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

[Eugene Syromiatnikov <esyr@redhat.com>]

On Fri, Sep 21, 2018 at 08:03:45AM -0700, Yu-cheng Yu wrote:
> 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                    | 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)

Hm, these defeinitions aren't much different comparing to NT_*
definitions in include/uapi/linux/elf.h, is it expected that those
properties have to be parsed individually for each architecture?

> +
> +#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

Same thing here, enablement of program properties per se seems rather generic.

> 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];
> + *	}[];
> + */
> +
> +#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));
> +}
> +
> +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);
> +	size = round_up(size + n->n_descsz, align);
> +
> +	if (buf + size < buf)
> +		return NULL;
> +	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;
> +
> +	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) ||
> +	    (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)
> +{
> +	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(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;
> +
> +	*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);
> +
> +			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;
> +
> +		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
> +	}
> +
> +	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;
> +}

There's a lot of similar code with bpf stackmap .build-id code (commit
v4.17-rc1~148^2~156^2~3^2~1), it might be worthy generalising some ELF
traversal routines, since there's general need of parsing ELF property
segments.