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From:   Arvind Sankar <nivedita@alum.mit.edu>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-efi@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v2 3/7] x86/boot: Reload GDTR after copying to the end of the buffer
Date:   Sun,  2 Feb 2020 12:13:49 -0500
Message-Id: <20200202171353.3736319-4-nivedita@alum.mit.edu>
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 <20200202171353.3736319-1-nivedita@alum.mit.edu>
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The GDT may get overwritten during the copy or during extract_kernel,
which will cause problems if any segment register is touched before the
GDTR is reloaded by the decompressed kernel. For safety update the GDTR
to point to the GDT within the copied kernel.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 arch/x86/boot/compressed/head_64.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index c56b30bd9c7b..27eb2a6786db 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -439,6 +439,16 @@ trampoline_return:
 	cld
 	popq	%rsi
 
+	/*
+	 * The GDT may get overwritten either during the copy we just did or
+	 * during extract_kernel below. To avoid any issues, repoint the GDTR
+	 * to the new copy of the GDT.
+	 */
+	leaq	gdt64(%rbx), %rax
+	subq	%rbp, 2(%rax)
+	addq	%rbx, 2(%rax)
+	lgdt	(%rax)
+
 /*
  * Jump to the relocated address.
  */
-- 
2.24.1