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Message-ID: <CAJcbSZGqY=btU=+eWfgdhF0je0Yn_GxkymFHn+OV-1BuzgbRYw@mail.gmail.com>
Date: Wed, 19 Jul 2017 08:40:14 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>,
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Subject: Re: [RFC 06/22] kvm: Adapt assembly for PIE support
On Tue, Jul 18, 2017 at 7:49 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Tue, Jul 18, 2017 at 6:33 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> Change the assembly code to use only relative references of symbols for the
>> kernel to be PIE compatible. The new __ASM_GET_PTR_PRE macro is used to
>> get the address of a symbol on both 32 and 64-bit with PIE support.
>>
>> 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/include/asm/kvm_host.h | 6 ++++--
>> arch/x86/kernel/kvm.c | 6 ++++--
>> arch/x86/kvm/svm.c | 4 ++--
>> 3 files changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 87ac4fba6d8e..3041201a3aeb 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -1352,9 +1352,11 @@ asmlinkage void kvm_spurious_fault(void);
>> ".pushsection .fixup, \"ax\" \n" \
>> "667: \n\t" \
>> cleanup_insn "\n\t" \
>> - "cmpb $0, kvm_rebooting \n\t" \
>> + "cmpb $0, kvm_rebooting" __ASM_SEL(,(%%rip)) " \n\t" \
>> "jne 668b \n\t" \
>> - __ASM_SIZE(push) " $666b \n\t" \
>> + __ASM_SIZE(push) "%%" _ASM_AX " \n\t" \
>> + __ASM_GET_PTR_PRE(666b) "%%" _ASM_AX "\n\t" \
>> + "xchg %%" _ASM_AX ", (%%" _ASM_SP ") \n\t" \
>> "call kvm_spurious_fault \n\t" \
>> ".popsection \n\t" \
>> _ASM_EXTABLE(666b, 667b)
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 71c17a5be983..53b8ad162589 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -618,8 +618,10 @@ asm(
>> ".global __raw_callee_save___kvm_vcpu_is_preempted;"
>> ".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
>> "__raw_callee_save___kvm_vcpu_is_preempted:"
>> -"movq __per_cpu_offset(,%rdi,8), %rax;"
>> -"cmpb $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
>> +"leaq __per_cpu_offset(%rip), %rax;"
>> +"movq (%rax,%rdi,8), %rax;"
>> +"addq " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rip), %rax;"
>
> This doesn't look right. It's accessing a per-cpu variable. The
> per-cpu section is an absolute, zero-based section and not subject to
> relocation.
>
PIE does not respect the zero-based section, it tries to have
everything relative. Patch 16/22 also adapt per-cpu to work with PIE
(while keeping the zero absolute design by default).
>> +"cmpb $0, (%rax);
>> "setne %al;"
>> "ret;"
>> ".popsection");
>
> --
> Brian Gerst
--
Thomas
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