KVM: x86: fix singlestepping over syscall

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 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 <ben@decadent.org.uk>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 35f8884817fa..3ef118df2547 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2310,6 +2310,7 @@ 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;
 }