Re: [PATCH v4 0/4] lib: introduce copy_struct_from_user() helper

On Tue, Oct 01, 2019 at 11:10:51AM +1000, Aleksa Sarai wrote:

Patch changelog:
 v4:
  * __always_inline copy_struct_from_user(). [Kees Cook]
  * Rework test_user_copy.ko changes. [Kees Cook]
 v3: <https://lore.kernel.org/lkml/20190930182810.6090-1-cyphar@cyphar.com/ (local)>
     <https://lore.kernel.org/lkml/20190930191526.19544-1-asarai@suse.de/ (local)>
 v2: <https://lore.kernel.org/lkml/20190925230332.18690-1-cyphar@cyphar.com/ (local)>
 v1: <https://lore.kernel.org/lkml/20190925165915.8135-1-cyphar@cyphar.com/ (local)>

This series was split off from the openat2(2) syscall discussion[1].
However, the copy_struct_to_user() helper has been dropped, because
after some discussion it appears that there is no really obvious
semantics for how copy_struct_to_user() should work on mixed-vintages
(for instance, whether [2] is the correct semantics for all syscalls).

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[3]). This series
implements the helper and ports several syscalls to use it.

Some in-kernel selftests are included in this patch. More complete
self-tests for copy_struct_from_user() are included in the openat2()
patchset.

[1]: https://lore.kernel.org/lkml/20190904201933.10736-1-cyphar@cyphar.com/ (local)

[2]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

[3]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Aleksa Sarai (4):
  lib: introduce copy_struct_from_user() helper
  clone3: switch to copy_struct_from_user()
  sched_setattr: switch to copy_struct_from_user()
  perf_event_open: switch to copy_struct_from_user()

 include/linux/bitops.h     |   7 ++
 include/linux/uaccess.h    |  70 +++++++++++++++++++
 include/uapi/linux/sched.h |   2 +
 kernel/events/core.c       |  47 +++----------
 kernel/fork.c              |  34 ++--------
 kernel/sched/core.c        |  43 ++----------
 lib/strnlen_user.c         |   8 +--
 lib/test_user_copy.c       | 136 +++++++++++++++++++++++++++++++++++--
 lib/usercopy.c             |  55 +++++++++++++++
 9 files changed, 288 insertions(+), 114 deletions(-)

I've picked this up now and it's sitting in
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user

It should show up in linux-next tomorrow. I will let this sit there for
a few days but overall this seems good to have in rc2.
If someone objects and prefers to take it through their tree I can drop
it.

Christian