Re: [PATCH 1/1] x86/elf: Add a new .note section containing Xfeatures information to x86 core files

[Kees Cook]

On Thu, Mar 14, 2024 at 04:53:28PM +0530, Vignesh Balasubramanian wrote:
> Add a new .note section containing type, size, offset and flags of
> every xfeature that is present.
> 
> This information will be used by the debuggers to understand the XSAVE
> layout of the machine where the core file is dumped, and to read XSAVE
> registers, especially during cross-platform debugging.

I see binutils in CC. Can someone from gdb confirm that this solution
can be used?

> 
> Co-developed-by: Jini Susan George <jinisusan.george@xxxxxxx>
> Signed-off-by: Jini Susan George <jinisusan.george@xxxxxxx>
> Signed-off-by: Vignesh Balasubramanian <vigbalas@xxxxxxx>
> ---
>  arch/Kconfig                   |   9 +++
>  arch/powerpc/Kconfig           |   1 +
>  arch/powerpc/include/asm/elf.h |   2 -
>  arch/x86/Kconfig               |   1 +
>  arch/x86/include/asm/elf.h     |   7 +++
>  arch/x86/kernel/fpu/xstate.c   | 101 +++++++++++++++++++++++++++++++++
>  include/linux/elf.h            |   2 +-
>  include/uapi/linux/elf.h       |   1 +
>  8 files changed, 121 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index fd18b7db2c77..3bd8a0b2bba1 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -502,6 +502,15 @@ config MMU_LAZY_TLB_SHOOTDOWN
>  config ARCH_HAVE_NMI_SAFE_CMPXCHG
>  	bool
>  
> +config ARCH_HAVE_EXTRA_ELF_NOTES
> +	bool
> +	help
> +	  An architecture should select this in order to enable adding an
> +	  arch-specific ELF note section to core files. It must provide two
> +	  functions: elf_coredump_extra_notes_size() and
> +	  elf_coredump_extra_notes_write() which are invoked by the ELF core
> +	  dumper.
> +
>  config ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
>  	bool
>  
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index a91cb070ca4a..3b31bd7490e2 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -156,6 +156,7 @@ config PPC
>  	select ARCH_HAS_UACCESS_FLUSHCACHE
>  	select ARCH_HAS_UBSAN
>  	select ARCH_HAVE_NMI_SAFE_CMPXCHG
> +	select ARCH_HAVE_EXTRA_ELF_NOTES        if SPU_BASE
>  	select ARCH_KEEP_MEMBLOCK
>  	select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE	if PPC_RADIX_MMU
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
> diff --git a/arch/powerpc/include/asm/elf.h b/arch/powerpc/include/asm/elf.h
> index 79f1c480b5eb..bb4b94444d3e 100644
> --- a/arch/powerpc/include/asm/elf.h
> +++ b/arch/powerpc/include/asm/elf.h
> @@ -127,8 +127,6 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
>  /* Notes used in ET_CORE. Note name is "SPU/<fd>/<filename>". */
>  #define NT_SPU		1
>  
> -#define ARCH_HAVE_EXTRA_ELF_NOTES
> -
>  #endif /* CONFIG_SPU_BASE */
>  
>  #ifdef CONFIG_PPC64
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 78050d5d7fac..35e8d1201099 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -104,6 +104,7 @@ config X86
>  	select ARCH_HAS_DEBUG_WX
>  	select ARCH_HAS_ZONE_DMA_SET if EXPERT
>  	select ARCH_HAVE_NMI_SAFE_CMPXCHG
> +	select ARCH_HAVE_EXTRA_ELF_NOTES
>  	select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
>  	select ARCH_MIGHT_HAVE_ACPI_PDC		if ACPI
>  	select ARCH_MIGHT_HAVE_PC_PARPORT
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 1fb83d47711f..1b9f0b4bf6bc 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -13,6 +13,13 @@
>  #include <asm/auxvec.h>
>  #include <asm/fsgsbase.h>
>  
> +struct xfeat_component {
> +	u32 xfeat_type;
> +	u32 xfeat_sz;
> +	u32 xfeat_off;
> +	u32 xfeat_flags;
> +} __packed;

While it is currently true, just for robustness, can you add
a _Static_assert that sizeof(struct xfeat_component) % 4 == 0 ?
Notes must be 4-byte aligned.

