From: Sean Christopherson seanjc@google.com
commit 8646e53633f314e4d746a988240d3b951a92f94a upstream.
Invoke rseq's NOTIFY_RESUME handler when processing the flag prior to transferring to a KVM guest, which is roughly equivalent to an exit to userspace and processes many of the same pending actions. While the task cannot be in an rseq critical section as the KVM path is reachable only by via ioctl(KVM_RUN), the side effects that apply to rseq outside of a critical section still apply, e.g. the current CPU needs to be updated if the task is migrated.
Clearing TIF_NOTIFY_RESUME without informing rseq can lead to segfaults and other badness in userspace VMMs that use rseq in combination with KVM, e.g. due to the CPU ID being stale after task migration.
Fixes: 72c3c0fe54a3 ("x86/kvm: Use generic xfer to guest work function") Reported-by: Peter Foley pefoley@google.com Bisected-by: Doug Evans dje@google.com Acked-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Shakeel Butt shakeelb@google.com Cc: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20210901203030.1292304-2-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/entry/kvm.c | 4 +++- kernel/rseq.c | 14 +++++++++++--- 2 files changed, 14 insertions(+), 4 deletions(-)
--- a/kernel/entry/kvm.c +++ b/kernel/entry/kvm.c @@ -19,8 +19,10 @@ static int xfer_to_guest_mode_work(struc if (ti_work & _TIF_NEED_RESCHED) schedule();
- if (ti_work & _TIF_NOTIFY_RESUME) + if (ti_work & _TIF_NOTIFY_RESUME) { tracehook_notify_resume(NULL); + rseq_handle_notify_resume(NULL, NULL); + }
ret = arch_xfer_to_guest_mode_handle_work(vcpu, ti_work); if (ret) --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -282,9 +282,17 @@ void __rseq_handle_notify_resume(struct
if (unlikely(t->flags & PF_EXITING)) return; - ret = rseq_ip_fixup(regs); - if (unlikely(ret < 0)) - goto error; + + /* + * regs is NULL if and only if the caller is in a syscall path. Skip + * fixup and leave rseq_cs as is so that rseq_sycall() will detect and + * kill a misbehaving userspace on debug kernels. + */ + if (regs) { + ret = rseq_ip_fixup(regs); + if (unlikely(ret < 0)) + goto error; + } if (unlikely(rseq_update_cpu_id(t))) goto error; return;