[PATCH 12/30] x86, kaiser: map GDT into user page tables

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

The GDT is used to control the x86 segmentation mechanism.  It
must be virtually mapped when switching segments or at IRET
time when switching between userspace and kernel.

The original KAISER patch did not do this.  I have no ide how
it ever worked.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
Cc: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
---

 b/arch/x86/kernel/cpu/common.c |   15 +++++++++++++++
 b/arch/x86/mm/kaiser.c         |   10 ++++++++++
 2 files changed, 25 insertions(+)

diff -puN arch/x86/kernel/cpu/common.c~kaiser-user-map-gdt-pages arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~kaiser-user-map-gdt-pages	2017-11-08 10:45:32.375681387 -0800
+++ b/arch/x86/kernel/cpu/common.c	2017-11-08 10:45:32.380681387 -0800
@@ -5,6 +5,7 @@
 #include <linux/export.h>
 #include <linux/percpu.h>
 #include <linux/string.h>
+#include <linux/kaiser.h>
 #include <linux/ctype.h>
 #include <linux/delay.h>
 #include <linux/sched/mm.h>
@@ -487,6 +488,20 @@ static inline void setup_fixmap_gdt(int
 #endif
 
 	__set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
+
+	/* CPU 0's mapping is done in kaiser_init() */
+	if (cpu) {
+		int ret;
+
+		ret = kaiser_add_mapping((unsigned long) get_cpu_gdt_ro(cpu),
+					 PAGE_SIZE, __PAGE_KERNEL_RO);
+		/*
+		 * We do not have a good way to fail CPU bringup.
+		 * Just WARN about it and hope we boot far enough
+		 * to get a good log out.
+		 */
+		WARN_ON(ret);
+	}
 }
 
 /* Load the original GDT from the per-cpu structure */
diff -puN arch/x86/mm/kaiser.c~kaiser-user-map-gdt-pages arch/x86/mm/kaiser.c
--- a/arch/x86/mm/kaiser.c~kaiser-user-map-gdt-pages	2017-11-08 10:45:32.376681387 -0800
+++ b/arch/x86/mm/kaiser.c	2017-11-08 10:45:32.380681387 -0800
@@ -361,6 +361,16 @@ void __init kaiser_init(void)
 	kaiser_add_user_map_early((void *)idt_descr.address,
 				  sizeof(gate_desc) * NR_VECTORS,
 				  __PAGE_KERNEL_RO | _PAGE_GLOBAL);
+
+	/*
+	 * We could theoretically do this in setup_fixmap_gdt().
+	 * But, we would need to rewrite the above page table
+	 * allocation code to use the bootmem allocator.  The
+	 * buddy allocator is not available at the time that we
+	 * call setup_fixmap_gdt() for CPU 0.
+	 */
+	kaiser_add_user_map_early(get_cpu_gdt_ro(0), PAGE_SIZE,
+				  __PAGE_KERNEL_RO | _PAGE_GLOBAL);
 }
 
 int kaiser_add_mapping(unsigned long addr, unsigned long size,
_