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

[David Rientjes <rientjes@google.com>]

On Wed, 4 Jan 2023, Tom Lendacky 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.
> 

Makes sense, thanks for the clarification.  Does that translate into an 
acked-by? :)