This is a note to let you know that I've just added the patch titled
KVM: x86: fix singlestepping over syscall
to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: kvm-x86-fix-singlestepping-over-syscall.patch and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Sun Nov 19 11:12:05 CET 2017
From: Paolo Bonzini pbonzini@redhat.com Date: Wed, 7 Jun 2017 15:13:14 +0200 Subject: KVM: x86: fix singlestepping over syscall
From: Paolo Bonzini pbonzini@redhat.com
commit c8401dda2f0a00cd25c0af6a95ed50e478d25de4 upstream.
TF is handled a bit differently for syscall and sysret, compared to the other instructions: TF is checked after the instruction completes, so that the OS can disable #DB at a syscall by adding TF to FMASK. When the sysret is executed the #DB is taken "as if" the syscall insn just completed.
KVM emulates syscall so that it can trap 32-bit syscall on Intel processors. Fix the behavior, otherwise you could get #DB on a user stack which is not nice. This does not affect Linux guests, as they use an IST or task gate for #DB.
This fixes CVE-2017-7518.
Reported-by: Andy Lutomirski luto@kernel.org Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Radim Krčmář rkrcmar@redhat.com [bwh: Backported to 4.4: - kvm_vcpu_check_singlestep() sets some flags differently - Drop changes to kvm_skip_emulated_instruction()] Signed-off-by: Ben Hutchings ben.hutchings@codethink.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/kvm_emulate.h | 1 arch/x86/kvm/emulate.c | 1 arch/x86/kvm/x86.c | 52 +++++++++++++++---------------------- 3 files changed, 24 insertions(+), 30 deletions(-)
--- a/arch/x86/include/asm/kvm_emulate.h +++ b/arch/x86/include/asm/kvm_emulate.h @@ -296,6 +296,7 @@ struct x86_emulate_ctxt {
bool perm_ok; /* do not check permissions if true */ bool ud; /* inject an #UD if host doesn't support insn */ + bool tf; /* TF value before instruction (after for syscall/sysret) */
bool have_exception; struct x86_exception exception; --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -2726,6 +2726,7 @@ static int em_syscall(struct x86_emulate ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF); }
+ ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0; return X86EMUL_CONTINUE; }
--- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5095,6 +5095,8 @@ static void init_emulate_ctxt(struct kvm kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
ctxt->eflags = kvm_get_rflags(vcpu); + ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0; + ctxt->eip = kvm_rip_read(vcpu); ctxt->mode = (!is_protmode(vcpu)) ? X86EMUL_MODE_REAL : (ctxt->eflags & X86_EFLAGS_VM) ? X86EMUL_MODE_VM86 : @@ -5315,37 +5317,26 @@ static int kvm_vcpu_check_hw_bp(unsigned return dr6; }
-static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r) +static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r) { struct kvm_run *kvm_run = vcpu->run;
- /* - * rflags is the old, "raw" value of the flags. The new value has - * not been saved yet. - * - * This is correct even for TF set by the guest, because "the - * processor will not generate this exception after the instruction - * that sets the TF flag". - */ - if (unlikely(rflags & X86_EFLAGS_TF)) { - if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) { - kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | - DR6_RTM; - kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip; - kvm_run->debug.arch.exception = DB_VECTOR; - kvm_run->exit_reason = KVM_EXIT_DEBUG; - *r = EMULATE_USER_EXIT; - } else { - vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF; - /* - * "Certain debug exceptions may clear bit 0-3. The - * remaining contents of the DR6 register are never - * cleared by the processor". - */ - vcpu->arch.dr6 &= ~15; - vcpu->arch.dr6 |= DR6_BS | DR6_RTM; - kvm_queue_exception(vcpu, DB_VECTOR); - } + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) { + kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM; + kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip; + kvm_run->debug.arch.exception = DB_VECTOR; + kvm_run->exit_reason = KVM_EXIT_DEBUG; + *r = EMULATE_USER_EXIT; + } else { + vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF; + /* + * "Certain debug exceptions may clear bit 0-3. The + * remaining contents of the DR6 register are never + * cleared by the processor". + */ + vcpu->arch.dr6 &= ~15; + vcpu->arch.dr6 |= DR6_BS | DR6_RTM; + kvm_queue_exception(vcpu, DB_VECTOR); } }
@@ -5500,8 +5491,9 @@ restart: toggle_interruptibility(vcpu, ctxt->interruptibility); vcpu->arch.emulate_regs_need_sync_to_vcpu = false; kvm_rip_write(vcpu, ctxt->eip); - if (r == EMULATE_DONE) - kvm_vcpu_check_singlestep(vcpu, rflags, &r); + if (r == EMULATE_DONE && + (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP))) + kvm_vcpu_do_singlestep(vcpu, &r); if (!ctxt->have_exception || exception_type(ctxt->exception.vector) == EXCPT_TRAP) __kvm_set_rflags(vcpu, ctxt->eflags);
Patches currently in stable-queue which might be from pbonzini@redhat.com are
queue-4.4/kvm-x86-fix-singlestepping-over-syscall.patch