Re: [patch] crypto: ccp - Avoid page allocation failure warning for SEV_GET_ID2

[David Rientjes <rientjes@google.com>]

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.