Linux Virtualization - Oct 2017 - [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support

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

Position Independent Executable (PIE) support will allow to extended the
KASLR randomization range below the -2G memory limit.

Signed-off-by: Thomas Garnier <thgarnie at google.com>
---
 arch/x86/entry/entry_64.S | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 49167258d587..15bd5530d2ae 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -194,12 +194,15 @@ entry_SYSCALL_64_fastpath:
 	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
 	movq	%r10, %rcx
 
+	/* Ensures the call is position independent */
+	leaq	sys_call_table(%rip), %r11
+
 	/*
 	 * This call instruction is handled specially in stub_ptregs_64.
 	 * It might end up jumping to the slow path.  If it jumps, RAX
 	 * and all argument registers are clobbered.
 	 */
-	call	*sys_call_table(, %rax, 8)
+	call	*(%r11, %rax, 8)
 .Lentry_SYSCALL_64_after_fastpath_call:
 
 	movq	%rax, RAX(%rsp)
@@ -334,7 +337,8 @@ ENTRY(stub_ptregs_64)
 	 * RAX stores a pointer to the C function implementing the syscall.
 	 * IRQs are on.
 	 */
-	cmpq	$.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
+	leaq	.Lentry_SYSCALL_64_after_fastpath_call(%rip), %r11
+	cmpq	%r11, (%rsp)
 	jne	1f
 
 	/*
@@ -1172,7 +1176,8 @@ ENTRY(error_entry)
 	movl	%ecx, %eax			/* zero extend */
 	cmpq	%rax, RIP+8(%rsp)
 	je	.Lbstep_iret
-	cmpq	$.Lgs_change, RIP+8(%rsp)
+	leaq	.Lgs_change(%rip), %rcx
+	cmpq	%rcx, RIP+8(%rsp)
 	jne	.Lerror_entry_done
 
 	/*
@@ -1383,10 +1388,10 @@ ENTRY(nmi)
 	 * resume the outer NMI.
 	 */
 
-	movq	$repeat_nmi, %rdx
+	leaq	repeat_nmi(%rip), %rdx
 	cmpq	8(%rsp), %rdx
 	ja	1f
-	movq	$end_repeat_nmi, %rdx
+	leaq	end_repeat_nmi(%rip), %rdx
 	cmpq	8(%rsp), %rdx
 	ja	nested_nmi_out
 1:
@@ -1440,7 +1445,8 @@ nested_nmi:
 	pushq	%rdx
 	pushfq
 	pushq	$__KERNEL_CS
-	pushq	$repeat_nmi
+	leaq	repeat_nmi(%rip), %rdx
+	pushq	%rdx
 
 	/* Put stack back */
 	addq	$(6*8), %rsp
@@ -1479,7 +1485,9 @@ first_nmi:
 	addq	$8, (%rsp)	/* Fix up RSP */
 	pushfq			/* RFLAGS */
 	pushq	$__KERNEL_CS	/* CS */
-	pushq	$1f		/* RIP */
+	pushq	%rax		/* Support Position Independent Code */
+	leaq	1f(%rip), %rax	/* RIP */
+	xchgq	%rax, (%rsp)	/* Restore RAX, put 1f */
 	INTERRUPT_RETURN	/* continues at repeat_nmi below */
 	UNWIND_HINT_IRET_REGS
 1:
-- 
2.15.0.rc0.271.g36b669edcc-goog

On Fri, Oct 20, 2017 at 1:26 AM, Ingo Molnar <mingo at kernel.org>
wrote:
>
> * Thomas Garnier <thgarnie at google.com> wrote:
>
>> Change the assembly code to use only relative references of symbols for
the
>> kernel to be PIE compatible.
>>
>> Position Independent Executable (PIE) support will allow to extended
the
>> KASLR randomization range below the -2G memory limit.
>>
>> Signed-off-by: Thomas Garnier <thgarnie at google.com>
>> ---
>>  arch/x86/entry/entry_64.S | 22 +++++++++++++++-------
>>  1 file changed, 15 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 49167258d587..15bd5530d2ae 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -194,12 +194,15 @@ entry_SYSCALL_64_fastpath:
>>       ja      1f                              /* return -ENOSYS
(already in pt_regs->ax) */
>>       movq    %r10, %rcx
>>
>> +     /* Ensures the call is position independent */
>> +     leaq    sys_call_table(%rip), %r11
>> +
>>       /*
>>        * This call instruction is handled specially in stub_ptregs_64.
>>        * It might end up jumping to the slow path.  If it jumps, RAX
>>        * and all argument registers are clobbered.
>>        */
>> -     call    *sys_call_table(, %rax, 8)
>> +     call    *(%r11, %rax, 8)
>>  .Lentry_SYSCALL_64_after_fastpath_call:
>>
>>       movq    %rax, RAX(%rsp)
>> @@ -334,7 +337,8 @@ ENTRY(stub_ptregs_64)
>>        * RAX stores a pointer to the C function implementing the
syscall.
>>        * IRQs are on.
>>        */
>> -     cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
>> +     leaq    .Lentry_SYSCALL_64_after_fastpath_call(%rip), %r11
>> +     cmpq    %r11, (%rsp)
>>       jne     1f
>>
>>       /*
>> @@ -1172,7 +1176,8 @@ ENTRY(error_entry)
>>       movl    %ecx, %eax                      /* zero extend */
>>       cmpq    %rax, RIP+8(%rsp)
>>       je      .Lbstep_iret
>> -     cmpq    $.Lgs_change, RIP+8(%rsp)
>> +     leaq    .Lgs_change(%rip), %rcx
>> +     cmpq    %rcx, RIP+8(%rsp)
>>       jne     .Lerror_entry_done
>>
>>       /*
>> @@ -1383,10 +1388,10 @@ ENTRY(nmi)
>>        * resume the outer NMI.
>>        */
>>
>> -     movq    $repeat_nmi, %rdx
>> +     leaq    repeat_nmi(%rip), %rdx
>>       cmpq    8(%rsp), %rdx
>>       ja      1f
>> -     movq    $end_repeat_nmi, %rdx
>> +     leaq    end_repeat_nmi(%rip), %rdx
>>       cmpq    8(%rsp), %rdx
>>       ja      nested_nmi_out
>>  1:
>> @@ -1440,7 +1445,8 @@ nested_nmi:
>>       pushq   %rdx
>>       pushfq
>>       pushq   $__KERNEL_CS
>> -     pushq   $repeat_nmi
>> +     leaq    repeat_nmi(%rip), %rdx
>> +     pushq   %rdx
>>
>>       /* Put stack back */
>>       addq    $(6*8), %rsp
>> @@ -1479,7 +1485,9 @@ first_nmi:
>>       addq    $8, (%rsp)      /* Fix up RSP */
>>       pushfq                  /* RFLAGS */
>>       pushq   $__KERNEL_CS    /* CS */
>> -     pushq   $1f             /* RIP */
>> +     pushq   %rax            /* Support Position Independent Code */
>> +     leaq    1f(%rip), %rax  /* RIP */
>> +     xchgq   %rax, (%rsp)    /* Restore RAX, put 1f */
>>       INTERRUPT_RETURN        /* continues at repeat_nmi below */
>>       UNWIND_HINT_IRET_REGS
>
> This patch seems to add extra overhead to the syscall fast-path even when
PIE is
> disabled, right?
It does add extra instructions when one is not possible, I preferred
that over ifdefing but I can change it.
>
> Thanks,
>
>         Ingo


-- 
Thomas