From db153db08846d038a6fd23fee7719dfee5d27ffc Mon Sep 17 00:00:00 2001 From: Jan Beulich Date: Wed, 23 Nov 2016 15:25:35 +0100 Subject: [PATCH] x86/HVM: limit writes to incoming TSS during task switch The only field modified (and even that conditionally) is the back link. Write only that field, and only when it actually has been written to. Take the opportunity and also ditch the pointless initializer from the "tss" local variable, which gets completely filled anyway by reading from guest memory. Signed-off-by: Jan Beulich Reviewed-by: Andrew Cooper Release-acked-by: Wei Liu --- xen/arch/x86/hvm/hvm.c | 26 +++++++++++++------------- 1 file changed, 13 insertions(+), 13 deletions(-) diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c index 1f9666a533..bde7640b1d 100644 --- a/xen/arch/x86/hvm/hvm.c +++ b/xen/arch/x86/hvm/hvm.c @@ -2890,7 +2890,7 @@ void hvm_task_switch( u32 cr3, eip, eflags, eax, ecx, edx, ebx, esp, ebp, esi, edi; u16 es, _3, cs, _4, ss, _5, ds, _6, fs, _7, gs, _8, ldt, _9; u16 trace, iomap; - } tss = { 0 }; + } tss; hvm_get_segment_register(v, x86_seg_gdtr, &gdt); hvm_get_segment_register(v, x86_seg_tr, &prev_tr); @@ -3010,12 +3010,6 @@ void hvm_task_switch( regs->esi = tss.esi; regs->edi = tss.edi; - if ( (taskswitch_reason == TSW_call_or_int) ) - { - regs->eflags |= X86_EFLAGS_NT; - tss.back_link = prev_tr.sel; - } - exn_raised = 0; if ( hvm_load_segment_selector(x86_seg_es, tss.es, tss.eflags) || hvm_load_segment_selector(x86_seg_cs, tss.cs, tss.eflags) || @@ -3025,12 +3019,18 @@ void hvm_task_switch( hvm_load_segment_selector(x86_seg_gs, tss.gs, tss.eflags) ) exn_raised = 1; - rc = hvm_copy_to_guest_virt( - tr.base, &tss, sizeof(tss), PFEC_page_present); - if ( rc == HVMCOPY_bad_gva_to_gfn ) - exn_raised = 1; - else if ( rc != HVMCOPY_okay ) - goto out; + if ( taskswitch_reason == TSW_call_or_int ) + { + regs->eflags |= X86_EFLAGS_NT; + tss.back_link = prev_tr.sel; + + rc = hvm_copy_to_guest_virt(tr.base + offsetof(typeof(tss), back_link), + &tss.back_link, sizeof(tss.back_link), 0); + if ( rc == HVMCOPY_bad_gva_to_gfn ) + exn_raised = 1; + else if ( rc != HVMCOPY_okay ) + goto out; + } if ( (tss.trace & 1) && !exn_raised ) hvm_inject_hw_exception(TRAP_debug, HVM_DELIVER_NO_ERROR_CODE); -- 2.39.5