On Wed, Feb 6, 2019 at 11:56 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Thu, Jan 31, 2019 at 11:24:26AM -0800, Thomas Garnier wrote:
> > Change the assembly code to use only relative references of symbols for the
> > kernel to be PIE compatible. The new __ASM_MOVABS 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 extend the
> > KASLR randomization range below 0xffffffff80000000.
> >
> > Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxxxx>
> > ---
> > arch/x86/include/asm/kvm_host.h | 8 ++++++--
> > arch/x86/kernel/kvm.c | 6 ++++--
> > arch/x86/kvm/svm.c | 4 ++--
> > arch/x86/kvm/vmx/vmx.c | 2 +-
> > 4 files changed, 13 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 4660ce90de7f..fdb3307d5fe1 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1498,9 +1498,13 @@ 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) "$0 \n\t" \
> > + __ASM_SIZE(push) "%%" _ASM_AX " \n\t" \
> > + _ASM_MOVABS " $666b, %%" _ASM_AX "\n\t" \
> > + _ASM_MOV " %%" _ASM_AX ", " __ASM_SEL(4, 8) "(%%" _ASM_SP ") \n\t" \
> > + __ASM_SIZE(pop) "%%" _ASM_AX " \n\t" \
>
> This blob isn't very intuitive to begin with, and the extra stack
> shenanigans are a bit much when PIE is disabled. What about breaking
> out the behavior to separate helper macros to keep the simpler code
> for non-PIE and to make the code somewhat self-documenting? E.g.:
>
> #ifndef CONFIG_X86_PIE
> #define KVM_PUSH_FAULTING_INSN_RIP __ASM_SIZE(push) " $666b \n\t"
> #else
> #define KVM_PUSH_FAULTING_INSN_RIP \
> __ASM_SIZE(push) "$0 \n\t" \
> __ASM_SIZE(push) "%%" _ASM_AX " \n\t" \
> _ASM_MOVABS " $666b, %%" _ASM_AX "\n\t" \
> _ASM_MOV " %%" _ASM_AX ", " __ASM_SEL(4, 8) "(%%" _ASM_SP ") \n\t" \
> __ASM_SIZE(pop) "%%" _ASM_AX " \n\t"
> #endif
>
> #define ____kvm_handle_fault_on_reboot(insn, cleanup_insn) \
> "666: " insn "\n\t" \
> "668: \n\t" \
> ".pushsection .fixup, \"ax\" \n" \
> "667: \n\t" \
> cleanup_insn "\n\t" \
> "cmpb $0, kvm_rebooting" __ASM_SEL(, (%%rip)) " \n\t" \
> "jne 668b \n\t" \
> KVM_PUSH_FAULTING_INSN_RIP \
> "jmp kvm_spurious_fault \n\t" \
> ".popsection \n\t" \
> _ASM_EXTABLE(666b, 667b)
>
> > "jmp 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 5c93a65ee1e5..f6eb02004e43 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
>
> This change to arch/x86/kernel/kvm.c should be done in a separate patch
> as it affects the kernel itself when running as a guest under KVM,
> whereas arch/x86/kvm/**/* and arch/x86/include/asm/kvm_host.h affect
> KVM as a host, i.e. the KVM module. Case in point, the below bug causes
> a kernel panic when running as a KVM guest but has no impact on the KVM
> module.
Got it, will split in next iteration.
>
> > @@ -826,8 +826,10 @@ 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 is wrong, it's directly accessing the per-cpu offset of 'steal_time'
> as a virtual address, e.g. without PIE enabled:
>
> 0xffffffff8104820b <+11>: add 0x7efccffe(%rip),%rax # 0x15210 <steal_time+16>
>
> This results in kernel panics due to unhandled page faults:
>
> [ 0.001453] BUG: unable to handle kernel paging request at 0000000000015210
> [ 0.001453] #PF error: [normal kernel read fault]
Yes, I think something went wrong in rebasing. Thanks for pointing it out.
>
> I think you want something like the following, except that the whole
> point of handcoded assembly is to avoid modifying registers other than
> RAX, i.e. modifying RDI is a no-no.
>
> "leaq " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rip), %rdi;"
> "cmpb $0, (%rax,%rdi,1);"
>
>
> And similar to the comment on ____kvm_handle_fault_on_reboot(), what
> about wrapping the PIE-specific version in an ifdef?
I will look into this and try your approach.
>
> > +"cmpb $0, (%rax);"
> > "setne %al;"
> > "ret;"
> > ".popsection");
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index f13a3a24d360..26abb82b1b67 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -706,12 +706,12 @@ static u32 svm_msrpm_offset(u32 msr)
> >
> > static inline void clgi(void)
> > {
> > - asm volatile (__ex("clgi"));
> > + asm volatile (__ex("clgi") : :);
> > }
> >
> > static inline void stgi(void)
> > {
> > - asm volatile (__ex("stgi"));
> > + asm volatile (__ex("stgi") : :);
> > }
> >
> > static inline void invlpga(unsigned long addr, u32 asid)
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 4341175339f3..3275761a7375 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2161,7 +2161,7 @@ static void vmclear_local_loaded_vmcss(void)
> > */
> > static void kvm_cpu_vmxoff(void)
> > {
> > - asm volatile (__ex("vmxoff"));
> > + asm volatile (__ex("vmxoff") :::);
> >
> > intel_pt_handle_vmx(0);
> > cr4_clear_bits(X86_CR4_VMXE);
> > --
> > 2.20.1.495.gaa96b0ce6b-goog
> >
Thanks for the feedback.
