Re: [bug report] KVM: SVM: prevent DBG_DECRYPT and DBG_ENCRYPT overflow

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 29, 2021, Dan Carpenter wrote:
> Hello David Rientjes,
> 
> The patch b86bc2858b38: "KVM: SVM: prevent DBG_DECRYPT and
> DBG_ENCRYPT overflow" from Mar 25, 2019, leads to the following
> static checker warning:
> 
> 	arch/x86/kvm/svm/sev.c:960 sev_dbg_crypt()
> 	error: uninitialized symbol 'ret'.
> 
> arch/x86/kvm/svm/sev.c
>    879  static int sev_dbg_crypt(struct kvm *kvm, struct kvm_sev_cmd *argp, bool dec)
>    880  {
>    881          unsigned long vaddr, vaddr_end, next_vaddr;
>                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
>    882          unsigned long dst_vaddr;
>                 ^^^^^^^^^^^^^^^^^^^^^^^^
> 
> These are unsigned long
> 
>    883          struct page **src_p, **dst_p;
>    884          struct kvm_sev_dbg debug;
>    885          unsigned long n;
>    886          unsigned int size;
>    887          int ret;
>    888  
>    889          if (!sev_guest(kvm))
>    890                  return -ENOTTY;
>    891  
>    892          if (copy_from_user(&debug, (void __user *)(uintptr_t)argp->data, sizeof(debug)))
>    893                  return -EFAULT;
>    894  
>    895          if (!debug.len || debug.src_uaddr + debug.len < debug.src_uaddr)
>                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> But these are u64 so this could still overflow on 32 bit.  Do we care?

Not really.  sev_guest() will always be false for CONFIG_KVM_AMD_SEV=n, and
CONFIG_KVM_AMD_SEV is dependent on CONFIG_X86_64=y.  This code is compiled for
32-bit only because everyone has been too lazy to stub out sev.c.