Re: [PATCH] x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks

Sean Christopherson
2018-07-13T16:46:54Z

On Wed, Jul 11, 2018 at 07:37:18PM +0200, Vitaly Kuznetsov wrote:
> When we switched from doing rdmsr() to reading FS/GS base values from
> current->thread we completely forgot about legacy 32-bit userspaces which
> we still support in KVM (why?). task->thread.{fsbase,gsbase} are only
> synced for 64-bit processes, calling save_fsgs_for_kvm() and using
> its result from current is illegal for legacy processes.
>
> There's no ARCH_SET_FS/GS prctls for legacy applications. Base MSRs are,
> however, not always equal to zero. Intel's manual says (3.4.4 Segment
> Loading Instructions in IA-32e Mode):
>
> "In order to set up compatibility mode for an application, segment-load
> instructions (MOV to Sreg, POP Sreg) work normally in 64-bit mode. An
> entry is read from the system descriptor table (GDT or LDT) and is loaded
> in the hidden portion of the segment register.
> ...
> The hidden descriptor register fields for FS.base and GS.base are
> physically mapped to MSRs in order to load all address bits supported by
> a 64-bit implementation.
> "
>
> The issue was found by strace test suite where 32-bit ioctl_kvm_run test
> started segfaulting.
>
> Reported-by: Dmitry V. Levin <ldv@altlinux.org>
> Bisected-by: Masatake YAMATO <yamato@redhat.com>
> Fixes: 42b933b59721 ("x86/kvm/vmx: read MSR_{FS,KERNEL_GS}_BASE from current->thread")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 25 +++++++++++++++++--------
> 1 file changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 559a12b6184d..65968649b365 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2560,6 +2560,7 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> #ifdef CONFIG_X86_64
> int cpu = raw_smp_processor_id();
> + unsigned long fsbase, kernel_gsbase;

Because bikeshedding is fun, what do you think about using fs_base and
kernel_gs_base for these names? I have a series that touches this
code and also adds local variables for {FS,GS}.base and {FS,GS}.sel.
I used {fs,gs}_base and {fs,gs}_sel to be consistent with the
vmx->host_state nomenclature (the local variables are used to update
the associated vmx->host_state variables), but I'll change my patches
if you have a strong preference for omitting the underscore.

> #endif
> int i;
>
> @@ -2575,12 +2576,20 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
> vmx->host_state.gs_ldt_reload_needed = vmx->host_state.ldt_sel;
>
> #ifdef CONFIG_X86_64
> - save_fsgs_for_kvm();
> - vmx->host_state.fs_sel = current->thread.fsindex;
> - vmx->host_state.gs_sel = current->thread.gsindex;
> -#else
> - savesegment(fs, vmx->host_state.fs_sel);
> - savesegment(gs, vmx->host_state.gs_sel);
> + if (likely(is_64bit_mm(current->mm))) {
> + save_fsgs_for_kvm();
> + vmx->host_state.fs_sel = current->thread.fsindex;
> + vmx->host_state.gs_sel = current->thread.gsindex;
> + fsbase = current->thread.fsbase;
> + kernel_gsbase = current->thread.gsbase;
> + } else {
> +#endif
> + savesegment(fs, vmx->host_state.fs_sel);
> + savesegment(gs, vmx->host_state.gs_sel);
> +#ifdef CONFIG_X86_64
> + fsbase = read_msr(MSR_FS_BASE);
> + kernel_gsbase = read_msr(MSR_KERNEL_GS_BASE);
> + }
> #endif
> if (!(vmx->host_state.fs_sel & 7)) {
> vmcs_write16(HOST_FS_SELECTOR, vmx->host_state.fs_sel);
> @@ -2600,10 +2609,10 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
> savesegment(ds, vmx->host_state.ds_sel);
> savesegment(es, vmx->host_state.es_sel);
>
> - vmcs_writel(HOST_FS_BASE, current->thread.fsbase);
> + vmcs_writel(HOST_FS_BASE, fsbase);
> vmcs_writel(HOST_GS_BASE, cpu_kernelmode_gs_base(cpu));
>
> - vmx->msr_host_kernel_gs_base = current->thread.gsbase;
> + vmx->msr_host_kernel_gs_base = kernel_gsbase;
> if (is_long_mode(&vmx->vcpu))
> wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
> #else
> --
> 2.14.4
>