[RFC PATCH v6 22/26] x86/cet/shstk: ELF header parsing of Shadow Stack

Yu-cheng Yu
2018-11-20T08:20:12Z

Look in .note.gnu.property of an ELF file and check if Shadow Stack needs
to be enabled for the task.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/Kconfig | 4 +
arch/x86/include/asm/elf.h | 5 +
arch/x86/include/uapi/asm/elf_property.h | 15 +
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/elf.c | 358 +++++++++++++++++++++++
fs/binfmt_elf.c | 15 +
include/uapi/linux/elf.h | 1 +
7 files changed, 400 insertions(+)
create mode 100644 arch/x86/include/uapi/asm/elf_property.h
create mode 100644 arch/x86/kernel/elf.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 86fb68f496a6..73dfb94cde71 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1925,6 +1925,9 @@ config X86_INTEL_CET
config ARCH_HAS_SHSTK
def_bool n

+config ARCH_HAS_PROGRAM_PROPERTIES
+ def_bool n
+
config X86_INTEL_SHADOW_STACK_USER
prompt "Intel Shadow Stack for user-mode"
def_bool n
@@ -1932,6 +1935,7 @@ config X86_INTEL_SHADOW_STACK_USER
select ARCH_USES_HIGH_VMA_FLAGS
select X86_INTEL_CET
select ARCH_HAS_SHSTK
+ select ARCH_HAS_PROGRAM_PROPERTIES
---help---
Shadow stack provides hardware protection against program stack
corruption. Only when all the following are true will an application
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f892e310..557ed0ba71c7 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -381,4 +381,9 @@ struct va_alignment {

extern struct va_alignment va_align;
extern unsigned long align_vdso_addr(unsigned long);
+
+#ifdef CONFIG_ARCH_HAS_PROGRAM_PROPERTIES
+extern int arch_setup_features(void *ehdr, void *phdr, struct file *file,
+ bool interp);
+#endif
#endif /* _ASM_X86_ELF_H */
diff --git a/arch/x86/include/uapi/asm/elf_property.h b/arch/x86/include/uapi/asm/elf_property.h
new file mode 100644
index 000000000000..af361207718c
--- /dev/null
+++ b/arch/x86/include/uapi/asm/elf_property.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _UAPI_ASM_X86_ELF_PROPERTY_H
+#define _UAPI_ASM_X86_ELF_PROPERTY_H
+
+/*
+ * pr_type
+ */
+#define GNU_PROPERTY_X86_FEATURE_1_AND (0xc0000002)
+
+/*
+ * Bits for GNU_PROPERTY_X86_FEATURE_1_AND
+ */
+#define GNU_PROPERTY_X86_FEATURE_1_SHSTK (0x00000002)
+
+#endif /* _UAPI_ASM_X86_ELF_PROPERTY_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index fbb2d91fb756..36b14ef410c8 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -141,6 +141,8 @@ obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o

