[PATCH] KVM: arm64: Fix buffer overflow in kvm_arm_set_fw_reg()

[Steven Price]

On 19/04/2023 10:48, Dan Carpenter wrote:
> On Wed, Apr 19, 2023 at 09:48:41AM +0100, Steven Price wrote:
>> On 19/04/2023 09:06, Dan Carpenter wrote:
>>> The KVM_REG_SIZE() comes from the ioctl and it can be a power of two
>>> between 0-32768 but if it is more than sizeof(long) this will corrupt
>>> memory.
>>>
>>> Fixes: 99adb567632b ("KVM: arm/arm64: Add save/restore support for firmware workaround state")
>>> Signed-off-by: Dan Carpenter <dan.carpenter at linaro.org>
>>
>> Reviewed-by: Steven Price <steven.price at arm.com>
>>
>> Although there might be something to be said for rejecting anything
>> where KVM_REG_SIZE(reg->id) != sizeof(u64), as for smaller sizes the top
>> bits of val would be undefined which would require the code to mask the
>> top bits out to be safe. Given that all registers are currently u64 (and
>> I don't expect that to change), perhaps the below would be clearer?
>>
>> 	if (KVM_REG_SIZE(reg->id) != sizeof(val))
>> 		return -EINVAL;
>> 	if (copy_from_user(&val, uaddr, sizeof(val)))
>> 		return -EFAULT;
> 
> I was thinking that zero might be a valid size?

Well any value of reg->id which doesn't match in the switch statement
will cause a -ENOENT return currently, so a zero size would fail that
way as it stands. So I don't think any size other than u64 is valid in
the current code.

There is obviously a question of return value - perhaps returning
-ENOENT would be more appropriate if the size doesn't match (as a later
kernel could decide to implement registers of different sizes)?

Steve