linux-kernel - Re: [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions

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Date:   Thu, 17 Dec 2020 16:28:51 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     ira.weiny@...el.com, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-doc@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions

On Fri, Nov 06 2020 at 15:29, ira weiny wrote:
> +#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
> +/*
> + * PKRS is a per-logical-processor MSR which overlays additional protection for
> + * pages which have been mapped with a protection key.
> + *
> + * The register is not maintained with XSAVE so we have to maintain the MSR
> + * value in software during context switch and exception handling.
> + *
> + * Context switches save the MSR in the task struct thus taking that value to
> + * other processors if necessary.
> + *
> + * To protect against exceptions having access to this memory we save the
> + * current running value and set the PKRS value for the duration of the
> + * exception.  Thus preventing exception handlers from having the elevated
> + * access of the interrupted task.
> + */
> +noinstr void irq_save_set_pkrs(irqentry_state_t *irq_state, u32 val)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_PKS))
> +		return;
> +
> +	irq_state->thread_pkrs = current->thread.saved_pkrs;
> +	write_pkrs(INIT_PKRS_VALUE);

Why is this noinstr? Just because it's called from a noinstr function?

Of course the function itself violates the noinstr constraints:

  vmlinux.o: warning: objtool: write_pkrs()+0x36: call to do_trace_write_msr() leaves .noinstr.text section

There is absolutely no reason to have this marked noinstr.

Thanks,

        tglx