[PATCH v9 08/11] x86/boot/64: Adapt assembly for PIE support

Thomas Garnier
2019-07-30T19:13:44Z

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 below 0xffffffff80000000.

Signed-off-by: Thomas Garnier <thgarnie@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
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 f3d3e9646a99..9a3f96566eb2 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -88,8 +88,10 @@ 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
/*
@@ -118,7 +120,8 @@ 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 */
@@ -136,7 +139,7 @@ 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:
@@ -233,11 +236,12 @@ 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
xorl %ebp, %ebp # 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)
--
2.22.0.770.g0f2c4a37fd-goog