Re: [PATCH v5.5 02/30] KVM: Disallow user memslot with size that exceeds "unsigned long"

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On 04.11.2021 01:25, Sean Christopherson wrote:
Reject userspace memslots whose size exceeds the storage capacity of an
"unsigned long".  KVM's uAPI takes the size as u64 to support large slots
on 64-bit hosts, but does not account for the size being truncated on
32-bit hosts in various flows.  The access_ok() check on the userspace
virtual address in particular casts the size to "unsigned long" and will
check the wrong number of bytes.

KVM doesn't actually support slots whose size doesn't fit in an "unsigned
long", e.g. KVM's internal kvm_memory_slot.npages is an "unsigned long",
not a "u64", and misc arch specific code follows that behavior.

Fixes: fa3d315a4ce2 ("KVM: Validate userspace_addr of memslot when registered")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
  virt/kvm/kvm_main.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 99e69375c4c9..83287730389f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1689,7 +1689,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
  	id = (u16)mem->slot;
/* General sanity checks */
-	if (mem->memory_size & (PAGE_SIZE - 1))
+	if ((mem->memory_size & (PAGE_SIZE - 1)) ||
+	    (mem->memory_size != (unsigned long)mem->memory_size))
  		return -EINVAL;
  	if (mem->guest_phys_addr & (PAGE_SIZE - 1))
  		return -EINVAL;

Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>

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