Re: [PATCH] KVM: SVM: Use kzalloc for sev ioctl interfaces to prevent kernel memory leak.

[Sean Christopherson <seanjc@google.com>]

On Fri, May 13, 2022, Peter Gonda wrote:
> On Thu, May 12, 2022 at 4:23 PM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
> >
> > From: Ashish Kalra <ashish.kalra@amd.com>
> >
> > For some sev ioctl interfaces, the length parameter that is passed maybe
> > less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data
> > that PSP firmware returns. In this case, kmalloc will allocate memory
> > that is the size of the input rather than the size of the data.
> > Since PSP firmware doesn't fully overwrite the allocated buffer, these
> > sev ioctl interface may return uninitialized kernel slab memory.
> >
> > Reported-by: Andy Nguyen <theflow@google.com>
> > Suggested-by: David Rientjes <rientjes@google.com>
> > Suggested-by: Peter Gonda <pgonda@google.com>
> > Cc: kvm@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> > ---
> >  arch/x86/kvm/svm/sev.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> 
> Can we just update all the kmalloc()s that buffers get given to the
> PSP? For instance doesn't sev_send_update_data() have an issue?
> Reading the PSP spec it seems like a user can call this ioctl with a
> large hdr_len and the PSP will only fill out what's actually required
> like in these fixed up cases? This is assuming the PSP is written to
> spec (and just the current version). I'd rather have all of these
> instances updated.

Agreed, the kernel should explicitly initialize any copy_to_user() to source and
never rely on the PSP to fill the entire blob unless there's an ironclad guarantee
the entire struct/blob will be written.  E.g. it's probably ok to skip zeroing
"data" in sev_ioctl_do_platform_status(), but even then it might be wortwhile as
defense-in-depth.

Looking through other copy_to_user() calls:

  - "blob" in sev_ioctl_do_pek_csr()
  - "id_blob" in sev_ioctl_do_get_id2()
  - "pdh_blob" and "cert_blob" in sev_ioctl_do_pdh_export()

The last one is probably fine since the copy length comes from the PSP, but it's
not like these ioctls are performance critical...

	/* If we query the length, FW responded with expected data. */
	input.cert_chain_len = data.cert_chain_len;
	input.pdh_cert_len = data.pdh_cert_len;