linux-kernel - [PATCH v4 10/27] x86/boot/64: Adapt assembly for PIE support

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Message-Id: <20180529221625.33541-11-thgarnie@google.com>
Date:   Tue, 29 May 2018 15:15:11 -0700
From:   Thomas Garnier <thgarnie@...gle.com>
To:     kernel-hardening@...ts.openwall.com
Cc:     Thomas Garnier <thgarnie@...gle.com>,
        Skip Tom Lendacky <thomas.lendacky@....com>,
        Skip Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v4 10/27] x86/boot/64: Adapt assembly for PIE support

Change the assembly code to use only relative references of symbols for the
kernel to be PIE compatible.

Early at boot, the kernel is mapped at a temporary address while preparing
the page table. To know the changes needed for the page table with KASLR,
the boot code calculate the difference between the expected address of the
kernel and the one chosen by KASLR. It does not work with PIE because all
symbols in code are relatives. Instead of getting the future relocated
virtual address, you will get the current temporary mapping.
Instructions were changed to have absolute 64-bit references.

Position Independent Executable (PIE) support will allow to extend the
KASLR randomization range 0xffffffff80000000.

Signed-off-by: Thomas Garnier <thgarnie@...gle.com>
---
 arch/x86/kernel/head_64.S | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 8344dd2f310a..7fca19e1f556 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -89,8 +89,10 @@ startup_64:
 	popq	%rsi
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
-	addq	$(early_top_pgt - __START_KERNEL_map), %rax
+	movabs  $(early_top_pgt - __START_KERNEL_map), %rcx
+	addq    %rcx, %rax
 	jmp 1f
+
 ENTRY(secondary_startup_64)
 	UNWIND_HINT_EMPTY
 	/*
@@ -119,7 +121,8 @@ ENTRY(secondary_startup_64)
 	popq	%rsi
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
-	addq	$(init_top_pgt - __START_KERNEL_map), %rax
+	movabs	$(init_top_pgt - __START_KERNEL_map), %rcx
+	addq    %rcx, %rax
 1:
 
 	/* Enable PAE mode, PGE and LA57 */
@@ -137,7 +140,7 @@ ENTRY(secondary_startup_64)
 	movq	%rax, %cr3
 
 	/* Ensure I am executing from virtual addresses */
-	movq	$1f, %rax
+	movabs  $1f, %rax
 	ANNOTATE_RETPOLINE_SAFE
 	jmp	*%rax
 1:
@@ -234,11 +237,12 @@ ENTRY(secondary_startup_64)
 	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
 	 *		address given in m16:64.
 	 */
-	pushq	$.Lafter_lret	# put return address on stack for unwinder
+	movabs  $.Lafter_lret, %rax
+	pushq	%rax		# put return address on stack for unwinder
 	xorq	%rbp, %rbp	# clear frame pointer
-	movq	initial_code(%rip), %rax
+	leaq	initial_code(%rip), %rax
 	pushq	$__KERNEL_CS	# set correct cs
-	pushq	%rax		# target address in negative space
+	pushq	(%rax)		# target address in negative space
 	lretq
 .Lafter_lret:
 END(secondary_startup_64)
-- 
2.17.0.921.gf22659ad46-goog