Re: [PATCH] KVM: s390: Fix potential spectre warnings

Eric Farman
2019-04-17T14:23:20Z

On 4/17/19 3:49 AM, David Hildenbrand wrote:
> On 17.04.19 02:54, Eric Farman wrote:
>> Fix some warnings from smatch:
>>
>> arch/s390/kvm/interrupt.c:2310 get_io_adapter() warn: potential spectre issue 'kvm->arch.adapters' [r] (local cap)
>> arch/s390/kvm/interrupt.c:2341 register_io_adapter() warn: potential spectre issue 'dev->kvm->arch.adapters' [w]
>>
>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>> ---
>> A recent patch from Paolo [1] acted as a reminder (thanks, Christian!)
>> that I had one for the s390 KVM code after some code reviews [2].
>> Let's clean that up.
>>
>> [1] https://patchwork.kernel.org/patch/10895463/
>> [2] https://patchwork.kernel.org/patch/10788565/#22484223
>> ---
>> arch/s390/kvm/interrupt.c | 11 +++++++++--
>> 1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
>> index 82162867f378..bfd55ad34a3e 100644
>> --- a/arch/s390/kvm/interrupt.c
>> +++ b/arch/s390/kvm/interrupt.c
>> @@ -14,6 +14,7 @@
>> #include <linux/kvm_host.h>
>> #include <linux/hrtimer.h>
>> #include <linux/mmu_context.h>
>> +#include <linux/nospec.h>
>> #include <linux/signal.h>
>> #include <linux/slab.h>
>> #include <linux/bitmap.h>
>> @@ -2307,6 +2308,7 @@ static struct s390_io_adapter *get_io_adapter(struct kvm *kvm, unsigned int id)
>> {
>> if (id >= MAX_S390_IO_ADAPTERS)
>> return NULL;
>> + id = array_index_nospec(id, MAX_S390_IO_ADAPTERS);
>> return kvm->arch.adapters[id];
>
> return kvm->arch.adapters[array_index_nospec(id, MAX_S390_IO_ADAPTERS)];
>
> should exactly fit into a single line if I am not wrong.

Yeah, just. As Paolo pointed out, that's not common usage. Though of
the four other hits I see, only one of them is the same as this
instance, in that "id" is passed as a variable and then we immediately
return with an array entry (even if NULL) rather than doing something
else in that function. So maybe all-in-one-line here is a little cleaner.

>
>> }
>>
>> @@ -2320,8 +2322,13 @@ static int register_io_adapter(struct kvm_device *dev,
>> (void __user *)attr->addr, sizeof(adapter_info)))
>> return -EFAULT;
>>
>> - if ((adapter_info.id >= MAX_S390_IO_ADAPTERS) ||
>> - (dev->kvm->arch.adapters[adapter_info.id] != NULL))
>> + if (adapter_info.id >= MAX_S390_IO_ADAPTERS)
>> + return -EINVAL;
>> +
>> + adapter_info.id = array_index_nospec(adapter_info.id,
>> + MAX_S390_IO_ADAPTERS);
>
> I dislike that we are modifying adapter_info here. Can you use a local
> variable instead?

I guess, but adapter_info is a local variable too. So sanitization this
way seems fine to me. But if you dislike it more than I don't care,
I'll add another local variable. :)

>
>> +
>> + if (dev->kvm->arch.adapters[adapter_info.id] != NULL)
>> return -EINVAL;
>>
>> adapter = kzalloc(sizeof(*adapter), GFP_KERNEL);
>>
>