Merge tag 'kernel-7.2-rc1.task_exec_state' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull task_exec_state updates from Christian Brauner:
 "This introduces a new per-task task_exec_state structure and relocates
  the dumpable mode and the user namespace captured at execve() from
  mm_struct onto it. It stays attached to the task for its full
  lifetime.

  __ptrace_may_access() and several /proc owner and visibility checks
  need to consult two pieces of state for any observable task, including
  zombies that have already gone through exit_mm(): the dumpable mode
  and the user namespace captured at execve(). Both live on mm_struct
  today, which exit_mm() clears from the task long before the task is
  reaped. A reader that races with do_exit() observes task->mm == NULL
  and either fails the check or falls back to init_user_ns - which
  denies legitimate access to non-dumpable zombies that were running in
  a nested user namespace.

  mm_struct loses ->user_ns and the dumpability bits in ->flags.
  MMF_DUMPABLE_BITS is reserved so the MMF_DUMP_FILTER_* layout exposed
  via /proc/<pid>/coredump_filter stays stable. task->user_dumpable and
  its exit_mm() snapshot are removed.

  task_exec_state is the privilege domain established by an execve().
  Within a thread group it is shared via refcount; across thread groups
  each task has its own:

   - CLONE_VM siblings (thread-group members, io_uring workers)
     refcount-share the parent's exec_state.

   - Non-CLONE_VM clones (fork(), vfork() without CLONE_VM) allocate a
     fresh exec_state inheriting the parent's dumpable mode and user_ns.

   - execve() in the child allocates a fresh instance and installs it
     under task_lock + exec_update_lock via task_exec_state_replace().

   - Credential changes (setresuid, capset, ...) and
     prctl(PR_SET_DUMPABLE) update dumpability on the current task's
     exec_state, i.e., on the thread group's shared instance.

  On top of this exec_mmap() no longer tears down the old mm while
  holding exec_update_lock for writing and cred_guard_mutex. Neither
  lock is needed for that: exec_update_lock only exists to make the mm
  swap atomic with the later commit_creds() and all its readers operate
  on the new mm; none looks at the detached old mm.

  The cost was real: __mmput() runs exit_mmap() over the entire old
  address space and can block in exit_aio() waiting for in-flight AIO,
  so execve() of a large process blocked ptrace_attach() and every
  exec_update_lock reader for the duration of the teardown.

  The old mm is now stashed in bprm->old_mm and released from
  setup_new_exec() after both locks are dropped, with a backstop in
  free_bprm() for the error paths"

* tag 'kernel-7.2-rc1.task_exec_state' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  exec: free the old mm outside the exec locks
  exec_state: relocate dumpable information
  ptrace: add ptracer_access_allowed()
  exec: introduce struct task_exec_state
  sched/coredump: introduce enum task_dumpable

1 files changed, 3 insertions, 3 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ee06cba5c6f5..258cb075478d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -85,6 +85,7 @@ struct seq_file;
 struct sighand_struct;
 struct signal_struct;
 struct task_delay_info;
+struct task_exec_state;
 struct task_group;
 struct task_struct;
 struct timespec64;
@@ -962,6 +963,8 @@ struct task_struct {
 	struct mm_struct		*mm;
 	struct mm_struct		*active_mm;
 
+	struct task_exec_state __rcu	*exec_state;
+
 	int				exit_state;
 	int				exit_code;
 	int				exit_signal;
@@ -1002,9 +1005,6 @@ struct task_struct {
 	unsigned			sched_rt_mutex:1;
 #endif
 
-	/* Save user-dumpable when mm goes away */
-	unsigned			user_dumpable:1;
-
 	/* Bit to tell TOMOYO we're in execve(): */
 	unsigned			in_execve:1;
 	unsigned			in_iowait:1;