[PATCH perf/core 3/4] uprobes: ensure return_instance is detached from the list before freeing

From: Andrii Nakryiko
Date: Thu Dec 05 2024 - 19:25:05 EST


Ensure that by the time we call free_ret_instance() to clean up an
instance of struct return_instance it isn't reachable from
utask->return_instances anymore.

free_ret_instance() is called in a few different situations, all but one
of which already are fine w.r.t. return_instance visibility:
- uprobe_free_utask() guarantees that ri_timer() won't be called
(through timer_delete_sync() call), and so there is no need to
unlink anything, because entire utask is being freed;
- uprobe_handle_trampoline() is already unlinking to-be-freed
return_instance with rcu_assign_pointer() before calling
free_ret_instance().

Only cleanup_return_instances() violates this property, which so far is
not causing problems due to RCU-delayed freeing of return_instance,
which we'll change in the next patch. So make sure we unlink
return_instance before passing it into free_ret_instance(), as otherwise
reuse will be unsafe.

Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
kernel/events/uprobes.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index cca1fe4a3fb1..2345aeb63d3b 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2116,12 +2116,12 @@ static void cleanup_return_instances(struct uprobe_task *utask, bool chained,

while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
ri_next = ri->next;
+ rcu_assign_pointer(utask->return_instances, ri_next);
utask->depth--;

free_ret_instance(ri, true /* cleanup_hprobe */);
ri = ri_next;
}
- rcu_assign_pointer(utask->return_instances, ri);
}

static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs,
--
2.43.5