KVM: Harden guest memory APIs against out-of-bounds accesses

When reading or writing a guest page, WARN and bail if offset+len would
result in a read to a different page so that KVM bugs are more likely to
be detected, and so that any such bugs are less likely to escalate to an
out-of-bounds access.  E.g. if userspace isn't using guard pages and the
target page is at the end of a memslot.

Note, KVM already hardens itself in similar APIs, e.g. in the "cached"
variants, it's just the vanilla APIs that are playing with fire.

Link: https://lore.kernel.org/r/20240829191413.900740-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 04011b9..d51357f 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3275,6 +3275,9 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
        int r;
        unsigned long addr;
 
+       if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+               return -EFAULT;
+
        addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
        if (kvm_is_error_hva(addr))
                return -EFAULT;
@@ -3348,6 +3351,9 @@ static int __kvm_read_guest_atomic(struct kvm_memory_slot *slot, gfn_t gfn,
        int r;
        unsigned long addr;
 
+       if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+               return -EFAULT;
+
        addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
        if (kvm_is_error_hva(addr))
                return -EFAULT;
@@ -3378,6 +3384,9 @@ static int __kvm_write_guest_page(struct kvm *kvm,
        int r;
        unsigned long addr;
 
+       if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+               return -EFAULT;
+
        addr = gfn_to_hva_memslot(memslot, gfn);
        if (kvm_is_error_hva(addr))
                return -EFAULT;