linux-kernel - [RFC][PATCH 20/24] x86/pti: Execute NMI handler on the kernel stack

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Date:   Mon,  9 Nov 2020 15:44:21 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, jroedel@...e.de
Cc:     konrad.wilk@...cle.com, jan.setjeeilers@...cle.com,
        junaids@...gle.com, oweisse@...gle.com, rppt@...ux.vnet.ibm.com,
        graf@...zon.de, mgross@...ux.intel.com, kuzuno@...il.com,
        alexandre.chartre@...cle.com
Subject: [RFC][PATCH 20/24] x86/pti: Execute NMI handler on the kernel stack

After a NMI from userland, the kernel is entered and it switches
the stack to the PTI stack which is mapped both in the kernel and in
the user page-table. When executing the NMI handler, switch to the
kernel stack (which is mapped only in the kernel page-table) so that
no kernel data leak to the userland through the stack.

Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
---
 arch/x86/kernel/nmi.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 4bc77aaf1303..be0f654c3095 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -506,8 +506,18 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 
 	inc_irq_stat(__nmi_count);
 
-	if (!ignore_nmis)
-		default_do_nmi(regs);
+	if (!ignore_nmis) {
+		if (user_mode(regs)) {
+			/*
+			 * If we come from userland then we are on the
+			 * trampoline stack, switch to the kernel stack
+			 * to execute the NMI handler.
+			 */
+			run_idt(default_do_nmi, regs);
+		} else {
+			default_do_nmi(regs);
+		}
+	}
 
 	idtentry_exit_nmi(regs, irq_state);
 
-- 
2.18.4