Re: [PATCH v2 4/6] KEYS: trusted: Move tpm2_key_decode() to the TPM driver

[Jarkko Sakkinen]

On Wed May 22, 2024 at 12:59 AM EEST, James Bottomley wrote:
> On Tue, 2024-05-21 at 22:44 +0100, David Howells wrote:
> > Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> > 
> > > On Tue May 21, 2024 at 9:18 PM EEST, James Bottomley wrote:
> > > ...
> > > You don't save a single byte of memory with any constant that
> > > dictates the size requirements for multiple modules in two disjoint
> > > subsystems.
> > 
> > I think James is just suggesting you replace your limit argument with
> > a constant not that you always allocate that amount of memory.
>
> Exactly.  All we use it for is the -E2BIG check to ensure user space
> isn't allowed to run away with loads of kernel memory.

Not true.

It did return -EINVAL. This patch changes it to -E2BIG.

>
> > What the limit should be, OTOH, is up for discussion, but PAGE_SIZE
> > seems not unreasonable.
>
> A page is fine currently (MAX_BLOB_SIZE is 512).  However, it may be
> too small for some of the complex policies when they're introduced. 
> I'm not bothered about what it currently is, I just want it to be able
> to be increased easily when the time comes.

MAX_BLOB_SIZE would be used to cap key blob, not the policy.

And you are ignoring it yourself too in the driver.


> James


BR, Jarkko