[PATCH v3 0/2] kernfs, sysfs: refuse splice on /sys/power/resume

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

This series refuses splice on writable opens of /sys/power/resume and
gives those opens separate lockdep classes, as Tejun suggested in the
v1 discussion.

The reported cycle is assembled from two operations. Splice into an
ordinary writable kernfs file records pipe->mutex before of->mutex. A
normal write to /sys/power/resume records of->mutex before the autofs
pipe locks when resume_store() resolves a relative path.

Patch 1 adds SYSFS_NO_SPLICE and kernfs_ops->no_splice. Writable opens
of a marked file refuse both splice directions. Their of->mutex and
seq_file lock use separate classes. Read-only opens retain splice
support.

Patch 2 marks /sys/power/resume. Normal reads and writes stay unchanged,
including a write which resolves a relative path through autofs. Rafael
acked it to go together with patch 1.

Other stores also look up paths. They remain outside this minimal series
and can be marked in follow-up patches.

Tested on v7.4-rc1 in a QEMU guest with the syzbot kernel config.
Without the series, the C and syz reproducers trigger the reported
lockdep cycle, and a splice from the autofs daemon pipe into
/sys/power/resume deadlocks. With the series, neither reproducer
triggers a lockdep report although autofs receives the same requests,
and that splice fails with EINVAL. Plain reads and writes of
/sys/power/resume, read-only splice and splice on other sysfs and
cgroup files work as before.

v3:
- Rename SYSFS_LOOKUP and ->may_lookup to SYSFS_NO_SPLICE and
  ->no_splice (Tejun).
- Drop the mmap, prealloc and binary attribute checks (Tejun).
- Add Rafael's Acked-by to patch 2.

v2:
- Replace the v1 locking changes with per-attribute splice control.
- Preserve splice on read-only opens.
- Split both the open-file mutex and seq_file lock classes.
- Reject incompatible mmap and preallocated-buffer combinations.
- Reject SYSFS_LOOKUP on binary attributes.
- Rebase onto v7.4-rc1.

v2: https://lore.kernel.org/r/cover.1793715774.git.krystianmkaniewski@gmail.com
v1: https://lore.kernel.org/r/cover.1792577179.git.krystianmkaniewski@gmail.com

Krystian Kaniewski (2):
  kernfs, sysfs: add SYSFS_NO_SPLICE to refuse splice on writable opens
  PM: hibernate: refuse splice on /sys/power/resume

 fs/kernfs/file.c         | 46 +++++++++++++++++++++++++++++++++++++---
 fs/sysfs/file.c          | 18 ++++++++++++++++
 fs/sysfs/group.c         |  4 ++--
 include/linux/kernfs.h   |  9 ++++++++
 include/linux/sysfs.h    | 22 +++++++++++++------
 kernel/power/hibernate.c |  3 ++-
 6 files changed, 90 insertions(+), 12 deletions(-)


base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.53.0