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

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Date:   Mon, 25 Jul 2022 09:49:19 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        kernel-team@...com, Josh Poimboeuf <jpoimboe@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Breno Leitao <leitao@...ian.org>
Subject: [PATCH v3] livepatch: fix race between fork and
 klp_reverse_transition

On Mon, 25 Jul 2022 15:32:22 +0200
Petr Mladek <pmladek@...e.com> wrote:

> We should update the commit message and mention also the other
> two locations where the state is manipulated without tasklist_lock.
> I am sorry that I did not mention it on Friday.

Done. Thank you for reviewing this patch so carefully!
I had looked at those other places in the code as well, but
do not have as complete a picture of the KLP code as you.

v2: a better approach, suggested by Petr (thank you)
v3: update changelog (thank you Petr)
---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.

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.

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.

Signed-off-by: Rik van Riel <riel@...riel.com>
Reported-by: Breno Leitao <leitao@...ian.org>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
 kernel/livepatch/transition.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 5d03a2ad1066..30187b1d8275 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -610,9 +610,23 @@ void klp_reverse_transition(void)
 /* Called from copy_process() during fork */
 void klp_copy_process(struct task_struct *child)
 {
-	child->patch_state = current->patch_state;

-	/* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
+	/*
+	 * The parent process may have gone through a KLP transition since
+	 * the thread flag was copied in setup_thread_stack earlier. Bring
+	 * the task flag up to date with the parent here.
+	 *
+	 * The operation is serialized against all klp_*_transition()
+	 * operations by the tasklist_lock. The only exception is
+	 * klp_update_patch_state(current), but we cannot race with
+	 * that because we are current.
+	 */
+	if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
+		set_tsk_thread_flag(child, TIF_PATCH_PENDING);
+	else
+		clear_tsk_thread_flag(child, TIF_PATCH_PENDING);
+
+	child->patch_state = current->patch_state;
 }

 /*
-- 
2.35.1