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mappings for the pages used for the kernel and some initial allocations used for the page table. It maps the kernel and the blocks used for these initial allocations using 2MB pages. However, if the kernel does not end on a 2MB boundary, it still maps the last portion using a 2MB page, but reports that the unused 4K blocks within this 2MB allocation are free physical blocks. This means that these same physical blocks could also be mapped elsewhere - for example, into a user process. Given the proximity to the kernel text and data area, it seems wise to avoid allowing someone to write data to physical blocks also mapped into these virtual addresses. (Note that this isn't a security vulnerability: the direct map makes most/all memory on the system mapped into kernel space. And, nothing in the kernel should be trying to access these pages, as the virtual addresses are unused. It simply seems wise to avoid reusing these physical blocks while they are mapped to virtual addresses so close to the kernel text and data area.) Consequently, let's reserve the physical blocks covered by the page-table mappings for these initial allocations. Reviewed by: kib, markj MFC after: 2 weeks Sponsored by: Netflix Differential Revision: https://reviews.freebsd.org/D14268
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