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Peter Anvin" , 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 In-Reply-To: <20230614143732.GW3635807@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: BD49E140022 X-Stat-Signature: gsd6qn63af31w5sqco91iue38d9feq8h X-Rspam-User: X-HE-Tag: 1686753928-364111 X-HE-Meta: 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 X7ZJdXyh zlMckhaqh4lW3NJXWtVRPQesX6/0UYRyz1b5yqFR4ij6sJD6JNKwW9209XAC2p3Yi7/4IDS0vVrhVazWkdd8cjJczG5rLUxB2BiwEvpAHCpWyREhU7ZO6BucOlKzjPwHawBtD7GEkNlXNF6SgjpY/DhIAPA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.