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Peter Anvin" , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: x86: pgtable / kaslr initialisation (OOB) help Message-ID: <20230614150615.GX3635807@google.com> References: <20230614132339.GS3635807@google.com> <20230614141654.GA1640123@hirez.programming.kicks-ass.net> <20230614143732.GW3635807@google.com> <0cefb67a-6fae-daa2-c871-ae35b96aac08@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0cefb67a-6fae-daa2-c871-ae35b96aac08@intel.com> X-Rspamd-Queue-Id: 8F5BEC02AA X-Rspam-User: X-Stat-Signature: rz69anaj784t5uf1t6tzyd4nkz3krqwh X-Rspamd-Server: rspam03 X-HE-Tag: 1686755182-304329 X-HE-Meta: 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 9V9JapP5 Ka+svwVn4xCZ7K987+4MKCmz6BIZ7CbW232iCsr67Uwe/lFyvonK/6+j59+EC07JknpRLqrOt9P3qVdDEj8IRGhjFjd/4bUogh5b/AhCXWZ7Pr4/KWJ8rdFHym3vbctWxDlXyxi5XClEngthdGY4dp7Mbl7gd9DcHt2v78ucUjisfysMLLBcLXo8gUSpYzTEZRQWXdUAnmusbxFBsTTjzLWeugrLGs4YffKarFolUpJw7or6uWP1XZVPn8g== 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: Thanks for chiming in Dave. I hoped you would. On Wed, 14 Jun 2023, Dave Hansen wrote: > 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. Okay. Is this what you're suggesting? diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c v index d336bb0cb38b..803595c7dcc8 100644 --- a/arch/x86/mm/kaslr.c +++ b/arch/x86/mm/kaslr.c @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void) set_pgd(&trampoline_pgd_entry, __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp))); } else { - set_pgd(&trampoline_pgd_entry, - __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); + trampoline_pgd_entry = + __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp); } } If so, I'll take it for a spin right now. -- Lee Jones [李琼斯]