x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks
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: NDmitry V. Levin <ldv@altlinux.org> Bisected-by: NMasatake YAMATO <yamato@redhat.com> Fixes: 42b933b5 ("x86/kvm/vmx: read MSR_{FS,KERNEL_GS}_BASE from current->thread") Cc: stable@vger.kernel.org Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com> Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
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