KVM: SVM: Flush pages under kvm->lock to fix UAF in svm_register_enc_region()

Do the cache flush of converted pages in svm_register_enc_region() before
dropping kvm->lock to fix use-after-free issues where region and/or its
array of pages could be freed by a different task, e.g. if userspace has
__unregister_enc_region_locked() already queued up for the region.

Note, the "obvious" alternative of using local variables doesn't fully
resolve the bug, as region->pages is also dynamically allocated. I.e. the
region structure itself would be fine, but region->pages could be freed.

Flushing multiple pages under kvm->lock is unfortunate, but the entire
flow is a rare slow path, and the manual flush is only needed on CPUs that
lack coherency for encrypted memory.

Fixes: 19a23da53932 ("Fix unsynchronized access to sev members through svm_register_enc_region")
Reported-by: Gabe Kirkpatrick <[e-mail]>
Cc: Josh Eads <[e-mail]>
Cc: Peter Gonda <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/svm/sev.c

···
 		goto e_free;
 	}
 
+	/*
+	 * The guest may change the memory encryption attribute from C=0 -> C=1
+	 * or vice versa for this memory range. Lets make sure caches are
+	 * flushed to ensure that guest data gets written into memory with
+	 * correct C-bit.  Note, this must be done before dropping kvm->lock,
+	 * as region and its array of pages can be freed by a different task
+	 * once kvm->lock is released.
+	 */
+	sev_clflush_pages(region->pages, region->npages);
+
 	region->uaddr = range->addr;
 	region->size = range->size;
 
 	list_add_tail(&region->list, &sev->regions_list);
 	mutex_unlock(&kvm->lock);
-
-	/*
-	 * The guest may change the memory encryption attribute from C=0 -> C=1
-	 * or vice versa for this memory range. Lets make sure caches are
-	 * flushed to ensure that guest data gets written into memory with
-	 * correct C-bit.
-	 */
-	sev_clflush_pages(region->pages, region->npages);
 
 	return ret;