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
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