[RFC PATCH v9 7/7] x86/cet/ibt: Introduce arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE)

Yu-cheng Yu
2020-02-05T18:23:29Z

When Indirect Branch Tracking (IBT) is active, non-IBT-compatible legacy
code can be executed if its address range is specified in the legacy code
bitmap. Each bit in the bitmap indicates a 4-KB legacy code page.

The bitmap is allocated when the first time an application calls
arch_prctl(ARCH_X86_MARK_LEGACY_CODE). It is setup as a special mapping.
The application can read the bitmap but not write to; it manages the bitmap
with the arch_prctl() being introduced:

arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE, unsigned long *buf)
Mark an address range as IBT legacy code.

*buf: starting linear address
*(buf + 1): size of the legacy code
*(buf + 2): set (1); clear (0)

v9:
- Split out special mapping zero page handling to the previous patch.
- Change the bitmap from a pre-defined address to get_unmapped_area().

v8:
- Change legacy bitmap to a special mapping.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/include/asm/cet.h | 2 +
arch/x86/include/uapi/asm/prctl.h | 3 +
arch/x86/kernel/Makefile | 2 +-
arch/x86/kernel/cet_bitmap.c | 226 ++++++++++++++++++++++++++++++
arch/x86/kernel/cet_prctl.c | 15 ++
5 files changed, 247 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/kernel/cet_bitmap.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index d3f0d50d51ec..a9677bcdeb5c 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -4,6 +4,7 @@

#ifndef __ASSEMBLY__
#include <linux/types.h>
+#include <asm/processor.h>

struct task_struct;
struct sc_ext;
@@ -30,6 +31,7 @@ void cet_disable_free_shstk(struct task_struct *p);
int cet_restore_signal(bool ia32, struct sc_ext *sc);
int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
int cet_setup_ibt(void);
+int cet_mark_legacy_code(unsigned long addr, unsigned long size, unsigned long set);
void cet_disable_ibt(void);
#else
static inline int prctl_cet(int option, unsigned long arg2) { return -EINVAL; }
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index d962f0ec9ccf..da39d4bde4e1 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -18,5 +18,8 @@
#define ARCH_X86_CET_DISABLE 0x3002
#define ARCH_X86_CET_LOCK 0x3003
#define ARCH_X86_CET_ALLOC_SHSTK 0x3004
+#define ARCH_X86_CET_GET_LEGACY_BITMAP 0x3005 /* deprecated */
+#define ARCH_X86_CET_SET_LEGACY_BITMAP 0x3006 /* deprecated */
+#define ARCH_X86_CET_MARK_LEGACY_CODE 0x3007

#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 69a19957e200..0261ea015e45 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -142,7 +142,7 @@ obj-$(CONFIG_UNWINDER_ORC) += unwind_orc.o
obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

-obj-$(CONFIG_X86_INTEL_CET) += cet.o cet_prctl.o
+obj-$(CONFIG_X86_INTEL_CET) += cet.o cet_prctl.o cet_bitmap.o

