Re: [PATCH] kernel: Expose SYS_kcmp by default

[Michel Dänzer]

On 2021-02-08 2:34 p.m., Daniel Vetter wrote:
> On Mon, Feb 8, 2021 at 12:49 PM Michel Dänzer <michel@xxxxxxxxxxx> wrote:
> > On 2021-02-05 9:53 p.m., Daniel Vetter wrote:
> > > On Fri, Feb 5, 2021 at 7:37 PM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > > > > Cc: Will Drewry <wad@xxxxxxxxxxxx>
> > > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > > Cc: Dave Airlie <airlied@xxxxxxxxx>
> > > > > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > > > > Cc: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> > > > > ---
> > > > >    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)
>
> Yeah that one is fine, but I thought we've discussed (irc or
> something) more uses for de-duping dma-buf and stuff like that. But
> quick grep says that hasn't landed yet, so I got a bit confused (or
> just dreamt). Looking at this again I'm kinda surprised the drmfd
> de-duping blows up on normal linux distros, but I guess it can all
> happen.

One example: GEM handle name-spaces are per file description. If 
user-space incorrectly assumes two DRM fds are independent, when they 
actually reference the same file description, closing a GEM handle with 
one file descriptor will make it unusable with the other file descriptor 
as well.


> > > Ofc we can leave the default n, but the select if CONFIG_DRM is
> > > unfortunately needed I think.
> >
> > Per above, not sure this is really true.
>
> We seem to be going boom on linux distros now, maybe userspace got
> more creative in abusing stuff?

I don't know what you're referring to. I've only seen maybe two or three 
reports from people who didn't enable CHECKPOINT_RESTORE in their 
self-built kernels.


> The entire thing is small enough that imo we don't really have to care,
> e.g. we also unconditionally select dma-buf, despite that on most
> systems there's only 1 gpu, and you're never going to end up with a
> buffer sharing case that needs any of that code (aside from the
> "here's an fd" part).
>
> But I guess we can limit to just KCMP_FILE like you suggest in another
> reply. Just feels a bit like overkill.

Making KCMP_FILE gated by DRM makes as little sense to me as by 
CHECKPOINT_RESTORE.


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