On Mon, May 20, 2019 at 04:19:34PM -0700, Thomas Garnier wrote: > From: Thomas Garnier <thgar...@google.com> > > 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 <thgar...@google.com>
Reviewed-by: Kees Cook <keesc...@chromium.org> -Kees > --- > 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 bcd206c8ac90..64a4f0a22b20 100644 > --- a/arch/x86/kernel/head_64.S > +++ b/arch/x86/kernel/head_64.S > @@ -90,8 +90,10 @@ 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 > /* > @@ -120,7 +122,8 @@ 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 */ > @@ -138,7 +141,7 @@ 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: > @@ -235,11 +238,12 @@ 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) > -- > 2.21.0.1020.gf2820cf01a-goog > -- Kees Cook
