[PATCH v3 0/5] liveupdate: serialization safety and race fixes

[Pasha Tatashin]

This series addresses several issues related to the synchronization
between the reboot process and LUO session management.

Changes in v3:
- Refined the session mutation blocking to use a dedicated global
  rwsem (luo_session_serialize_rwsem) instead of pinning individual
  mutexes.
- Fixed a use-after-free race in luo_file_unpreserve_files() where
  a module could be released before its file handler ID was erased.
- Fixed a TOCTOU race in luo_session_retrieve() by extending the
  lock scope to overlap with session mutex acquisition.
- Removed an unused 'ser' field from struct luo_session.
- Dropped the KHO skip patch as it was not needed.

1. Skip LUO serialization for context-preserving kexec: A
preserve_context kexec returns to the current kernel, which is unrelated
to live update where state is passed to the next kernel. Skipping
serialization avoids unnecessary work and prevents sessions from being
left in a frozen state upon return.

2. Block session mutations during reboot: During the reboot() syscall,
user processes may still be running concurrently and attempting to
mutate sessions. To prevent this, we introduce luo_session_serialize_rwsem.
All mutation operations (create, retrieve, release, ioctl) hold the
read lock. The serialization process holds the write lock indefinitely
on success, effectively freezing the subsystem.

3. Fix use-after-free in luo_file_unpreserve_files(): Reorder module_put()
to ensure the file handler module remains pinned while its operations
are being accessed during cleanup.

4. Fix TOCTOU race in luo_session_retrieve(): Extend the rwsem lock
scope to prevent a session from being released between lookup and
mutex acquisition.

5. Remove unused ser field from struct luo_session: Clean up the
session structure by removing a field that was never utilized.

Tree: git.kernel.org/pub/scm/linux/kernel/git/tatashin/linux.git Branch:
luo-reboot-sync/v3

Pasha Tatashin (5):
  liveupdate: skip serialization for context-preserving kexec
  liveupdate: block session mutations during reboot
  liveupdate: fix u-a-f in luo_file_unpreserve_files() and
    luo_file_finish()
  liveupdate: fix TOCTOU race in luo_session_retrieve()
  liveupdate: Remove unused ser field from struct luo_session

 kernel/kexec_core.c              |  8 +++++---
 kernel/liveupdate/luo_file.c     |  5 +++--
 kernel/liveupdate/luo_internal.h |  2 --
 kernel/liveupdate/luo_session.c  | 27 ++++++++++++++++++++-------
 4 files changed, 28 insertions(+), 14 deletions(-)


base-commit: 7b0b68b2b95606e65594958686833e53423f58f2
-- 
2.53.0