[PATCH 5.10] KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM guest

[Sean Christopherson]

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@xxxxxxxxxx>
Bisected-by: Doug Evans <dje@xxxxxxxxxx>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20210901203030.1292304-2-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[sean: Resolve benign conflict due to unrelated access_ok() check in 5.10]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 kernel/entry/kvm.c |  4 +++-
 kernel/rseq.c      | 13 ++++++++++---
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
index b6678a5e3cf6..2a3139dab109 100644
--- a/kernel/entry/kvm.c
+++ b/kernel/entry/kvm.c
@@ -16,8 +16,10 @@ 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 a4f86a9d6937..0077713bf240 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -268,9 +268,16 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 		return;
 	if (unlikely(!access_ok(t->rseq, sizeof(*t->rseq))))
 		goto error;
-	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;
-- 
2.33.0.685.g46640cef36-goog