ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL

putreg() assumes that the tracee is not running and pt_regs_access() can
safely play with its stack.  However a killed tracee can return from
ptrace_stop() to the low-level asm code and do RESTORE_REST, this means
that debugger can actually read/modify the kernel stack until the tracee
does SAVE_REST again.

set_task_blockstep() can race with SIGKILL too and in some sense this
race is even worse, the very fact the tracee can be woken up breaks the
logic.

As Linus suggested we can clear TASK_WAKEKILL around the arch_ptrace()
call, this ensures that nobody can ever wakeup the tracee while the
debugger looks at it.  Not only this fixes the mentioned problems, we
can do some cleanups/simplifications in arch_ptrace() paths.

Probably ptrace_unfreeze_traced() needs more callers, for example it
makes sense to make the tracee killable for oom-killer before
access_process_vm().

While at it, add the comment into may_ptrace_stop() to explain why
ptrace_stop() still can't rely on SIGKILL and signal_pending_state().

Reported-by: Salman Qazi <sqazi@google.com>
Reported-by: Suleiman Souhlal <suleiman@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 6e97aa6fa32c..3d09cf6cde75 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1794,6 +1794,10 @@ static inline int may_ptrace_stop(void)
          * If SIGKILL was already sent before the caller unlocked
          * ->siglock we must see ->core_state != NULL. Otherwise it
          * is safe to enter schedule().
+         *
+         * This is almost outdated, a task with the pending SIGKILL can't
+         * block in TASK_TRACED. But PTRACE_EVENT_EXIT can be reported
+         * after SIGKILL was already dequeued.
          */
         if (unlikely(current->mm->core_state) &&
             unlikely(current->mm == current->parent->mm))
@@ -1919,6 +1923,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
                 if (gstop_done)
                         do_notify_parent_cldstop(current, false, why);
 
+                /* tasklist protects us from ptrace_freeze_traced() */
                 __set_current_state(TASK_RUNNING);
                 if (clear_code)
                         current->exit_code = 0;