Re: [PATCH v15 05/23] x86/virt/tdx: Handle SEAMCALL no entropy error in common code

Isaku Yamahata
2023-11-15T19:27:11Z

On Wed, Nov 15, 2023 at 10:41:46AM +0000,
"Huang, Kai" <kai.huang@intel.com> wrote:

>
> > > +#include <asm/archrandom.h>
> > > +
> > > +typedef u64 (*sc_func_t)(u64 fn, struct tdx_module_args *args);
> > > +
> > > +static inline u64 sc_retry(sc_func_t func, u64 fn,
> > > + struct tdx_module_args *args)
> > > +{
> > > + int retry = RDRAND_RETRY_LOOPS;
> > > + u64 ret;
> > > +
> > > + do {
> > > + ret = func(fn, args);
> > > + } while (ret == TDX_RND_NO_ENTROPY && --retry);
> >
> > This loop assumes that args isn't touched when TDX_RND_NO_ENTRYPOY is returned.
> > It's not true. TDH.SYS.INIT() and TDH.SYS.LP.INIT() clear RCX, RDX, etc on
> > error including TDX_RND_NO_ENTRY. Because TDH.SYS.INIT() takes RCX as input,
> > this wrapper doesn't work. TDH.SYS.LP.INIT() doesn't use RCX, RDX ... as
> > input. So it doesn't matter.
> >
> > Other SEAMCALLs doesn't touch registers on the no entropy error.
> > TDH.EXPORTS.STATE.IMMUTABLE(), TDH.IMPORTS.STATE.IMMUTABLE(), TDH.MNG.ADDCX(),
> > and TDX.MNG.CREATE(). TDH.SYS.INIT() is an exception.
>
> If I am reading the spec (TDX module 1.5 ABI) correctly the TDH.SYS.INIT doesn't
> return TDX_RND_NO_ENTROPY.

The next updated spec would fix it.

> TDH.SYS.LP.INIT indeed can return NO_ENTROPY but as
> you said it doesn't take any register as input. So technically the code works
> fine. (Even the TDH.SYS.INIT can return NO_ENTROPY the code still works fine
> because the RCX must be 0 for TDH.SYS.INIT.)

Ah yes, I agree with you. So it doesn't matter.

> Also, I can hardly think out of any reason why TDX module needs to clobber input
> registers in case of NO_ENTROPY for *ANY* SEAMCALL. But despite that, I am not
> opposing the idea that it *MIGHT* be better to "not assume" NO_ENTROPY will
> never clobber registers either, e.g., for the sake of future extendibility. In
> this case, the below diff should address:

Now we agreed that TDH.SYS.INIT() and TDH.SYS.LP.INIT() doesn't matter,
I'm fine with this patch. (TDX KVM handles other SEAMCALLS itself.)

Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
--
Isaku Yamahata <isaku.yamahata@linux.intel.com>