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

[Peter Gonda]

On Fri, May 13, 2022 at 2:11 PM Ashish Kalra <ashkalra@xxxxxxx> wrote:
>
> Hello Sean & Peter,
>
> On 5/13/22 14:49, Sean Christopherson wrote:
> > On Fri, May 13, 2022, Peter Gonda wrote:
> >> On Thu, May 12, 2022 at 4:23 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
> >>> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> >>>
> >>> 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@xxxxxxxxxx>
> >>> Suggested-by: David Rientjes <rientjes@xxxxxxxxxx>
> >>> Suggested-by: Peter Gonda <pgonda@xxxxxxxxxx>
> >>> Cc: kvm@xxxxxxxxxxxxxxx
> >>> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >>> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> >>> ---
> >>>   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.
>
> Yes, this function is also vulnerable as it allocates the return buffer
> using kmalloc() and copies back to user the buffer sized as per the user
> provided length (and not the FW returned length), so it surely needs fixup.
>
> I will update all these instances to use kzalloc() instead of kmalloc().

Do we need the alloc_page() in __sev_dbg_encrypt_user() to have __GFP_ZERO too?


>
> > 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()
>
> These functions are part of the ccp driver and a fix for them has
> already been sent upstream to linux-crypto@xxxxxxxxxxxxxxx and
> linux-kernel@xxxxxxxxxxxxxxx:
>
> [PATCH] crypto: ccp - Use kzalloc for sev ioctl interfaces to prevent
> kernel memory leak
>
> Thanks,
>
> Ashish
>
> >
> > 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;