KVM: x86: fix singlestepping over syscall
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.
Cc: stable@vger.kernel.org
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0816ab2..80890de 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2742,6 +2742,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF);
}
+ ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
return X86EMUL_CONTINUE;
}