linux-kernel - [PATCH 1/1] x86/boot/compressed/32: Fix reloading of GDTR post-relocation

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Message-Id: <20200226204515.2752095-2-nivedita@alum.mit.edu>
Date:   Wed, 26 Feb 2020 15:45:15 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] x86/boot/compressed/32: Fix reloading of GDTR post-relocation

Commit ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
introduced GDT setup into startup_32, and reloads the GDTR after
relocating the kernel for paranoia's sake.

The GDTR is adjusted by init_size - _end, however this may not be the
correct offset to apply if the kernel was loaded at a misaligned address
or below LOAD_PHYSICAL_ADDR, as in that case the decompression buffer
has an additional offset from the original load address.

This should never happen for a conformant bootloader, but we're being
paranoid anyway, so just store the new GDT address in there instead of
adding any offsets, which is simpler as well.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
Fixes: ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
---
 arch/x86/boot/compressed/head_32.S | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 356060c5332c..2f8138b71ea9 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -139,12 +139,11 @@ SYM_FUNC_START(startup_32)
 	/*
 	 * 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. EAX still contains the previously
-	 * calculated relocation offset of init_size - _end.
+	 * to the new copy of the GDT.
 	 */
-	leal	gdt(%ebx), %edx
-	addl	%eax, 2(%edx)
-	lgdt	(%edx)
+	leal	gdt(%ebx), %eax
+	movl	%eax, 2(%eax)
+	lgdt	(%eax)

 /*
  * Jump to the relocated address.
-- 
2.24.1