KVM: SVM: fix svn_pin_memory()'s use of get_user_pages_fast()

KVM: SVM: fix svn_pin_memory()'s use of get_user_pages_fast()

There are two problems in svn_pin_memory():

1) The return value of get_user_pages_fast() is stored in an
unsigned long, although the declared return value is of type int.
This will not cause any symptoms, but it is misleading.
Fix this by changing the type of npinned to "int".

2) The number of pages passed into get_user_pages_fast() is stored
in an unsigned long, even though get_user_pages_fast() accepts an
int. This means that it is possible to silently overflow the number
of pages.

Fix this by adding a WARN_ON_ONCE() and an early error return. The
npages variable is left as an unsigned long for convenience in
checking for overflow.

Fixes: 89c505809052 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_UPDATE_DATA command")
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: x86@kernel.org
Cc: kvm@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Message-Id: <20200526062207.1360225-2-jhubbard@nvidia.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>