Re: [PATCH v6 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

1 Sep 2022


      On Mon, Aug 29, 2022, Kyle Huey wrote:
...
@@ -1246,6 +1246,21 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
   	}
   }

/*
* Update the user protection key storage. Allow KVM to


* pass in a NULL pkru pointer if the mask bit is unset


* for its legacy ABI behavior.


*/


if (pkru)
*pkru = 0;



if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
struct pkru_state *xpkru;



xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);


*pkru = xpkru->pkru;


}

What about writing this as:
if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
    	...
*pkru = xpkru->pkru;
    } else if (pkru) {
    	*pkru = 0;
    }
to make it slightly more obvious that @pkru must be non-NULL if the feature flag
is enabled?
Or we could be paranoid, though I'm not sure this is worthwhile.
if ((hdr.xfeatures & XFEATURE_MASK_PKRU) &&
        !WARN_ON_ONCE(!pkru)) {
    	...
*pkru = xpkru->pkru;
    } else if (pkru) {
    	*pkru = 0;
    }
Otherwise, looks good from a KVM perspective.  Thanks!

    