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

[Mike Rapoport <rppt@kernel.org>]

On Fri, May 15, 2026 at 12:37:17AM +0000, Pasha Tatashin wrote:
> 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.

Sashiko is still unhappy:
https://sashiko.dev/#/patchset/20260515003722.938123-1-pasha.tatashin@soleen.com

Didn't verify it's actually right, but its complaints seem legit.

Among other things sashiko noted a TOCTOU issue and then found it's fixed
by a later patch, maybe move the TOCTOU fix earlier in the series?
 
> 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
> 

-- 
Sincerely yours,
Mike.