From 2c2ca2d12f5c7b7a961555029dcddd1154cc98e5 Mon Sep 17 00:00:00 2001 From: Nadav Amit Date: Sun, 2 Nov 2014 11:54:57 +0200 Subject: [PATCH] KVM: x86: Remove redundant and incorrect cpl check on task-switch Task-switch emulation checks the privilege level prior to performing the task-switch. This check is incorrect in the case of task-gates, in which the tss.dpl is ignored, and can cause superfluous exceptions. Moreover this check is unnecassary, since the CPU checks the privilege levels prior to exiting. Intel SDM 25.4.2 says "If CALL or JMP accesses a TSS descriptor directly outside IA-32e mode, privilege levels are checked on the TSS descriptor" prior to exiting. AMD 15.14.1 says "The intercept is checked before the task switch takes place but after the incoming TSS and task gate (if one was involved) have been checked for correctness." This patch removes the CPL checks for CALL and JMP. Signed-off-by: Nadav Amit Signed-off-by: Paolo Bonzini --- arch/x86/kvm/emulate.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index fab270801809..765f1897814b 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -2824,7 +2824,8 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt, * * 1. jmp/call/int to task gate: Check against DPL of the task gate * 2. Exception/IRQ/iret: No check is performed - * 3. jmp/call to TSS: Check against DPL of the TSS + * 3. jmp/call to TSS/task-gate: No check is performed since the + * hardware checks it before exiting. */ if (reason == TASK_SWITCH_GATE) { if (idt_index != -1) { @@ -2841,13 +2842,8 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt, if ((tss_selector & 3) > dpl || ops->cpl(ctxt) > dpl) return emulate_gp(ctxt, (idt_index << 3) | 0x2); } - } else if (reason != TASK_SWITCH_IRET) { - int dpl = next_tss_desc.dpl; - if ((tss_selector & 3) > dpl || ops->cpl(ctxt) > dpl) - return emulate_gp(ctxt, tss_selector); } - desc_limit = desc_limit_scaled(&next_tss_desc); if (!next_tss_desc.p || ((desc_limit < 0x67 && (next_tss_desc.type & 8)) || -- 2.20.1