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

Dan Carpenter
2018-05-19T06:02:22Z

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.

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 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.

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