linux-kernel - Re: [PATCH v3] livepatch: fix race between fork and klp_reverse

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Message-ID: <YuKpQXl2P3jkOpEy@alley>
Date:   Thu, 28 Jul 2022 17:20:33 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, kernel-team@...com,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Breno Leitao <leitao@...ian.org>
Subject: Re: [PATCH v3] livepatch: fix race between fork and
 klp_reverse_transition

On Tue 2022-07-26 20:26:41, Rik van Riel wrote:
> On Tue, 2022-07-26 at 17:10 -0700, Josh Poimboeuf wrote:
> > On Mon, Jul 25, 2022 at 09:49:19AM -0400, Rik van Riel wrote:
> > > When a KLP fails to apply, klp_reverse_transition will clear the
> > > TIF_PATCH_PENDING flag on all tasks, except for newly created tasks
> > > which are not on the task list yet.
> > 
> > This paragraph and $SUBJECT both talk about a reverse transition. 
> > Isn't
> > it also possible to race on a normal (forward) transition?
> 
> I don't know whether the race is also possible on a forward
> transition.  If the parent task has transitioned, will
> the child have, as well, by the time we reach the end of fork?

I think that the race should be possible also with the forward
transition. I do not see what would prevent it.


> I suppose the only way the parent task can transition while
> inside fork would be if none of the functions in its stack
> need to be transitioned, and at that point the child process
> would automatically be safe, too?

IMHO, these races might be dangerous only when fork() calls
a function on the way out that is livepatched but it was not
on the stack when the process was copied.

Anyway, the patch should make sure that task->patch_state and
TIF_PATCH_PENTING are always consitent when the child is added
to the global task list. So, we should always be on the safe side.


> However, we have only observed this warning on reverse transitions
> for some reason.

IMHO, it is because the race during the forward transition is
kind of "self-healing":

parent:				worker:

  fork()
    #copy set TIF_PATCH_PENDING
    # schedule


				  klp_try_complete_transition()
				     clear_bit(parent, TIF_PATCH_PENDING);
				     parent->patch_state = klp_target_state;

   # running again
   # copy already migrated parent->patch_state



later:

     clear_bit(child, TIF_PATCH_PENDING);
     child->patch_state = klp_target_state;

As a result, child->patch_state will be updated twice
to klp_target_state.



The problematic situation during revert:


parent:					another process:

   # migrate parent
   clear_bit(parent, TIF_PATCH_PENDING);
   parent->patch_state = klp_target_state;

   fork()
     #copy cleared TIF_PATCH_PENDING

					klp_revert_patch()
					    # invert @klp_target_state
					    set_bit(parent, TIF_PATCH_PENDING)

     # copy parent->patch_state that
       needs migration once again

   # migrated once again after revert
   clear_bit(parent, TIF_PATCH_PENDING);
   parent->patch_state = klp_target_state;

WARNING: child will never get migrated because it copied the cleared
	 TIF_PATCH_PENDING before @klp_target_state was inverted

Resume:

It is great that the race was found and fixed.

Best Regards,
Petr