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

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Date:   Tue, 26 Jul 2022 17:10:40 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Petr Mladek <pmladek@...e.com>, 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 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?

> Meanwhile, fork will copy over the TIF_PATCH_PENDING flag from the
> parent to the child early on, in dup_task_struct -> setup_thread_stack.
> 
> Much later, klp_copy_process will set child->patch_state to match
> that of the parent.
> 
> However, the parent's patch_state may have been changed by KLP loading
> or unloading since it was initially copied over into the child.
> 
> This results in the KLP code occasionally hitting this warning in
> klp_complete_transition:
> 
>         for_each_process_thread(g, task) {
>                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
>                 task->patch_state = KLP_UNDEFINED;
>         }
> 
> This patch will set, or clear, the TIF_PATCH_PENDING flag in the child
> process depending on whether or not it is needed at the time
> klp_copy_process is called, at a point in copy_process where the
> tasklist_lock is held exclusively, preventing races with the KLP
> code.

Use imperative language, i.e. no "This patch".  See
Documentation/process/submitting-patches.rst
> 
> The KLP code does have a few places where the state is changed
> without the tasklist_lock held, but those should not cause
> problems because klp_update_patch_state(current) cannot be
> called while the current task is in the middle of fork,
> klp_check_and_switch_task() which is called under the pi_lock,
> which prevents rescheduling, and manipulation of the patch
> state of idle tasks, which do not fork.
> 
> This should prevent this warning from triggering again in the
> future.
> 

Fixes: d83a7cb375ee ("livepatch: change to a per-task consistency model")

> Signed-off-by: Rik van Riel <riel@...riel.com>
> Reported-by: Breno Leitao <leitao@...ian.org>
> Reviewed-by: Petr Mladek <pmladek@...e.com>

With the above minor things fixed:

Acked-by: Josh Poimboeuf <jpoimboe@...nel.org>

-- 
Josh