From 336dabf9938641377d7ee84da1a3fd18c7fba8b4 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Wed, 1 Sep 2021 13:30:26 -0700
Subject: [PATCH] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest
Git-commit: 8646e53633f314e4d746a988240d3b951a92f94a
Patch-mainline: v5.15-rc3
References: stable-5.14.9

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 kernel/entry/kvm.c |  4 +++-
 kernel/rseq.c      | 14 +++++++++++---
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index 49972ee99aff..049fd06b4c3d 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -19,8 +19,10 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
 		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)
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 35f7bd0fced0..6d45ac3dae7f 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -282,9 +282,17 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 
 	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;
-- 
2.26.2