[PATCH] kernel: Expose SYS_kcmp by default

[Daniel Vetter]

On Fri, Feb 5, 2021 at 7:37 PM Kees Cook <keescook at 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 at chris-wilson.co.uk>
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Andy Lutomirski <luto at amacapital.net>
> > Cc: Will Drewry <wad at chromium.org>
> > Cc: Andrew Morton <akpm at linux-foundation.org>
> > Cc: Dave Airlie <airlied at gmail.com>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > Cc: Lucas Stach <l.stach at 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. Was maybe not the brighest
ideas, but since enough distros had this enabled by defaults, it
wasn't really discovered, and now we're shipping this everywhere.

Ofc we can leave the default n, but the select if CONFIG_DRM is
unfortunately needed I think. For that part:

Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Also adding Dave Airlie for his take.
-Daniel

>
> -Kees
>
> > +     help
> > +       Enable the file descriptor comparison system call. It provides
> > +       user-space with the ability to compare two fd to see if they
> > +       point to the same file, and check other attributes.
> > +
> > +       If unsure, say Y.
> > +
> >  config RSEQ
> >       bool "Enable rseq() system call" if EXPERT
> >       default y
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index aa7368c7eabf..320f1f3941b7 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -51,7 +51,7 @@ obj-y += livepatch/
> >  obj-y += dma/
> >  obj-y += entry/
> >
> > -obj-$(CONFIG_CHECKPOINT_RESTORE) += kcmp.o
> > +obj-$(CONFIG_KCMP) += kcmp.o
> >  obj-$(CONFIG_FREEZER) += freezer.o
> >  obj-$(CONFIG_PROFILING) += profile.o
> >  obj-$(CONFIG_STACKTRACE) += stacktrace.o
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index 26c72f2b61b1..1b6c7d33c4ff 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -315,7 +315,7 @@ TEST(kcmp)
> >       ret = __filecmp(getpid(), getpid(), 1, 1);
> >       EXPECT_EQ(ret, 0);
> >       if (ret != 0 && errno == ENOSYS)
> > -             SKIP(return, "Kernel does not support kcmp() (missing CONFIG_CHECKPOINT_RESTORE?)");
> > +             SKIP(return, "Kernel does not support kcmp() (missing CONFIG_KCMP?)");
> >  }
> >
> >  TEST(mode_strict_support)
> > --
> > 2.20.1
> >
>
> --
> Kees Cook



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch