[PATCH v1] x86/mce: Fix endless loop when run task works after #MC

[Ding Hui]

Recently we encounter multi #MC on the same task when it's
task_work_run() has not been called, current->mce_kill_me was
added to task_works list more than once, that make a circular
linked task_works, so task_work_run() will do a endless loop.

More seriously, the SIGBUS signal can not be delivered to the
userspace task which tigger the #MC and I met #MC flood.

I borrowed mce_kill_me.func to check whether current->mce_kill_me
has been added to task_works, prevent duplicate addition. When
work function be called, the task_works must has been taken,
so it is safe to be cleared in callback.

Fixed: commit 5567d11c21a1 ("x86/mce: Send #MC singal from task work")
Cc: <stable@xxxxxxxxxxxxxxx> #v5.8+
Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/mce/core.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 22791aadc085..32fb9ded6b85 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1250,6 +1250,7 @@ static void __mc_scan_banks(struct mce *m, struct pt_regs *regs, struct mce *fin
 
 static void kill_me_now(struct callback_head *ch)
 {
+	WRITE_ONCE(ch->func, NULL);
 	force_sig(SIGBUS);
 }
 
@@ -1259,6 +1260,8 @@ static void kill_me_maybe(struct callback_head *cb)
 	int flags = MF_ACTION_REQUIRED;
 	int ret;
 
+	WRITE_ONCE(cb->func, NULL);
+
 	pr_err("Uncorrected hardware memory error in user-access at %llx", p->mce_addr);
 
 	if (!p->mce_ripv)
@@ -1289,17 +1292,20 @@ static void kill_me_maybe(struct callback_head *cb)
 
 static void queue_task_work(struct mce *m, int kill_current_task)
 {
+	struct callback_head ch;
+
 	current->mce_addr = m->addr;
 	current->mce_kflags = m->kflags;
 	current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
 	current->mce_whole_page = whole_page(m);
 
 	if (kill_current_task)
-		current->mce_kill_me.func = kill_me_now;
+		ch.func = kill_me_now;
 	else
-		current->mce_kill_me.func = kill_me_maybe;
+		ch.func = kill_me_maybe;
 
-	task_work_add(current, &current->mce_kill_me, TWA_RESUME);
+	if (!cmpxchg(&current->mce_kill_me.func, NULL, ch.func))
+		task_work_add(current, &current->mce_kill_me, TWA_RESUME);
 }
 
 /*
-- 
2.17.1