[PATCH v13 08/35] KVM: Introduce KVM_SET_USER_MEMORY_REGION2

[Paolo Bonzini]

On 10/30/23 21:25, Sean Christopherson wrote:
> On Mon, Oct 30, 2023, Paolo Bonzini wrote:
>> On 10/27/23 20:21, Sean Christopherson wrote:
>>>
>>> +		if (ioctl == KVM_SET_USER_MEMORY_REGION)
>>> +			size = sizeof(struct kvm_userspace_memory_region);
>>
>> This also needs a memset(&mem, 0, sizeof(mem)), otherwise the out-of-bounds
>> access of the commit message becomes a kernel stack read.
> 
> Ouch.  There's some irony.  Might be worth doing memset(&mem, -1, sizeof(mem))
> though as '0' is a valid file descriptor and a valid file offset.

Either is okay, because unless the flags check is screwed up it should
not matter.  The memset is actually unnecessary, though it may be a good
idea anyway to keep it, aka belt-and-suspenders.

>> Probably worth adding a check on valid flags here.
> 
> Definitely needed.  There's a very real bug here.  But rather than duplicate flags
> checking or plumb @ioctl all the way to __kvm_set_memory_region(), now that we
> have the fancy guard(mutex) and there are no internal calls to kvm_set_memory_region(),
> what if we:
> 
>    1. Acquire/release slots_lock in __kvm_set_memory_region()
>    2. Call kvm_set_memory_region() from x86 code for the internal memslots
>    3. Disallow *any* flags for internal memslots
>    4. Open code check_memory_region_flags in kvm_vm_ioctl_set_memory_region()

I dislike this step, there is a clear point where all paths meet
(ioctl/internal, locked/unlocked) and that's __kvm_set_memory_region().
I think that's the place where flags should be checked.  (I don't mind
the restriction on internal memslots; it's just that to me it's not a
particularly natural way to structure the checks).

On the other hand, the place where to protect from out-of-bounds
accesses, is the place where you stop caring about struct
kvm_userspace_memory_region vs kvm_userspace_memory_region2 (and
your code gets it right, by dropping "ioctl" as soon as possible).

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 87f45aa91ced..fe5a2af14fff 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1635,6 +1635,14 @@ bool __weak kvm_arch_dirty_log_supported(struct kvm *kvm)
  	return true;
  }
  
+/*
+ * Flags that do not access any of the extra space of struct
+ * kvm_userspace_memory_region2.  KVM_SET_USER_MEMORY_REGION_FLAGS
+ * only allows these.
+ */
+#define KVM_SET_USER_MEMORY_REGION_FLAGS \
+	(KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY)
+
  static int check_memory_region_flags(struct kvm *kvm,
  				     const struct kvm_userspace_memory_region2 *mem)
  {
@@ -5149,10 +5149,16 @@ static long kvm_vm_ioctl(struct file *filp,
  		struct kvm_userspace_memory_region2 mem;
  		unsigned long size;
  
-		if (ioctl == KVM_SET_USER_MEMORY_REGION)
+		if (ioctl == KVM_SET_USER_MEMORY_REGION) {
+			/*
+			 * Fields beyond struct kvm_userspace_memory_region shouldn't be
+			 * accessed, but avoid leaking kernel memory in case of a bug.
+			 */
+			memset(&mem, 0, sizeof(mem));
  			size = sizeof(struct kvm_userspace_memory_region);
-		else
+		} else {
  			size = sizeof(struct kvm_userspace_memory_region2);
+		}
  
  		/* Ensure the common parts of the two structs are identical. */
  		SANITY_CHECK_MEM_REGION_FIELD(slot);
@@ -5165,6 +5167,11 @@ static long kvm_vm_ioctl(struct file *filp,
  		if (copy_from_user(&mem, argp, size))
  			goto out;
  
+		r = -EINVAL;
+		if (ioctl == KVM_SET_USER_MEMORY_REGION &&
+		    (mem->flags & ~KVM_SET_USER_MEMORY_REGION_FLAGS))
+			goto out;
+
  		r = kvm_vm_ioctl_set_memory_region(kvm, &mem);
  		break;
  	}


That's a kind of patch that you can't really get wrong (though I have
the brown paper bag ready).

Maintainance-wise it's fine, since flags are being added at a pace of
roughly one every five years, and anyway it's also future proof: I placed
the #define near check_memory_region_flags so that in five years we remember
to keep it up to date.  But worst case, the new flags will only be allowed
by KVM_SET_USER_MEMORY_REGION2 unnecessarily; there are no security issues
waiting to bite us.

In sum, this is exactly the only kind of fix that should be in the v13->v14
delta.

Paolo