[SRU][T][PATCH 0/1] CVE-2017-7518 - Incorrect debug exception emulation

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[SRU][T][PATCH 0/1] CVE-2017-7518 - Incorrect debug exception emulation

Tyler Hicks-2
https://people.canonical.com/~ubuntu-security/cve/2017/CVE-2017-7518.html

 Andy Lutomirski discovered that the KVM implementation in the Linux kernel
 was vulnerable to a debug exception error when single-stepping through a
 syscall. A local attacker in a non-Linux guest vm could possibly use this
 to gain administrative privileges in the guest vm.

This is a cherry-picked patch from the 3.16 linux-stable tree.

Tyler


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[SRU][T][PATCH 1/1] KVM: x86: fix singlestepping over syscall

Tyler Hicks-2
From: Paolo Bonzini <[hidden email]>

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 <[hidden email]>
Signed-off-by: Paolo Bonzini <[hidden email]>
Signed-off-by: Radim Krčmář <[hidden email]>
[bwh: Backported to 3.16:
 - kvm_vcpu_check_singlestep() did not take an rflags parameter but
   called get_rflags() itself; delete that code
 - kvm_vcpu_check_singlestep() sets some flags differently
 - Drop changes to kvm_skip_emulated_instruction()]
Signed-off-by: Ben Hutchings <[hidden email]>

CVE-2017-7518

(cherry picked from commit 24ee2a286de7ac680ad4b2423c2dcee68444e567 linux-stable)
Signed-off-by: Tyler Hicks <[hidden email]>
---
 arch/x86/include/asm/kvm_emulate.h |  1 +
 arch/x86/kvm/emulate.c             |  1 +
 arch/x86/kvm/x86.c                 | 53 ++++++++++++++++----------------------
 3 files changed, 24 insertions(+), 31 deletions(-)

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index 24ec1216596e..71a1399903cc 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -275,6 +275,7 @@ struct x86_emulate_ctxt {
  bool guest_mode; /* guest running a nested guest */
  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;
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 29524529d3f5..fda743285155 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2274,6 +2274,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
  ctxt->eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF);
  }
 
+ ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
  return X86EMUL_CONTINUE;
 }
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f1c15ad80219..4ec7f1ee6357 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4866,6 +4866,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
  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 :
@@ -5056,38 +5058,26 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
  return dr6;
 }
 
-static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, int *r)
+static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
 {
  struct kvm_run *kvm_run = vcpu->run;
 
- /*
- * Use the "raw" value to see if TF was passed to the processor.
- * Note that the new value of the flags 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".
- */
- unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
-
- if (unlikely(rflags & X86_EFLAGS_TF)) {
- if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
- kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
- 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;
- kvm_queue_exception(vcpu, DB_VECTOR);
- }
+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+ kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
+ 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;
+ kvm_queue_exception(vcpu, DB_VECTOR);
  }
 }
 
@@ -5240,8 +5230,9 @@ restart:
  kvm_make_request(KVM_REQ_EVENT, vcpu);
  vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
  kvm_rip_write(vcpu, ctxt->eip);
- if (r == EMULATE_DONE)
- kvm_vcpu_check_singlestep(vcpu, &r);
+ if (r == EMULATE_DONE &&
+    (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
+ kvm_vcpu_do_singlestep(vcpu, &r);
  kvm_set_rflags(vcpu, ctxt->eflags);
  } else
  vcpu->arch.emulate_regs_need_sync_to_vcpu = true;
--
2.7.4


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