linux-kernel - [PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL

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Message-Id: <20200428151725.31091-13-joro@8bytes.org>
Date:   Tue, 28 Apr 2020 17:16:22 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     x86@...nel.org
Cc:     hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Joerg Roedel <joro@...tes.org>, Joerg Roedel <jroedel@...e.de>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL_CS after GDT is loaded

From: Joerg Roedel <jroedel@...e.de>

When the pre-decompression code loads its first GDT in startup_64, it is
still running on the CS value of the previous GDT. In the case of SEV-ES
this is the EFI GDT.

To make exception handling work (especially IRET) the CPU needs to
switch to a CS value in the current GDT, so jump to __KERNEL_CS after
the first GDT is loaded.

Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 arch/x86/boot/compressed/head_64.S | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 4f7e6b84be07..6b11060c3a0f 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -393,6 +393,14 @@ SYM_CODE_START(startup_64)
 	addq	%rax, 2(%rax)
 	lgdt	(%rax)
 
+	/* Reload CS so IRET returns to a CS actually in the GDT */
+	pushq	$__KERNEL_CS
+	leaq	.Lon_kernel_cs(%rip), %rax
+	pushq	%rax
+	lretq
+
+.Lon_kernel_cs:
+
 	/*
 	 * paging_prepare() sets up the trampoline and checks if we need to
 	 * enable 5-level paging.
-- 
2.17.1