linux-kernel - [RFC][PATCH 4/4] x86/entry, mce: Disallow #DB during #MC

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Date:   Fri, 22 May 2020 22:47:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, luto@...capital.net, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [RFC][PATCH 4/4] x86/entry, mce: Disallow #DB during #MC

#MC is fragile as heck, don't tempt fate.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/cpu/mce/core.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1943,22 +1943,34 @@ static __always_inline void exc_machine_
 /* MCE hit kernel mode */
 DEFINE_IDTENTRY_MCE(exc_machine_check)
 {
+	unsigned long dr7;
+
+	local_db_save(&dr7);
 	exc_machine_check_kernel(regs);
+	local_db_restore(dr7);
 }
 
 /* The user mode variant. */
 DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
 {
+	unsigned long dr7;
+
+	local_db_save(&dr7);
 	exc_machine_check_user(regs);
+	local_db_restore(dr7);
 }
 #else
 /* 32bit unified entry point */
 DEFINE_IDTENTRY_MCE(exc_machine_check)
 {
+	unsigned long dr7;
+
+	local_db_save(&dr7);
 	if (user_mode(regs))
 		exc_machine_check_user(regs);
 	else
 		exc_machine_check_kernel(regs);
+	local_db_restore(dr7);
 }
 #endif