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From:   Alexandre Chartre <alexandre.chartre@oracle.com>
To:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        x86@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
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Cc:     konrad.wilk@oracle.com, jan.setjeeilers@oracle.com,
        junaids@google.com, oweisse@google.com, rppt@linux.vnet.ibm.com,
        graf@amazon.de, mgross@linux.intel.com, kuzuno@gmail.com,
        alexandre.chartre@oracle.com
Subject: [RFC][PATCH v2 18/21] x86/pti: Execute NMI handler on the kernel stack
Date:   Mon, 16 Nov 2020 15:47:54 +0100
Message-Id: <20201116144757.1920077-19-alexandre.chartre@oracle.com>
In-Reply-To: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
Precedence: bulk

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@oracle.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