KVM: Check validity of resolved slot when searching memslots

commit b6467ab142b708dd076f6186ca274f14af379c72 upstream.

Check that the resolved slot (somewhat confusingly named 'start') is a
valid/allocated slot before doing the final comparison to see if the
specified gfn resides in the associated slot.  The resolved slot can be
invalid if the binary search loop terminated because the search index
was incremented beyond the number of used slots.

This bug has existed since the binary search algorithm was introduced,
but went unnoticed because KVM statically allocated memory for the max
number of slots, i.e. the access would only be truly out-of-bounds if
all possible slots were allocated and the specified gfn was less than
the base of the lowest memslot.  Commit 36947254e5f98 ("KVM: Dynamically
size memslot array based on number of used slots") eliminated the "all
possible slots allocated" condition and made the bug embarrasingly easy
to hit.

Fixes: 9c1a5d38780e6 ("kvm: optimize GFN to memslot lookup with large slots amount")
Reported-by: syzbot+d889b59b2bb87d4047a2@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200408064059.8957-2-sean.j.christopherson@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ab90a8541aaaf8ff5f905edcfe6da1abafb74257..bb4af1bfcaf431957c6df9155b15b1763a54ee05 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -914,7 +914,7 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
                        start = slot + 1;
        }
 
-       if (gfn >= memslots[start].base_gfn &&
+       if (start < slots->used_slots && gfn >= memslots[start].base_gfn &&
            gfn < memslots[start].base_gfn + memslots[start].npages) {
                atomic_set(&slots->lru_slot, start);
                return &memslots[start];