linux-kernel - Re: [PATCH v4] livepatch: fix race between fork and KLP transition

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Date:   Thu, 28 Jul 2022 17:37:26 +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 v4] livepatch: fix race between fork and KLP transition

On Wed 2022-07-27 10:24:37, Rik van Riel wrote:
> v4: address changelog comments by Josh (thank you)
> 
> ---8<---
> 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.

It actually is not true. klp_reverse_transtion() clears TIF_PATCH_FLAG only
temporary when it waits until all processes leave the ftrace
handler. It sets TIF_PATCH_FLAG once again for all tasks by calling
klp_start_transition().

The difference is important. The WARN_ON_ONCE() in
klp_complete_transition() will be printed when fork() copied
TIF_PATCH_FLAG before it was set again.

Anyway, the important thing is that TIF_PATCH_FLAG and task->patch_state
might be incompatible because fork() copies them at different times.

klp_copy_process() must make sure that they are in sync. And
it must be done under tasklist_lock when the child is added
to the global task list.

Best Regards,
Petr