###
# 64 bit specific files
diff --git a/arch/x86/kernel/cet_bitmap.c b/arch/x86/kernel/cet_bitmap.c
new file mode 100644
index 000000000000..2c9e76f9b3f6
--- /dev/null
+++ b/arch/x86/kernel/cet_bitmap.c
@@ -0,0 +1,226 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/bits.h>
+#include <linux/err.h>
+#include <linux/memcontrol.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+#include <linux/oom.h>
+#include <linux/pagemap.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <asm/cet.h>
+#include <asm/fpu/internal.h>
+
+#define MMAP_MAX (unsigned long)(test_thread_flag(TIF_ADDR32) ? \
+ TASK_SIZE : TASK_SIZE_MAX)
+#define IBT_BITMAP_SIZE (round_up(MMAP_MAX, PAGE_SIZE * BITS_PER_BYTE) / \
+ (PAGE_SIZE * BITS_PER_BYTE))
+
+/*
+ * For read fault, provide the zero page. For write fault coming from
+ * get_user_pages(), clear the page already allocated.
+ */
+static vm_fault_t bitmap_fault(const struct vm_special_mapping *sm,
+ struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+ if (!(vmf->flags & FAULT_FLAG_WRITE)) {
+ vmf->page = ZERO_PAGE(vmf->address);
+ return 0;
+ } else {
+ vm_fault_t r;
+
+ if (!vmf->cow_page)
+ return VM_FAULT_ERROR;
+
+ clear_user_highpage(vmf->cow_page, vmf->address);
+ __SetPageUptodate(vmf->cow_page);
+ r = finish_fault(vmf);
+ return r ? r : VM_FAULT_DONE_COW;
+ }
+}
+
+static int bitmap_mremap(const struct vm_special_mapping *sm,
+ struct vm_area_struct *vma)
+{
+ return -EINVAL;
+}
+
+static const struct vm_special_mapping bitmap_mapping = {
+ .name = "[ibt_bitmap]",
+ .fault = bitmap_fault,
+ .mremap = bitmap_mremap,
+};
+
+static int alloc_bitmap(void)
+{
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma;
+ unsigned long addr;
+ u64 msr_val;
+ int r = 0;
+
+ if (down_write_killable(&mm->mmap_sem))
+ return -EINTR;
+
+ addr = get_unmapped_area(NULL, 0, IBT_BITMAP_SIZE, 0, 0);
+ if (IS_ERR_VALUE(addr)) {
+ up_write(&mm->mmap_sem);
+ return PTR_ERR((void *)addr);
+ }
+
+ vma = _install_special_mapping(mm, addr, IBT_BITMAP_SIZE,
+ VM_READ | VM_MAYREAD | VM_MAYWRITE,
+ &bitmap_mapping);
+
+ if (IS_ERR(vma))
+ r = PTR_ERR(vma);
+
+ up_write(&mm->mmap_sem);
+
+ if (r)
+ return r;
+
+ fpregs_lock();
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ __fpregs_load_activate();
+
+ rdmsrl(MSR_IA32_U_CET, msr_val);
+ msr_val |= (addr | MSR_IA32_CET_LEG_IW_EN);
+ wrmsrl(MSR_IA32_U_CET, msr_val);
+ fpregs_unlock();
+ current->thread.cet.ibt_bitmap_used = 1;
+ current->thread.cet.ibt_bitmap_base = addr;
+ return 0;
+}
+
+/*
+ * Set bits in the IBT legacy code bitmap, which is read-only user memory.
+ */
+static int set_bits(unsigned long start_bit, unsigned long end_bit,
+ unsigned long set)
+{
+ unsigned long start_ul, end_ul, nr_ul;
+ unsigned long start_ul_addr, tmp_addr, len;
+ int i, j;
+
+ start_ul = start_bit / BITS_PER_LONG;
+ end_ul = end_bit / BITS_PER_LONG;
+ i = start_bit % BITS_PER_LONG;
+ j = end_bit % BITS_PER_LONG;
+
+ tmp_addr = current->thread.cet.ibt_bitmap_base;
+ start_ul_addr = tmp_addr + start_ul * sizeof(0UL);
+ nr_ul = end_ul - start_ul + 1;
+
+ tmp_addr = start_ul_addr;
+ len = nr_ul * sizeof(0UL);
+
+ down_read(&current->mm->mmap_sem);
+ while (len) {
+ unsigned long *first, *last, mask, bytes;
+ int ret, offset;
+ void *kern_page_addr;
+ struct page *page = NULL;
+
+ ret = get_user_pages(tmp_addr, 1, FOLL_WRITE | FOLL_FORCE,
+ &page, NULL);
+
+ if (ret <= 0) {
+ up_read(&current->mm->mmap_sem);
+ return ret;
+ }
+
+ kern_page_addr = kmap(page);
+
+ bytes = len;
+ offset = tmp_addr & (PAGE_SIZE - 1);
+
+ /* Is end_ul in this page? */
+ if (bytes > (PAGE_SIZE - offset)) {
+ bytes = PAGE_SIZE - offset;
+ last = NULL;
+ } else {
+ last = (unsigned long *)(kern_page_addr + offset + bytes) - 1;
+ }
+
+ /* Is start_ul in this page? */
+ if (tmp_addr == start_ul_addr)
+ first = (unsigned long *)(kern_page_addr + offset);
+ else
+ first = NULL;
+
+ if (nr_ul == 1) {
+ mask = GENMASK(j, i);
+
+ if (set)
+ *first |= mask;
+ else
+ *first &= ~mask;
+ } else {
+ if (first) {
+ mask = GENMASK(BITS_PER_LONG - 1, i);
+
+ if (set)
+ *first |= mask;
+ else
+ *first &= ~mask;
+ }
+
+ if (last) {
+ mask = GENMASK(j, 0);
+
+ if (set)
+ *last |= mask;
+ else
+ *last &= ~mask;
+ }
+
+ if (nr_ul > 2) {
+ void *p = kern_page_addr + offset;
+ int cnt = bytes;
+
+ if (first) {
+ p += sizeof(*first);
+ cnt -= sizeof(*first);
+ }
+
+ if (last)
+ cnt -= sizeof(*last);
+
+ if (set)
+ memset(p, 0xff, cnt);
+ else
+ memset(p, 0, cnt);
+ }
+ }
+
+ set_page_dirty_lock(page);
+ kunmap(page);
+ put_page(page);
+
+ len -= bytes;
+ tmp_addr += bytes;
+ }
+ up_read(&current->mm->mmap_sem);
+ return 0;
+}
+
+int cet_mark_legacy_code(unsigned long addr, unsigned long size, unsigned long set)
+{
+ int r;
+
+ if (!current->thread.cet.ibt_enabled)
+ return -EINVAL;
+
+ if ((addr >= MMAP_MAX) || (addr + size > MMAP_MAX))
+ return -EINVAL;
+
+ if (!current->thread.cet.ibt_bitmap_used) {
+ r = alloc_bitmap();
+ if (r)
+ return r;
+ }
+
+ return set_bits(addr / PAGE_SIZE, (addr + size - 1) / PAGE_SIZE, set);
+}
diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
index 2a3170786a3b..3fa5ce8d4938 100644
--- a/arch/x86/kernel/cet_prctl.c
+++ b/arch/x86/kernel/cet_prctl.c
@@ -54,6 +54,18 @@ static int handle_alloc_shstk(unsigned long arg2)
return 0;
}

+static int handle_mark_legacy_code(unsigned long arg2)
+{
+ unsigned long addr, size, set;
+
+ if (get_user(addr, (unsigned long __user *)arg2) ||
+ get_user(size, (unsigned long __user *)arg2 + 1) ||
+ get_user(set, (unsigned long __user *)arg2 + 2))
+ return -EFAULT;
+
+ return cet_mark_legacy_code(addr, size, set);
+}
+
int prctl_cet(int option, unsigned long arg2)
{
struct cet_status *cet = &current->thread.cet;
@@ -82,6 +94,9 @@ int prctl_cet(int option, unsigned long arg2)
case ARCH_X86_CET_ALLOC_SHSTK:
return handle_alloc_shstk(arg2);

+ case ARCH_X86_CET_MARK_LEGACY_CODE:
+ return handle_mark_legacy_code(arg2);
+
default:
return -EINVAL;
}
--
2.21.0