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authorDave Hansen2018-04-06 13:55:13 -0700
committerIngo Molnar2018-04-12 09:05:58 +0200
commit430d4005b8b41c19966dd3bfdb33004bdb2de01c (patch)
tree0c24ef41097badc4e85ce4d61d4fe2fd5af7a621 /arch/x86
parent1a54420aeb4da1ba5b28283aa5696898220c9a27 (diff)
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.hansen@linux.intel.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: David Woodhouse <dwmw2@infradead.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Hugh Dickins <hughd@google.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Kees Cook <keescook@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Nadav Amit <namit@vmware.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-mm@kvack.org Link: http://lkml.kernel.org/r/20180406205513.079BB265@viggo.jf.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/kernel/head_64.S11
1 files changed, 10 insertions, 1 deletions
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)