obj-$(CONFIG_X86_INTEL_CET) += cet.o

+obj-$(CONFIG_ARCH_HAS_PROGRAM_PROPERTIES) += elf.o
+
###
# 64 bit specific files
ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/elf.c b/arch/x86/kernel/elf.c
new file mode 100644
index 000000000000..60e396e2abe9
--- /dev/null
+++ b/arch/x86/kernel/elf.c
@@ -0,0 +1,358 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Look at an ELF file's .note.gnu.property and determine if the file
+ * supports shadow stack and/or indirect branch tracking.
+ * The path from the ELF header to the note section is the following:
+ * elfhdr->elf_phdr->elf_note->property[].
+ */
+
+#include <asm/cet.h>
+#include <asm/elf_property.h>
+#include <asm/prctl.h>
+#include <asm/processor.h>
+#include <uapi/linux/elf-em.h>
+#include <uapi/linux/prctl.h>
+#include <linux/binfmts.h>
+#include <linux/elf.h>
+#include <linux/slab.h>
+#include <linux/fs.h>
+#include <linux/uaccess.h>
+#include <linux/string.h>
+#include <linux/compat.h>
+
+/*
+ * The .note.gnu.property layout:
+ *
+ * struct elf_note {
+ * u32 n_namesz; --> sizeof(n_name[]); always (4)
+ * u32 n_ndescsz;--> sizeof(property[])
+ * u32 n_type; --> always NT_GNU_PROPERTY_TYPE_0
+ * };
+ * char n_name[4]; --> always 'GNU\0'
+ *
+ * struct {
+ * struct property_x86 {
+ * u32 pr_type;
+ * u32 pr_datasz;
+ * };
+ * u8 pr_data[pr_datasz];
+ * }[];
+ */
+
+#define BUF_SIZE (PAGE_SIZE / 4)
+
+struct property_x86 {
+ u32 pr_type;
+ u32 pr_datasz;
+};
+
+typedef bool (test_item_fn)(void *buf, u32 *arg);
+typedef void *(next_item_fn)(void *buf, u32 *arg);
+
+static inline bool test_note_type_0(void *buf, u32 *align)
+{
+ struct elf_note *n = buf;
+
+ return ((n->n_type == NT_GNU_PROPERTY_TYPE_0) && (n->n_namesz == 4) &&
+ (memcmp(n + 1, "GNU", 4) == 0));
+}
+
+static inline void *next_note(void *buf, u32 *align)
+{
+ struct elf_note *n = buf;
+ u64 size;
+
+ if (check_add_overflow((u64)sizeof(*n), (u64)n->n_namesz, &size))
+ return NULL;
+
+ size = round_up(size, *align);
+
+ if (check_add_overflow(size, (u64)n->n_descsz, &size))
+ return NULL;
+
+ size = round_up(size, *align);
+
+ if (buf + size < buf)
+ return NULL;
+ else
+ return (buf + size);
+}
+
+static inline bool test_property_x86(void *buf, u32 *max_type)
+{
+ struct property_x86 *pr = buf;
+
+ /*
+ * Property types must be in ascending order.
+ * Keep track of the max when testing each.
+ */
+ if (pr->pr_type > *max_type)
+ *max_type = pr->pr_type;
+
+ return (pr->pr_type == GNU_PROPERTY_X86_FEATURE_1_AND);
+}
+
+static inline void *next_property(void *buf, u32 *max_type)
+{
+ struct property_x86 *pr = buf;
+
+ if ((buf + sizeof(*pr) + pr->pr_datasz < buf) ||
+ (pr->pr_type > GNU_PROPERTY_X86_FEATURE_1_AND) ||
+ (pr->pr_type > *max_type))
+ return NULL;
+ else
+ return (buf + sizeof(*pr) + pr->pr_datasz);
+}
+
+/*
+ * Scan 'buf' for a pattern; return true if found.
+ * *pos is the distance from the beginning of buf to where
+ * the searched item or the next item is located.
+ */
+static int scan(u8 *buf, u32 buf_size, int item_size, test_item_fn test_item,
+ next_item_fn next_item, u32 *arg, u32 *pos)
+{
+ int found = 0;
+ u8 *p, *max;
+
+ max = buf + buf_size;
+ if (max < buf)
+ return 0;
+
+ p = buf;
+
+ while ((p + item_size < max) && (p + item_size > buf)) {
+ if (test_item(p, arg)) {
+ found = 1;
+ break;
+ }
+
+ p = next_item(p, arg);
+ }
+
+ *pos = (p + item_size <= buf) ? 0 : (u32)(p - buf);
+ return found;
+}
+
+/*
+ * Search a NT_GNU_PROPERTY_TYPE_0 for GNU_PROPERTY_X86_FEATURE_1_AND.
+ */
+static int find_feature_x86(struct file *file, unsigned long desc_size,
+ loff_t file_offset, u8 *buf, u32 *feature)
+{
+ u32 buf_pos;
+ unsigned long read_size;
+ unsigned long done;
+ int found = 0;
+ int ret = 0;
+ u32 last_pr = 0;
+
+ *feature = 0;
+ buf_pos = 0;
+
+ for (done = 0; done < desc_size; done += buf_pos) {
+ read_size = desc_size - done;
+ if (read_size > BUF_SIZE)
+ read_size = BUF_SIZE;
+
+ ret = kernel_read(file, buf, read_size, &file_offset);
+
+ if (ret != read_size)
+ return (ret < 0) ? ret : -EIO;
+
+ ret = 0;
+ found = scan(buf, read_size, sizeof(struct property_x86),
+ test_property_x86, next_property,
+ &last_pr, &buf_pos);
+
+ if ((!buf_pos) || found)
+ break;
+
+ file_offset += buf_pos - read_size;
+ }
+
+ if (found) {
+ struct property_x86 *pr =
+ (struct property_x86 *)(buf + buf_pos);
+
+ if (pr->pr_datasz == 4) {
+ u32 *max = (u32 *)(buf + read_size);
+ u32 *data = (u32 *)((u8 *)pr + sizeof(*pr));
+
+ if (data + 1 <= max) {
+ *feature = *data;
