On Thu 2021-09-23 15:28:56, Peter Zijlstra wrote:
On Thu, Sep 23, 2021 at 03:14:48PM +0200, Petr Mladek wrote:
quoted
IMHO, this is not safe:
CPU0 CPU1
klp_check_task(A)
if (context_tracking_state_cpu(task_cpu(task)) == CONTEXT_USER)
goto complete;
clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
# task switching to kernel space
klp_update_patch_state(A)
if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
//false
# calling kernel code with old task->patch_state
task->patch_state = klp_target_state;
BANG: CPU0 sets task->patch_state when task A is already running
kernel code on CPU1.
Why is that a problem? That is, who actually cares about
task->patch_state ? I was under the impression that state was purely for
klp itself, to track which task has observed the new state.
It is the other way. The patch_state is used in klp_ftrace_handler()
to decide which code must be used (old or new).
The state must change only when the given task is _not_ using
any patched function. Hence we do it when:
+ no patched function is on the stack (needed primary for kthreads)
+ entering/leaving kernel (reliable way for user space)
See "Consistency model" in Documentation/livepatch/livepatch.rst
for more details.
Best Regards,
Petr