Commit b062b794 authored Jul 11, 2018 by Vitaly Kuznetsov Committed by Paolo Bonzini Jul 15, 2018

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: Dmitry V. Levin <ldv@altlinux.org>
Bisected-by: Masatake 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: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent cd283252

arch/x86/kvm/vmx.c

+17 −8

Original line number	Diff line number	Diff line
		@@ -2365,6 +2365,7 @@ 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 fs_base, kernel_gs_base;
		#endif
		int i;

		@@ -2380,12 +2381,20 @@ 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
		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;
		#else
		fs_base = current->thread.fsbase;
		kernel_gs_base = current->thread.gsbase;
		} else {
		#endif
		savesegment(fs, vmx->host_state.fs_sel);
		savesegment(gs, vmx->host_state.gs_sel);
		#ifdef CONFIG_X86_64
		fs_base = read_msr(MSR_FS_BASE);
		kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE);
		}
		#endif
		if (!(vmx->host_state.fs_sel & 7)) {
		vmcs_write16(HOST_FS_SELECTOR, vmx->host_state.fs_sel);
		@@ -2405,10 +2414,10 @@ 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, fs_base);
		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_gs_base;
		if (is_long_mode(&vmx->vcpu))
		wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
		#else