Commit-ID: 4ddee6efdcd0259fdd5014312bd536325e6d3429 Gitweb: https://git.kernel.org/tip/4ddee6efdcd0259fdd5014312bd536325e6d3429 Author: Dave Hansen <dave.han...@linux.intel.com> AuthorDate: Fri, 6 Apr 2018 13:55:13 -0700 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 9 Apr 2018 18:27:33 +0200
x86/mm: Comment _PAGE_GLOBAL mystery I was mystified as to where the _PAGE_GLOBAL in the kernel page tables for kernel text came from. I audited all the places I could find, but I missed one: head_64.S. The page tables that we create in here live for a long time, and they also have _PAGE_GLOBAL set, despite whether the processor supports it or not. It's harmless, and we got *lucky* that the pageattr code accidentally clears it when we wipe it out of __supported_pte_mask and then later try to mark kernel text read-only. Comment some of these properties to make it easier to find and understand in the future. Signed-off-by: Dave Hansen <dave.han...@linux.intel.com> Cc: Andrea Arcangeli <aarca...@redhat.com> Cc: Andy Lutomirski <l...@kernel.org> Cc: Arjan van de Ven <ar...@linux.intel.com> Cc: Borislav Petkov <b...@alien8.de> Cc: Dan Williams <dan.j.willi...@intel.com> Cc: David Woodhouse <dw...@infradead.org> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Cc: Hugh Dickins <hu...@google.com> Cc: Josh Poimboeuf <jpoim...@redhat.com> Cc: Juergen Gross <jgr...@suse.com> Cc: Kees Cook <keesc...@google.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Nadav Amit <na...@vmware.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: linux...@kvack.org Link: http://lkml.kernel.org/r/20180406205513.079bb...@viggo.jf.intel.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- arch/x86/kernel/head_64.S | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S index 48385c1074a5..8344dd2f310a 100644 --- a/arch/x86/kernel/head_64.S +++ b/arch/x86/kernel/head_64.S @@ -399,8 +399,13 @@ NEXT_PAGE(level3_ident_pgt) .quad level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC .fill 511, 8, 0 NEXT_PAGE(level2_ident_pgt) - /* Since I easily can, map the first 1G. + /* + * Since I easily can, map the first 1G. * Don't set NX because code runs from these pages. + * + * Note: This sets _PAGE_GLOBAL despite whether + * the CPU supports it or it is enabled. But, + * the CPU should ignore the bit. */ PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD) #else @@ -431,6 +436,10 @@ NEXT_PAGE(level2_kernel_pgt) * (NOTE: at +512MB starts the module area, see MODULES_VADDR. * If you want to increase this then increase MODULES_VADDR * too.) + * + * This table is eventually used by the kernel during normal + * runtime. Care must be taken to clear out undesired bits + * later, like _PAGE_RW or _PAGE_GLOBAL in some cases. */ PMDS(0, __PAGE_KERNEL_LARGE_EXEC, KERNEL_IMAGE_SIZE/PMD_SIZE)
