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Subject[025/136] xen: make -fstack-protector work under Xen
2.6.31-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
commit 577eebeae34d340685d8985dfdb7dfe337c511e8 upstream.

-fstack-protector uses a special per-cpu "stack canary" value.
gcc generates special code in each function to test the canary to make
sure that the function's stack hasn't been overrun.

On x86-64, this is simply an offset of %gs, which is the usual per-cpu
base segment register, so setting it up simply requires loading %gs's
base as normal.

On i386, the stack protector segment is %gs (rather than the usual kernel
percpu %fs segment register). This requires setting up the full kernel
GDT and then loading %gs accordingly. We also need to make sure %gs is
initialized when bringing up secondary cpus too.

To keep things consistent, we do the full GDT/segment register setup on
both architectures.

Because we need to avoid -fstack-protected code before setting up the GDT
and because there's no way to disable it on a per-function basis, several
files need to have stack-protector inhibited.

[ Impact: allow Xen booting with stack-protector enabled ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
arch/x86/mm/Makefile | 4 +
arch/x86/xen/Makefile | 2
arch/x86/xen/enlighten.c | 131 ++++++++++++++++++++++++++++++++++++++++-------
arch/x86/xen/smp.c | 1
drivers/xen/Makefile | 3 +
5 files changed, 122 insertions(+), 19 deletions(-)
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -1,6 +1,10 @@
obj-y := init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
pat.o pgtable.o gup.o

+# Make sure __phys_addr has no stackprotector
+nostackp := $(call cc-option, -fno-stack-protector)
+CFLAGS_ioremap.o := $(nostackp)
+
obj-$(CONFIG_SMP) += tlb.o

obj-$(CONFIG_X86_32) += pgtable_32.o iomap_32.o
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -51,6 +51,7 @@
#include <asm/pgtable.h>
#include <asm/tlbflush.h>
#include <asm/reboot.h>
+#include <asm/stackprotector.h>

#include "xen-ops.h"
#include "mmu.h"
@@ -330,18 +331,28 @@ static void xen_load_gdt(const struct de
unsigned long frames[pages];
int f;

- /* A GDT can be up to 64k in size, which corresponds to 8192
- 8-byte entries, or 16 4k pages.. */
+ /*
+ * A GDT can be up to 64k in size, which corresponds to 8192
+ * 8-byte entries, or 16 4k pages..
+ */

BUG_ON(size > 65536);
BUG_ON(va & ~PAGE_MASK);

for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
int level;
- pte_t *ptep = lookup_address(va, &level);
+ pte_t *ptep;
unsigned long pfn, mfn;
void *virt;

+ /*
+ * The GDT is per-cpu and is in the percpu data area.
+ * That can be virtually mapped, so we need to do a
+ * page-walk to get the underlying MFN for the
+ * hypercall. The page can also be in the kernel's
+ * linear range, so we need to RO that mapping too.
+ */
+ ptep = lookup_address(va, &level);
BUG_ON(ptep == NULL);

pfn = pte_pfn(*ptep);
@@ -358,6 +369,44 @@ static void xen_load_gdt(const struct de
BUG();
}

+/*
+ * load_gdt for early boot, when the gdt is only mapped once
+ */
+static __init void xen_load_gdt_boot(const struct desc_ptr *dtr)
+{
+ unsigned long va = dtr->address;
+ unsigned int size = dtr->size + 1;
+ unsigned pages = (size + PAGE_SIZE - 1) / PAGE_SIZE;
+ unsigned long frames[pages];
+ int f;
+
+ /*
+ * A GDT can be up to 64k in size, which corresponds to 8192
+ * 8-byte entries, or 16 4k pages..
+ */
+
+ BUG_ON(size > 65536);
+ BUG_ON(va & ~PAGE_MASK);
+
+ for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
+ pte_t pte;
+ unsigned long pfn, mfn;
+
+ pfn = virt_to_pfn(va);
+ mfn = pfn_to_mfn(pfn);
+
+ pte = pfn_pte(pfn, PAGE_KERNEL_RO);
+
+ if (HYPERVISOR_update_va_mapping((unsigned long)va, pte, 0))
+ BUG();
+
+ frames[f] = mfn;
+ }
+
+ if (HYPERVISOR_set_gdt(frames, size / sizeof(struct desc_struct)))
+ BUG();
+}
+
static void load_TLS_descriptor(struct thread_struct *t,
unsigned int cpu, unsigned int i)
{
@@ -581,6 +630,29 @@ static void xen_write_gdt_entry(struct d
preempt_enable();
}

+/*
+ * Version of write_gdt_entry for use at early boot-time needed to
+ * update an entry as simply as possible.
+ */
+static __init void xen_write_gdt_entry_boot(struct desc_struct *dt, int entry,
+ const void *desc, int type)
+{
+ switch (type) {
+ case DESC_LDT:
+ case DESC_TSS:
+ /* ignore */
+ break;
+
+ default: {
+ xmaddr_t maddr = virt_to_machine(&dt[entry]);
+
+ if (HYPERVISOR_update_descriptor(maddr.maddr, *(u64 *)desc))
+ dt[entry] = *(struct desc_struct *)desc;
+ }
+
+ }
+}
+
static void xen_load_sp0(struct tss_struct *tss,
struct thread_struct *thread)
{
@@ -965,6 +1037,23 @@ static const struct machine_ops __initda
.emergency_restart = xen_emergency_restart,
};

