linux-kernel - Re: x86: pgtable / kaslr initialisation (OOB) help

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Date:   Wed, 14 Jun 2023 07:45:24 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Lee Jones <lee@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: x86: pgtable / kaslr initialisation (OOB) help

On 6/14/23 07:37, Lee Jones wrote:
> Still unsure how we (the kernel) can/should write to an area of memory
> that does not belong to it.  Should we allocate enough memory
> (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> in a more sane way?

No.

I think this:

                set_pgd(&trampoline_pgd_entry,
                        __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));

is bogus-ish.  set_pgd() wants to operate on a pgd_t inside a pgd
*PAGE*.  But it's just being pointed at a single  _entry_.  The address
of 'trampoline_pgd_entry' in your case  also just (unfortunately)
happens to pass the:

	__pti_set_user_pgtbl -> pgdp_maps_userspace()

test.  I _think_ we want these to just be something like:

	trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
				     __pa(p4d_page_tramp);

That'll keep us away from all of the set_pgd()-induced nastiness.