commit c8401dda 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>