svn commit: r332822 - stable/11/sys/amd64/amd64

[Jonathan T. Looney]

Author: jtl
Date: Fri Apr 20 14:55:13 2018
New Revision: 332822
URL: https://svnweb.freebsd.org/changeset/base/332822

Log:
  MFC r329071:
    On bootup, the amd64 pmap initialization code creates page-table
    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.
  
  Sponsored by:	Netflix, Inc.

Modified:
  stable/11/sys/amd64/amd64/pmap.c
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/amd64/amd64/pmap.c
==============================================================================
--- stable/11/sys/amd64/amd64/pmap.c	Fri Apr 20 14:47:02 2018	(r332821)
+++ stable/11/sys/amd64/amd64/pmap.c	Fri Apr 20 14:55:13 2018	(r332822)
@@ -957,6 +957,13 @@ create_pagetables(vm_paddr_t *firstaddr)
 		pd_p[i] = (i << PDRSHIFT) | X86_PG_RW | X86_PG_V | PG_PS |
 		    pg_g;
 
+	/*
+	 * Because we map the physical blocks in 2M pages, adjust firstaddr
+	 * to record the physical blocks we've actually mapped into kernel
+	 * virtual address space.
+	 */
+	*firstaddr = round_2mpage(*firstaddr);
+
 	/* And connect up the PD to the PDP (leaving room for L4 pages) */
 	pdp_p = (pdp_entry_t *)(KPDPphys + ptoa(KPML4I - KPML4BASE));
 	for (i = 0; i < nkpdpe; i++)