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Date: Fri, 20 Oct 2017 07:47:44 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
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Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v1 06/27] x86/entry/64: Adapt assembly for PIE support
On Fri, Oct 20, 2017 at 1:26 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Thomas Garnier <thgarnie@...gle.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@...gle.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
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