linux-kernel - Re: [PATCH v2] x86/fpu: Allow PKRU to be (once again) written by ptrace.

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Date:   Thu, 4 Aug 2022 11:23:19 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Robert O'Callahan <robert@...llahan.org>,
        David Manouchehri <david.manouchehri@...eup.net>,
        Borislav Petkov <bp@...e.de>, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] x86/fpu: Allow PKRU to be (once again) written by
 ptrace.


* Kyle Huey <me@...ehuey.com> wrote:

> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 0531d6a06df5..dfb79e2ee81f 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -406,16 +406,7 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
>  	if (ustate->xsave.header.xfeatures & ~xcr0)
>  		return -EINVAL;
>  
> -	ret = copy_uabi_from_kernel_to_xstate(kstate, ustate);
> -	if (ret)
> -		return ret;
> -
> -	/* Retrieve PKRU if not in init state */
> -	if (kstate->regs.xsave.header.xfeatures & XFEATURE_MASK_PKRU) {
> -		xpkru = get_xsave_addr(&kstate->regs.xsave, XFEATURE_PKRU);
> -		*vpkru = xpkru->pkru;
> -	}
> -	return 0;
> +	return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru);
>  }
>  EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);

So this removes 'ret' and 'xpkru' use from 
fpu_copy_uabi_to_guest_fpstate(), but leaves the variables in place, 
generating a build warning (and error with CONFIG_WERROR=y) ...

Thanks,

	Ingo