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Message-ID: <0cefb67a-6fae-daa2-c871-ae35b96aac08@intel.com>
Date:   Wed, 14 Jun 2023 07:45:24 -0700
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Subject: Re: x86: pgtable / kaslr initialisation (OOB) help
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To:     Lee Jones <lee@kernel.org>, Peter Zijlstra <peterz@infradead.org>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20230614132339.GS3635807@google.com>
 <20230614141654.GA1640123@hirez.programming.kicks-ass.net>
 <20230614143732.GW3635807@google.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20230614143732.GW3635807@google.com>
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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.