From a7315d2f3c6c4ff37b27c652dba70939d94d436c Mon Sep 17 00:00:00 2001 From: Nadav Amit Date: Wed, 19 Nov 2014 17:43:10 +0200 Subject: [PATCH] KVM: x86: Emulator performs privilege checks on __linearize When segment is accessed, real hardware does not perform any privilege level checks. In contrast, KVM emulator does. This causes some discrepencies from real hardware. For instance, reading from readable code segment may fail due to incorrect segment checks. In addition, it introduces unnecassary overhead. To reference Intel SDM 5.5 ("Privilege Levels"): "Privilege levels are checked when the segment selector of a segment descriptor is loaded into a segment register." The SDM never mentions privilege level checks during memory access, except for loading far pointers in section 5.10 ("Pointer Validation"). Those are actually segment selector loads and are emulated in the similarily (i.e., regardless to __linearize checks). This behavior was also checked using sysexit. A data-segment whose DPL=0 was loaded, and after sysexit (CPL=3) it is still accessible. Therefore, all the privilege level checks in __linearize are removed. Signed-off-by: Nadav Amit Signed-off-by: Paolo Bonzini --- arch/x86/kvm/emulate.c | 15 --------------- 1 file changed, 15 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index 1317560d0823..d9461e47a65d 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -663,7 +663,6 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt, ulong la; u32 lim; u16 sel; - unsigned cpl; la = seg_base(ctxt, addr.seg) + addr.ea; *max_size = 0; @@ -705,20 +704,6 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt, } if (size > *max_size) goto bad; - cpl = ctxt->ops->cpl(ctxt); - if (!fetch) { - /* data segment or readable code segment */ - if (cpl > desc.dpl) - goto bad; - } else if ((desc.type & 8) && !(desc.type & 4)) { - /* nonconforming code segment */ - if (cpl != desc.dpl) - goto bad; - } else if ((desc.type & 8) && (desc.type & 4)) { - /* conforming code segment */ - if (cpl < desc.dpl) - goto bad; - } break; } if (ctxt->mode != X86EMUL_MODE_PROT64) -- 2.20.1