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 <gkirkpatrick@google.com>
Cc: Josh Eads <josheads@google.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20240217013430.2079561-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f760106c31f8a58d2941dbabd82531b9779089fa..a132547fcfb52ffba57eb9c18111b0822004000e 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1975,20 +1975,22 @@ int sev_mem_enc_register_region(struct kvm *kvm,
                goto e_free;
        }
 
-       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.
+        * 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);
+
        return ret;
 
 e_free: