Re: [PATCH] KVM: X86: prevent integer overflows in KVM_MEMORY_ENCRYPT_REG_REGION

Brijesh Singh
2018-05-21T15:52:55Z

Hi Dan,

On 05/19/2018 01:01 AM, Dan Carpenter wrote:
> This is a fix from reviewing the code, but it looks like it might be
> able to lead to an Oops. It affects 32bit systems.
>

Please note that SEV is not available on 32bit systems.

> The KVM_MEMORY_ENCRYPT_REG_REGION ioctl uses a u64 for range->addr and
> range->size but the high 32 bits would be truncated away on a 32 bit
> system. This is harmless but it's also harmless to prevent it.
>
> Then in sev_pin_memory() the "uaddr + ulen" calculation can wrap around.
> The wrap around can happen on 32 bit or 64 bit systems, but I was only
> able to figure out a problem for 32 bit systems. We would pick a number
> which results in "npages" being zero. The sev_pin_memory() would then
> return ZERO_SIZE_PTR without allocating anything.
>
> I made it illegal to call sev_pin_memory() with "ulen" set to zero.
> Hopefully, that doesn't cause any problems.

I think this should be fine.

I also changed the type of
> "first" and "last" to long, just for cosmetic reasons. Otherwise on a
> 64 bit system you're saving "uaddr >> 12" in an int and it truncates the
> high 20 bits away. The math works in the current code so far as I can
> see but it's just weird.
>

This change looks good. thanks

> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>

> ---
> Again, this is a static checker fix. The most risky parts of this
> patch are blocking "ulen == 0" and changing the types of "first" and
> "last". I felt like those changes made the math easier to understand
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 220e5a89465a..de21d5c5168b 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1762,7 +1762,10 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
> unsigned long npages, npinned, size;
> unsigned long locked, lock_limit;
> struct page **pages;
> - int first, last;
> + unsigned long first, last;
> +
> + if (ulen == 0 || uaddr + ulen < uaddr)
> + return NULL;
>
> /* Calculate number of pages. */
> first = (uaddr & PAGE_MASK) >> PAGE_SHIFT;
> @@ -6925,6 +6928,9 @@ static int svm_register_enc_region(struct kvm *kvm,
> if (!sev_guest(kvm))
> return -ENOTTY;
>
> + if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
> + return -EINVAL;
> +
> region = kzalloc(sizeof(*region), GFP_KERNEL);
> if (!region)
> return -ENOMEM;
>