+ } else {
+ file_offset += buf_pos - read_size;
+ file_offset += sizeof(*pr);
+ ret = kernel_read(file, feature, 4,
+ &file_offset);
+ }
+ }
+ }
+
+ return ret;
+}
+
+/*
+ * Search a PT_NOTE segment for NT_GNU_PROPERTY_TYPE_0.
+ */
+static int find_note_type_0(struct file *file, unsigned long note_size,
+ loff_t file_offset, u32 align, u32 *feature)
+{
+ u8 *buf;
+ u32 buf_pos;
+ unsigned long read_size;
+ unsigned long done;
+ int found = 0;
+ int ret = 0;
+
+ buf = kmalloc(BUF_SIZE, GFP_KERNEL);
+ if (!buf)
+ return -ENOMEM;
+
+ *feature = 0;
+ buf_pos = 0;
+
+ for (done = 0; done < note_size; done += buf_pos) {
+ read_size = note_size - done;
+ if (read_size > BUF_SIZE)
+ read_size = BUF_SIZE;
+
+ ret = kernel_read(file, buf, read_size, &file_offset);
+
+ if (ret != read_size) {
+ ret = (ret < 0) ? ret : -EIO;
+ kfree(buf);
+ return ret;
+ }
+
+ /*
+ * item_size = sizeof(struct elf_note) + elf_note.n_namesz.
+ * n_namesz is 4 for the note type we look for.
+ */
+ ret = scan(buf, read_size, sizeof(struct elf_note) + 4,
+ test_note_type_0, next_note,
+ &align, &buf_pos);
+
+ file_offset += buf_pos - read_size;
+
+ if (ret && !found) {
+ struct elf_note *n =
+ (struct elf_note *)(buf + buf_pos);
+ u64 start = round_up(sizeof(*n) + n->n_namesz, align);
+ u64 total = 0;
+
+ if (check_add_overflow(start, (u64)n->n_descsz, &total)) {
+ ret = -EINVAL;
+ break;
+ }
+ total = round_up(total, align);
+
+ ret = find_feature_x86(file, n->n_descsz,
+ file_offset + start,
+ buf, feature);
+ found++;
+ file_offset += total;
+ buf_pos += total;
+ } else if (!buf_pos || ret) {
+ ret = 0;
+ *feature = 0;
+ break;
+ }
+ }
+
+ kfree(buf);
+ return ret;
+}
+
+#ifdef CONFIG_COMPAT
+static int check_notes_32(struct file *file, struct elf32_phdr *phdr,
+ int phnum, u32 *feature)
+{
+ int i;
+ int err = 0;
+
+ for (i = 0; i < phnum; i++, phdr++) {
+ if ((phdr->p_type != PT_NOTE) || (phdr->p_align != 4))
+ continue;
+
+ err = find_note_type_0(file, phdr->p_filesz, phdr->p_offset,
+ phdr->p_align, feature);
+ if (err)
+ return err;
+ }
+
+ return 0;
+}
+#endif
+
+#ifdef CONFIG_X86_64
+static int check_notes_64(struct file *file, struct elf64_phdr *phdr,
+ int phnum, u32 *feature)
+{
+ int i;
+ int err = 0;
+
+ for (i = 0; i < phnum; i++, phdr++) {
+ if ((phdr->p_type != PT_NOTE) || (phdr->p_align != 8))
+ continue;
+
+ err = find_note_type_0(file, phdr->p_filesz, phdr->p_offset,
+ phdr->p_align, feature);
+ if (err)
+ return err;
+ }
+
+ return 0;
+}
+#endif
+
+int arch_setup_features(void *ehdr_p, void *phdr_p,
+ struct file *file, bool interp)
+{
+ int err = 0;
+ u32 feature = 0;
+
+ struct elf64_hdr *ehdr64 = ehdr_p;
+
+ if (!cpu_x86_cet_enabled())
+ return 0;
+
+ if (ehdr64->e_ident[EI_CLASS] == ELFCLASS64) {
+ struct elf64_phdr *phdr64 = phdr_p;
+
+ err = check_notes_64(file, phdr64, ehdr64->e_phnum,
+ &feature);
+ if (err < 0)
+ goto out;
+ } else {
+#ifdef CONFIG_COMPAT
+ struct elf32_hdr *ehdr32 = ehdr_p;
+
+ if (ehdr32->e_ident[EI_CLASS] == ELFCLASS32) {
+ struct elf32_phdr *phdr32 = phdr_p;
+
+ err = check_notes_32(file, phdr32, ehdr32->e_phnum,
+ &feature);
+ if (err < 0)
+ goto out;
+ }
+#else
+ WARN_ONCE(1, "Exec of 32-bit app, but CONFIG_COMPAT is not enabled.\n");
+ return -ENOTSUPP;
+#endif
+ }
+
+ memset(&current->thread.cet, 0, sizeof(struct cet_status));
+
+ if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+ if (feature & GNU_PROPERTY_X86_FEATURE_1_SHSTK) {
+ err = cet_setup_shstk();
+ if (err < 0)
+ goto out;
+ }
+ }
+
+out:
+ return err;
+}
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 54207327f98f..007ff0fbae84 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1081,6 +1081,21 @@ static int load_elf_binary(struct linux_binprm *bprm)
goto out_free_dentry;
}

+#ifdef CONFIG_ARCH_HAS_PROGRAM_PROPERTIES
+ if (interpreter) {
+ retval = arch_setup_features(&loc->interp_elf_ex,
+ interp_elf_phdata,
+ interpreter, true);
+ } else {
+ retval = arch_setup_features(&loc->elf_ex,
+ elf_phdata,
+ bprm->file, false);
+ }
+
+ if (retval < 0)
+ goto out_free_dentry;
+#endif
+
if (elf_interpreter) {
unsigned long interp_map_addr = 0;

diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index c5358e0ae7c5..5ef25a565e88 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -372,6 +372,7 @@ typedef struct elf64_shdr {
#define NT_PRFPREG 2
#define NT_PRPSINFO 3
#define NT_TASKSTRUCT 4
+#define NT_GNU_PROPERTY_TYPE_0 5
#define NT_AUXV 6
/*
* Note to userspace developers: size of NT_SIGINFO note may increase
--
2.17.1