
>>>>        bool
>>>>
>>>> +config KCMP
>>>> +     bool "Enable kcmp() system call" if EXPERT
>>>> +     default y
>>>
>>> I would expect this to be not default-y, especially if
>>> CHECKPOINT_RESTORE does a "select" on it.
>>>
>>> This is a really powerful syscall, but it is bounded by ptrace access
>>> controls, and uses pointer address obfuscation, so it may be okay to
>>> expose this. As it is, at least Ubuntu already has
>>> CONFIG_CHECKPOINT_RESTORE, so really, there's probably not much
>>> difference on exposure.
>>>
>>> So, if you drop the "default y", I'm fine with this.
>>
>> It was maybe stupid, but our userspace started relying on fd
>> comaprison through sys_kcomp. So for better or worse, if you want to
>> run the mesa3d gl/vk stacks, you need this.
> 
> That's overstating things somewhat. The vast majority of applications 
> will work fine regardless (as they did before Mesa started using this 
> functionality). Only some special ones will run into issues, because the 
> user-space drivers incorrectly assume two file descriptors reference 
> different descriptions.
> 
> 
>> Was maybe not the brighest ideas, but since enough distros had this
>> enabled by defaults,
> 
> Right, that (and the above) is why I considered it fair game to use. 
> What should I have done instead? (TBH I was surprised that this 
> functionality isn't generally available)

In that spirit, an alternative might be to make KCMP_FILE available 
unconditionally, and the rest of SYS_kcmp only with CHECKPOINT_RESTORE 
as before. (Or maybe other parts of SYS_kcmp are generally useful as well?)


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer