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SubjectRe: [patch] crypto: ccp - Avoid page allocation failure warning for SEV_GET_ID2
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On 1/3/23 17:18, David Rientjes wrote:
> On Tue, 3 Jan 2023, Tom Lendacky wrote:
>
>> On 12/30/22 16:18, David Rientjes wrote:
>>> For SEV_GET_ID2, the user provided length does not have a specified
>>> limitation because the length of the ID may change in the future. The
>>> kernel memory allocation, however, is implicitly limited to 4MB on x86 by
>>> the page allocator, otherwise the kzalloc() will fail.
>>>
>>> When this happens, it is best not to spam the kernel log with the warning.
>>> Simply fail the allocation and return ENOMEM to the user.
>>>
>>> Fixes: d6112ea0cb34 ("crypto: ccp - introduce SEV_GET_ID2 command")
>>> Reported-by: Andy Nguyen <theflow@google.com>
>>> Reported-by: Peter Gonda <pgonda@google.com>
>>> Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
>>> Signed-off-by: David Rientjes <rientjes@google.com>
>>> ---
>>> drivers/crypto/ccp/sev-dev.c | 9 ++++++++-
>>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>>> --- a/drivers/crypto/ccp/sev-dev.c
>>> +++ b/drivers/crypto/ccp/sev-dev.c
>>> @@ -881,7 +881,14 @@ static int sev_ioctl_do_get_id2(struct sev_issue_cmd
>>> *argp)
>>> input_address = (void __user *)input.address;
>>> if (input.address && input.length) {
>>> - id_blob = kzalloc(input.length, GFP_KERNEL);
>>> + /*
>>> + * The length of the ID shouldn't be assumed by software since
>>> + * it may change in the future. The allocation size is
>>> limited
>>> + * to 1 << (PAGE_SHIFT + MAX_ORDER - 1) by the page allocator.
>>> + * If the allocation fails, simply return ENOMEM rather than
>>> + * warning in the kernel log.
>>> + */
>>> + id_blob = kzalloc(input.length, GFP_KERNEL | __GFP_NOWARN);
>>
>> We could do this or we could have the driver invoke the API with a zero length
>> to get the minimum buffer size needed for the call. The driver could then
>> perform some validation checks comparing the supplied input.length to the
>> returned length. If the driver can proceed, then if input.length is exactly 2x
>> the minimum length, then kzalloc the 2 * minimum length, otherwise kzalloc the
>> minimum length. This is a bit more complicated, though, compared to this fix.
>>
>
> Thanks Tom. IIUC, this could be useful to identify situations where
> input.length != min_length and input.length != min_length*2 and, in those
> cases, return EINVAL? Or are there situations where this is actually a
> valid input.length?
>
> I was assuming that the user was always doing its own SEV_GET_ID2 first to
> determine the length and then use it for input.length, but perhaps that's
> not the case and they are passing a bogus value.

Except that if the user was always doing that, then we wouldn't be worried
about this case then. But, I think my method is overkill and the simple
approach of this patch is the way to go.

Thanks,
Tom

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