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

[Nadav Amit <namit@vmware.com>]

From: Peter Zijlstra
Sent: November 11, 2018 at 2:43:27 PM GMT
> To: Nadav Amit <namit@vmware.com>
> Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, x86@kernel.org, H. Peter Anvin <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, Kees Cook <keescook@chromium.org>, Dave Hansen <dave.hansen@intel.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.

Actually, now that we removed the fallback of patching without poking_mm, a
failure to allocate poking_mm should have had a BUG_ON().

For the second case, I think we still need either WARN_ON() or BUG_ON(), at
least as some sort of an in-code comment. I’ll change it to BUG_ON() if you
prefer.