linux-kernel - Re: [PATCH 08/30] x86, kaiser: unmap kernel from userspace page tables (core patch)

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Date:   Wed, 22 Nov 2017 14:45:54 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        richard.fellner@...dent.tugraz.at, moritz.lipp@...k.tugraz.at,
        daniel.gruss@...k.tugraz.at, michael.schwarz@...k.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 08/30] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

On 11/20/2017 09:21 AM, Thomas Gleixner wrote:
>> +	pgd = native_get_shadow_pgd(pgd_offset_k(0UL));
>> +	for (i = PTRS_PER_PGD / 2; i < PTRS_PER_PGD; i++) {
>> +		unsigned long addr = PAGE_OFFSET + i * PGDIR_SIZE;
> This looks wrong. The kernel address space gets incremented by PGDIR_SIZE
> and does not make a jump from PAGE_OFFSET to PAGE_OFFSET + 256 * PGDIR_SIZE
> 
> 	int i, j;
> 
> 	for (i = PTRS_PER_PGD / 2, j = 0; i < PTRS_PER_PGD; i++, j++) {
> 		unsigned long addr = PAGE_OFFSET + j * PGDIR_SIZE;
> 
> Not that is has any effect right now. Neither p4d_alloc_one() nor
> pud_alloc_one() are using the 'addr' argument.

Ahh, you're saying that 'i' is effectively starting *at* PAGE_OFFSET
since it's halfway up the address space already doing PTRS_PER_PGD/2.
Adding PAGE_OFFSET to PAGE_OFFSET is nonsense.

Would it just be simpler to do:

>         for (i = PTRS_PER_PGD / 2; i < PTRS_PER_PGD; i++) {
>                 unsigned long addr = i * PGDIR_SIZE;

?