[PATCH 0/9] fs: harden anon inodes

[Christian Brauner <brauner@kernel.org>]

* Anonymous inodes currently don't come with a proper mode causing
  issues in the kernel when we want to add useful VFS debug assert. Fix
  that by giving them a proper mode and masking it off when we report it
  to userspace which relies on them not having any mode.

* Anonymous inodes currently allow to change inode attributes because
  the VFS falls back to simple_setattr() if i_op->setattr isn't
  implemented. This means the ownership and mode for every single user
  of anon_inode_inode can be changed. Block that as it's either useless
  or actively harmful. If specific ownership is needed the respective
  subsystem should allocate anonymous inodes from their own private
  superblock.

* Port pidfs to the new anon_inode_{g,s}etattr() helpers.

* Add proper tests for anonymous inode behavior.

The anonymous inode specific fixes should ideally be backported to all
LTS kernels.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (9):
      anon_inode: use a proper mode internally
      pidfs: use anon_inode_getattr()
      anon_inode: explicitly block ->setattr()
      pidfs: use anon_inode_setattr()
      anon_inode: raise SB_I_NODEV and SB_I_NOEXEC
      selftests/filesystems: add first test for anonymous inodes
      selftests/filesystems: add second test for anonymous inodes
      selftests/filesystems: add third test for anonymous inodes
      selftests/filesystems: add fourth test for anonymous inodes

 fs/anon_inodes.c                                   | 45 ++++++++++++++
 fs/internal.h                                      |  5 ++
 fs/libfs.c                                         |  2 +-
 fs/pidfs.c                                         | 26 +-------
 tools/testing/selftests/filesystems/.gitignore     |  1 +
 tools/testing/selftests/filesystems/Makefile       |  2 +-
 .../selftests/filesystems/anon_inode_test.c        | 69 ++++++++++++++++++++++
 7 files changed, 124 insertions(+), 26 deletions(-)
---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250407-work-anon_inode-e22bb1a74992