Re: [PATCH v5 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

[Thomas Gleixner]

Kyle!

On Mon, Aug 08 2022 at 07:15, Kyle Huey wrote:
> When management of the PKRU register was moved away from XSTATE, emulation
> of PKRU's existence in XSTATE was added for APIs that read XSTATE, but not
> for APIs that write XSTATE. This can be seen by running gdb and executing
> `p $pkru`, `set $pkru = 42`, and `p $pkru`. On affected kernels (5.14+) the
> write to the PKRU register (which gdb performs through ptrace) is ignored.
>
> There are three relevant APIs: PTRACE_SETREGSET with NT_X86_XSTATE,
> sigreturn, and KVM_SET_XSAVE. KVM_SET_XSAVE has its own special handling to
> make PKRU writes take effect (in fpu_copy_uabi_to_guest_fpstate). Push that
> down into copy_uabi_to_xstate and have PTRACE_SETREGSET with NT_X86_XSTATE
> and sigreturn pass in pointers to the appropriate PKRU value.
>
> This also adds code to initialize the PKRU value to the hardware init value
> (namely 0) if the PKRU bit is not set in the XSTATE header to match XRSTOR.
> This is a change to the current KVM_SET_XSAVE behavior.

You are stating a fact here, but provide 0 justification why this is
correct.

>
> Changelog since v4:

Can you please put the change log past the --- seperator line, so it
gets stripped off when the patch is applied? That spares manual fixups.

>
> Signed-off-by: Kyle Huey <me@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx # For edge case behavior of KVM_SET_XSAVE
> Cc: stable@xxxxxxxxxxxxxxx # 5.14+
> Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")

Can you please use the documented tag ordering?

https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-submission-notes

> @@ -1235,6 +1235,24 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
>  	for (i = 0; i < XFEATURE_MAX; i++) {
>  		mask = BIT_ULL(i);
>  
> +		if (i == XFEATURE_PKRU) {
> +			/*
> +			 * Retrieve PKRU if not in init state, otherwise
> +			 * initialize it.
> +			 */
> +			if (hdr.xfeatures & mask) {
> +				struct pkru_state xpkru = {0};
> +
> +				if (copy_from_buffer(&xpkru, xstate_offsets[i],
> +						     sizeof(xpkru), kbuf, ubuf))
> +					return -EFAULT;
> +
> +				*pkru = xpkru.pkru;
> +			} else {
> +				*pkru = 0;
> +			}
> +		}

That's really horrible and there is no point in copying the stuff from
the buffer twice:

@@ -1246,6 +1246,15 @@ static int copy_uabi_to_xstate(struct fp
 		}
 	}
 
+	/* Update the user protection key storage */
+	*pkru = 0;
+	if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
+		struct pkru_state *xpkru;
+
+		xpkru = get_xsave_addr(xsave, XFEATURE_PKRU);
+		*pkru = xpkru->pkru;
+	}
+

Hmm?

Thanks,

        tglx