linux-kernel - Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for patching

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Date:   Sun, 11 Nov 2018 15:43:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for
 patching


I don't seem to have gotten patches 0-2,7 for some reason; I'll try and
dig them out of the LKML folder.

On Sat, Nov 10, 2018 at 03:17:27PM -0800, Nadav Amit wrote:
> +void __init poking_init(void)
> +{
> +	spinlock_t *ptl;
> +	pte_t *ptep;
> +
> +	poking_mm = copy_init_mm();
> +	if (!poking_mm) {
> +		pr_err("x86/mm: error setting a separate poking address space");
> +		return;
> +	}
> +
> +	/*
> +	 * Randomize the poking address, but make sure that the following page
> +	 * will be mapped at the same PMD. We need 2 pages, so find space for 3,
> +	 * and adjust the address if the PMD ends after the first one.
> +	 */
> +	poking_addr = TASK_UNMAPPED_BASE +
> +		(kaslr_get_random_long("Poking") & PAGE_MASK) %
> +		(TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
> +
> +	if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
> +		poking_addr += PAGE_SIZE;
> +
> +	/*
> +	 * We need to trigger the allocation of the page-tables that will be
> +	 * needed for poking now. Later, poking may be performed in an atomic
> +	 * section, which might cause allocation to fail.
> +	 */
> +	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> +	if (!WARN_ON(!ptep))
> +		pte_unmap_unlock(ptep, ptl);
> +}

The difference in how we deal with -ENOMEM here is weird. I think we
have a _lot_ of code that simply hard assumes we don't fail memory alloc
on init.

I for instance would not mind to simply remove both branches and let the
kernel crash and burn if we ever fail here.