Re: [RFC PATCH v4 21/27] x86/cet/shstk: ELF header parsing of Shadow Stack

Kees Cook
2018-10-15T23:40:59Z

On Fri, Sep 21, 2018 at 8:03 AM, Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> Look in .note.gnu.property of an ELF file and check if Shadow Stack needs
> to be enabled for the task.

Ah, I've been wanting this for other things too (see below).

>
> 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 | 340 +++++++++++++++++++++++
> fs/binfmt_elf.c | 15 +
> include/uapi/linux/elf.h | 1 +
> 7 files changed, 382 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 808aa3aecf3c..6377125543cc 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1919,12 +1919,16 @@ 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
> depends on CPU_SUP_INTEL && X86_64
> 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 0d157d2a1e2a..5b5f169c5c07 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -382,4 +382,9 @@ 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..2fddd0bc545b
> --- /dev/null
> +++ b/arch/x86/kernel/elf.c
> @@ -0,0 +1,340 @@
> +/* 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];
> + * }[];
> + */

Does NT_GNU_PROPERTY_TYPE_0 only ever contain property_x86 bytes? (I
assume not, since there is a pr_type?)

> +
> +#define BUF_SIZE (PAGE_SIZE / 4)
> +
> +struct property_x86 {
> + u32 pr_type;
> + u32 pr_datasz;
> +};
> +
> +typedef bool (test_fn)(void *buf, u32 *arg);
> +typedef void *(next_fn)(void *buf, u32 *arg);
> +
> +static inline bool test_note_type_0(void *buf, u32 *arg)
> +{
> + struct elf_note *n = buf;
> +
> + return ((n->n_namesz == 4) && (memcmp(n + 1, "GNU", 4) == 0) &&
> + (n->n_type == NT_GNU_PROPERTY_TYPE_0));

Cheaper to test n_type first...

> +}
> +
> +static inline void *next_note(void *buf, u32 *arg)
> +{
> + struct elf_note *n = buf;
> + u32 align = *arg;
> + int size;
> +
> + size = round_up(sizeof(*n) + n->n_namesz, align);

I think this could overflow: n_namesz can be u64 for elf64_note.

> + size = round_up(size + n->n_descsz, align);

Same here. You may want to use check_add_overflow(), etc, an u64 types.

> +
> + if (buf + size < buf)
> + return NULL;

I don't understand this. You want to check size not exceeding the
allocation, which isn't passed into this function. Checking for a full
unsigned address wrap around is not sufficient to detect overflow.

> + else
> + return (buf + size);
> +}
> +
> +static inline bool test_property_x86(void *buf, u32 *arg)
> +{
> + struct property_x86 *pr = buf;
> + u32 max_type = *arg;
> +
> + if (pr->pr_type > max_type)
> + *arg = pr->pr_type;

Why is *arg being updated? I don't see last_pr used outside of here --
are properties required to be pr_type-ordered?

> +
> + return (pr->pr_type == GNU_PROPERTY_X86_FEATURE_1_AND);
> +}
> +
> +static inline void *next_property(void *buf, u32 *arg)
> +{
> + struct property_x86 *pr = buf;
> + u32 max_type = *arg;
> +
> + if ((buf + sizeof(*pr) + pr->pr_datasz < buf) ||

Again, this "< buf" test doesn't look at all correct to me.

> + (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_fn test, next_fn next, u32 *arg, u32 *pos)

I'm not a fan of the short "scan", "test" and "next" names, and I
really don't like an arg named "arg". Something slightly more
descriptive for all of these would be nice, please.

> +{
> + 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)) {

These comparisons are safe due to the BUF_SIZE limit of buf_size and
the only used size of item_size, but if this becomes more generic, it
should be more defensive on the size calculations (e.g. make sure than
"item_size < max" and then here "p < max - item_size", etc).

I'd kind of rather this code walked the base type and check each for
the matching feature. What is the general specification for what
NT_GNU_PROPERTY_TYPE_0 contains?

> + if (test(p, arg)) {
> + found = 1;
> + break;
> + }
> +
> + p = next(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 the first 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;

Why kmalloc over stack variable? (Or, does BUF_SIZE here really need
to be 1024?)

> +
> + *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 = 0;
> + found += 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 (found == 1) {
> + struct elf_note *n =
> + (struct elf_note *)(buf + buf_pos);
> + u32 start = round_up(sizeof(*n) + n->n_namesz, align);
> + u32 total = round_up(start + n->n_descsz, align);

Same overflow notes from earlier...

> +
> + ret = find_feature_x86(file, n->n_descsz,
> + file_offset + start,
> + buf, feature);
> + file_offset += total;
> + buf_pos += total;
> + } else if (!buf_pos) {
> + *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;

Instead of a separate parser here, wouldn't it be a bit nicer to
attach this to the existing binfmt_elf program header parsing loop:

elf_ppnt = elf_phdata;
for (i = 0; i < loc->elf_ex.e_phnum; i++, elf_ppnt++)
switch (elf_ppnt->p_type) {
case PT_GNU_STACK:
...
case PT_LOPROC ... PT_HIPROC:
...

> +
> + 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_feature_enabled(X86_FEATURE_SHSTK))
> + 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;
> + }
> +#endif

Should there be an #else error here?

> + }
> +
> + memset(&current->thread.cet, 0, sizeof(struct cet_status));
> +
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {

The CPU feature was already tested at arch_setup_features() entry.

> + 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 efae2fb0930a..b891aa292b46 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
>

I'd like to be using this code for a few other cases too (not just
x86-specific). For example, for marking KASan binaries as needing a
"legacy" memory layouts[1]. Others might be setting things like
no_new_privs at exec time, etc.

-Kees

[1] https://lkml.kernel.org/r/CAGXu5jL1HRG7Dn9vraw8Hu7LF+69k3EDpztt1Ju7ijEzmvRdhA@mail.gmail.com

--
Kees Cook
Pixel Security