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

[Ashish Kalra <ashkalra@amd.com>]

Hello Peter,

On 5/13/22 20:09, Peter Gonda wrote:
> On Fri, May 13, 2022 at 2:11 PM Ashish Kalra <ashkalra@amd.com> 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@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.
>> 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?

Actually this is used to allocate intermediate buffers to do 
copy_from_user, so it should be safe as size here is used for actual 
guest memory size to be encrypted.

Thanks, Ashish
>
>
>>> 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@vger.kernel.org and
>> linux-kernel@vger.kernel.org:
>>
>> [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;