> +
>  typedef unsigned long elf_greg_t;
>  
>  #define ELF_NGREG (sizeof(struct user_regs_struct) / sizeof(elf_greg_t))
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 117e74c44e75..6e5ea483ec1d 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -13,6 +13,7 @@
>  #include <linux/seq_file.h>
>  #include <linux/proc_fs.h>
>  #include <linux/vmalloc.h>
> +#include <linux/coredump.h>
>  
>  #include <asm/fpu/api.h>
>  #include <asm/fpu/regset.h>
> @@ -1836,3 +1837,103 @@ int proc_pid_arch_status(struct seq_file *m, struct pid_namespace *ns,
>  	return 0;
>  }
>  #endif /* CONFIG_PROC_PID_ARCH_STATUS */
> +
> +/*
> + * Dump type, size, offset and flag values for every xfeature that is present.
> + */
> +static int dump_xsave_layout_desc(struct coredump_params *cprm)
> +{
> +
> +	struct xfeat_component xc;
> +	int num_records = 0;
> +	int i;
> +
> +	/* XFEATURE_FPU and XFEATURE_SSE, both are fixed legacy states. */
> +	for (i = 0; i < FIRST_EXTENDED_XFEATURE; i++) {
> +		xc.xfeat_type = i;
> +		xc.xfeat_sz = xstate_sizes[i];
> +		xc.xfeat_off = xstate_offsets[i];
> +		xc.xfeat_flags = xstate_flags[i];
> +
> +		if (!dump_emit(cprm, &xc, sizeof(struct xfeat_component)))
> +			return 0;
> +		num_records++;
> +	}
> +
> +	for_each_extended_xfeature(i, fpu_user_cfg.max_features) {
> +		xc.xfeat_type = i;
> +		xc.xfeat_sz = xstate_sizes[i];
> +		xc.xfeat_off = xstate_offsets[i];
> +		xc.xfeat_flags = xstate_flags[i];
> +
> +		if (!dump_emit(cprm, &xc, sizeof(struct xfeat_component)))
> +			return 0;
> +		num_records++;
> +	}
> +
> +	return num_records;
> +}
> +
> +static int get_xsave_desc_size(void)
> +{
> +	/* XFEATURE_FP and XFEATURE_SSE, both are fixed legacy states */
> +	int xfeatures_count = 2;
> +	int i;
> +
> +	for_each_extended_xfeature(i, fpu_user_cfg.max_features)
> +		xfeatures_count++;
> +
> +	return xfeatures_count * (sizeof(struct xfeat_component));
> +}
> +
> +int elf_coredump_extra_notes_write(struct coredump_params *cprm)
> +{
> +	const char *owner_name = "LINUX";

If you use an array instead of a pointer, there's no need for strlen(),
and you can make it a static outside of the function to refer to it
later.

static const char owner_name[] = "LINUX";

> +	int num_records = 0;
> +	struct elf_note en;
> +
> +	en.n_namesz = strlen(owner_name) + 1;

en.n_namesz = sizeof(owner_name);

> +	en.n_descsz = get_xsave_desc_size();
> +	en.n_type = NT_X86_XSAVE_LAYOUT;
> +
> +	if (!dump_emit(cprm, &en, sizeof(en)))
> +		return 1;
> +	if (!dump_emit(cprm, owner_name, en.n_namesz))
> +		return 1;
> +	if (!dump_align(cprm, 4))
> +		return 1;
> +
> +	num_records = dump_xsave_layout_desc(cprm);
> +	if (!num_records) {
> +		pr_warn("Error adding XSTATE layout ELF note. XSTATE buffer in the core file will be unparseable.");

Can you make this pr_warn_ratelimited() (and below)?

> +		return 1;
> +	}
> +
> +	/* Total size should be equal to the number of records */
> +	if ((sizeof(struct xfeat_component) * num_records) != en.n_descsz) {
> +		pr_warn("Error adding XSTATE layout ELF note. The size of the .note section does not match with the total size of the records.");
> +		return 1;
> +	}
> +
> +	if (!dump_align(cprm, 4))
> +		return 1;

I don't think this call is needed?

> +
> +	return 0;
> +}
> +
> +/*
> + * Return the size of new note.
> + */
> +int elf_coredump_extra_notes_size(void)
> +{
> +	const char *fullname = "LINUX";

Now this can be dropped.

> +	int size = 0;
> +
> +	/* NOTE Header */
> +	size += sizeof(struct elf_note);
> +	/* name + align */
> +	size += roundup(strlen(fullname) + 1, 4);

	size += roundup(sizeof(owner_name), 4);

> +	size += get_xsave_desc_size();
> +
> +	return size;
> +}
> diff --git a/include/linux/elf.h b/include/linux/elf.h
> index c9a46c4e183b..5c402788da19 100644
> --- a/include/linux/elf.h
> +++ b/include/linux/elf.h
> @@ -65,7 +65,7 @@ extern Elf64_Dyn _DYNAMIC [];
>  struct file;
>  struct coredump_params;
>  
> -#ifndef ARCH_HAVE_EXTRA_ELF_NOTES
> +#ifndef CONFIG_ARCH_HAVE_EXTRA_ELF_NOTES
>  static inline int elf_coredump_extra_notes_size(void) { return 0; }
>  static inline int elf_coredump_extra_notes_write(struct coredump_params *cprm) { return 0; }
>  #else
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index 9417309b7230..3325488cb39b 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -411,6 +411,7 @@ typedef struct elf64_shdr {
>  #define NT_X86_XSTATE	0x202		/* x86 extended state using xsave */
>  /* Old binutils treats 0x203 as a CET state */
>  #define NT_X86_SHSTK	0x204		/* x86 SHSTK state */
> +#define NT_X86_XSAVE_LAYOUT	0x205	/* XSAVE layout description */
>  #define NT_S390_HIGH_GPRS	0x300	/* s390 upper register halves */
>  #define NT_S390_TIMER	0x301		/* s390 timer register */
>  #define NT_S390_TODCMP	0x302		/* s390 TOD clock comparator register */
> -- 
> 2.43.0
> 

Otherwise looks reasonable, though I see Dave has feedback to address
too. :)

Thanks for working on this!

-Kees

-- 
Kees Cook