Re: [PATCH 1/2] KVM: SVM: fix svn_pin_memory()'s use of get_user_pages_fast()

Vitaly Kuznetsov
2020-05-26T07:33:38Z

John Hubbard <jhubbard@nvidia.com> writes:

> 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>
> ---
> arch/x86/kvm/svm/sev.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 89f7f3aebd31..9693db1af57c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -313,7 +313,8 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
> int write)
> {
> struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> - unsigned long npages, npinned, size;
> + unsigned long npages, size;
> + int npinned;
> unsigned long locked, lock_limit;
> struct page **pages;
> unsigned long first, last;
> @@ -333,6 +334,9 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
> return NULL;
> }
>
> + if (WARN_ON_ONCE(npages > INT_MAX))
> + return NULL;
> +

I bit unrelated to this patch, but callers of sev_pin_memory() treat
NULL differently:

sev_launch_secret()/svm_register_enc_region() return -ENOMEM
sev_dbg_crypt() returns -EFAULT

Should we switch to ERR_PTR() to preserve the error?

> /* Avoid using vmalloc for smaller buffers. */
> size = npages * sizeof(struct page *);
> if (size > PAGE_SIZE)

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly