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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Sean Christopherson <seanjc@google.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Cc:     "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        "Guilherme G . Piccoli" <gpiccoli@igalia.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Subject: [PATCH] x86/nmi: Make register_nmi_handler() more robust
In-Reply-To: <20220511234332.3654455-1-seanjc@google.com>
References: <20220511234332.3654455-1-seanjc@google.com>
Date:   Fri, 13 May 2022 13:10:06 +0200
Message-ID: <87zgjlsn75.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

register_nmi_handler() has no sanity check whether a handler has been
registered already. Such an unintended double-add leads to list corruption
and hard to diagnose problems during the next NMI handling.

Init the list head in the static nmi action struct and check it for being
empty in register_nmi_handler().

Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/include/asm/nmi.h |    1 +
 arch/x86/kernel/nmi.c      |   10 +++++++---
 2 files changed, 8 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/nmi.h
+++ b/arch/x86/include/asm/nmi.h
@@ -47,6 +47,7 @@ struct nmiaction {
 #define register_nmi_handler(t, fn, fg, n, init...)	\
 ({							\
 	static struct nmiaction init fn##_na = {	\
+		.list = LIST_HEAD_INIT(fn##_na.list),	\
 		.handler = (fn),			\
 		.name = (n),				\
 		.flags = (fg),				\
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -157,7 +157,7 @@ int __register_nmi_handler(unsigned int
 	struct nmi_desc *desc = nmi_to_desc(type);
 	unsigned long flags;
 
-	if (!action->handler)
+	if (WARN_ON_ONCE(action->handler || !list_empty(&action->list)))
 		return -EINVAL;
 
 	raw_spin_lock_irqsave(&desc->lock, flags);
@@ -186,7 +186,7 @@ EXPORT_SYMBOL(__register_nmi_handler);
 void unregister_nmi_handler(unsigned int type, const char *name)
 {
 	struct nmi_desc *desc = nmi_to_desc(type);
-	struct nmiaction *n;
+	struct nmiaction *n, *found = NULL;
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&desc->lock, flags);
@@ -200,12 +200,16 @@ void unregister_nmi_handler(unsigned int
 			WARN(in_nmi(),
 				"Trying to free NMI (%s) from NMI context!\n", n->name);
 			list_del_rcu(&n->list);
+			found = n;
 			break;
 		}
 	}
 
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
-	synchronize_rcu();
+	if (found) {
+		synchronize_rcu();
+		INIT_LIST_HEAD(found);
+	}
 }
 EXPORT_SYMBOL_GPL(unregister_nmi_handler);