+/*
+ * Set up the GDT and segment registers for -fstack-protector. Until
+ * we do this, we have to be careful not to call any stack-protected
+ * function, which is most of the kernel.
+ */
+static void __init xen_setup_stackprotector(void)
+{
+ pv_cpu_ops.write_gdt_entry = xen_write_gdt_entry_boot;
+ pv_cpu_ops.load_gdt = xen_load_gdt_boot;
+
+ setup_stack_canary_segment(0);
+ switch_to_new_gdt(0);
+
+ pv_cpu_ops.write_gdt_entry = xen_write_gdt_entry;
+ pv_cpu_ops.load_gdt = xen_load_gdt;
+}
+
/* First C function to be called on Xen boot */
asmlinkage void __init xen_start_kernel(void)
{
@@ -983,13 +1072,28 @@ asmlinkage void __init xen_start_kernel(
pv_apic_ops = xen_apic_ops;
pv_mmu_ops = xen_mmu_ops;

-#ifdef CONFIG_X86_64
/*
- * Setup percpu state. We only need to do this for 64-bit
- * because 32-bit already has %fs set properly.
+ * Set up some pagetable state before starting to set any ptes.
*/
- load_percpu_segment(0);
-#endif
+
+ /* Prevent unwanted bits from being set in PTEs. */
+ __supported_pte_mask &= ~_PAGE_GLOBAL;
+ if (!xen_initial_domain())
+ __supported_pte_mask &= ~(_PAGE_PWT | _PAGE_PCD);
+
+ __supported_pte_mask |= _PAGE_IOMAP;
+
+ xen_setup_features();
+
+ /* Get mfn list */
+ if (!xen_feature(XENFEAT_auto_translated_physmap))
+ xen_build_dynamic_phys_to_machine();
+
+ /*
+ * Set up kernel GDT and segment registers, mainly so that
+ * -fstack-protector code can be executed.
+ */
+ xen_setup_stackprotector();

xen_init_irq_ops();
xen_init_cpuid_mask();
@@ -1001,8 +1105,6 @@ asmlinkage void __init xen_start_kernel(
set_xen_basic_apic_ops();
#endif

- xen_setup_features();
-
if (xen_feature(XENFEAT_mmu_pt_update_preserve_ad)) {
pv_mmu_ops.ptep_modify_prot_start = xen_ptep_modify_prot_start;
pv_mmu_ops.ptep_modify_prot_commit = xen_ptep_modify_prot_commit;
@@ -1019,17 +1121,8 @@ asmlinkage void __init xen_start_kernel(

xen_smp_init();

- /* Get mfn list */
- if (!xen_feature(XENFEAT_auto_translated_physmap))
- xen_build_dynamic_phys_to_machine();
-
pgd = (pgd_t *)xen_start_info->pt_base;

- /* Prevent unwanted bits from being set in PTEs. */
- __supported_pte_mask &= ~_PAGE_GLOBAL;
- if (!xen_initial_domain())
- __supported_pte_mask &= ~(_PAGE_PWT | _PAGE_PCD);
-
#ifdef CONFIG_X86_64
/* Work out if we support NX */
check_efer();
--- a/arch/x86/xen/Makefile
+++ b/arch/x86/xen/Makefile
@@ -8,6 +8,7 @@ endif
# Make sure early boot has no stackprotector
nostackp := $(call cc-option, -fno-stack-protector)
CFLAGS_enlighten.o := $(nostackp)
+CFLAGS_mmu.o := $(nostackp)

obj-y := enlighten.o setup.o multicalls.o mmu.o irq.o \
time.o xen-asm.o xen-asm_$(BITS).o \
@@ -16,3 +17,4 @@ obj-y := enlighten.o setup.o multicalls
obj-$(CONFIG_SMP) += smp.o
obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= spinlock.o
obj-$(CONFIG_XEN_DEBUG_FS) += debugfs.o
+
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -236,6 +236,7 @@ cpu_initialize_context(unsigned int cpu,
ctxt->user_regs.ss = __KERNEL_DS;
#ifdef CONFIG_X86_32
ctxt->user_regs.fs = __KERNEL_PERCPU;
+ ctxt->user_regs.gs = __KERNEL_STACK_CANARY;
#else
ctxt->gs_base_kernel = per_cpu_offset(cpu);
#endif
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -1,6 +1,9 @@
obj-y += grant-table.o features.o events.o manage.o
obj-y += xenbus/

+nostackp := $(call cc-option, -fno-stack-protector)
+CFLAGS_features.o := $(nostackp)
+
obj-$(CONFIG_HOTPLUG_CPU) += cpu_hotplug.o
obj-$(CONFIG_XEN_XENCOMM) += xencomm.o
obj-$(CONFIG_XEN_BALLOON) += balloon.o



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