> > +#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. 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.)
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:
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index a621721f63dd..962a7a6be721 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -97,12 +97,23 @@ 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)
{
+ struct tdx_module_args _args = *args;
int retry = RDRAND_RETRY_LOOPS;
u64 ret;
- do {
- ret = func(fn, args);
- } while (ret == TDX_RND_NO_ENTROPY && --retry);
+again:
+ ret = func(fn, args);
+ if (ret == TDX_RND_NO_ENTROPY && --retry) {
+ /*
+ * Do not assume TDX module will never clobber the input
+ * registers when any SEAMCALL fails with out of entropy.
+ * In this case the original input registers in @args
+ * are clobbered. Always restore the input registers
+ * before retrying the SEAMCALL.
+ */
+ *args = _args;
+ goto again;
+ }
return ret;
}
The downside is we will have an additional memory copy of 'struct
tdx_module_args' for each SEAMCALL, but I don't believe this will have any
difference in practice.
Or, we can go and ask TDX module guys to promise no input registers will be
clobbered in case of NO_ENTROPY.
Hi Dave,
Do you have any opinion?
On Wed, Nov 15, 2023 at 10:41:46AM +0000,
"Huang, Kai" <[email protected]> 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 <[email protected]>
--
Isaku Yamahata <[email protected]>