Re: [PATCH v3 10/35] lib: code tagging framework

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 12, 2024 at 01:38:56PM -0800, Suren Baghdasaryan wrote:
> Add basic infrastructure to support code tagging which stores tag common
> information consisting of the module name, function, file name and line
> number. Provide functions to register a new code tag type and navigate
> between code tags.
> 
> Co-developed-by: Kent Overstreet <kent.overstreet@linux.dev>
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/codetag.h |  71 ++++++++++++++
>  lib/Kconfig.debug       |   4 +
>  lib/Makefile            |   1 +
>  lib/codetag.c           | 199 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 275 insertions(+)
>  create mode 100644 include/linux/codetag.h
>  create mode 100644 lib/codetag.c
> 
> diff --git a/include/linux/codetag.h b/include/linux/codetag.h
> new file mode 100644
> index 000000000000..a9d7adecc2a5
> --- /dev/null
> +++ b/include/linux/codetag.h
> @@ -0,0 +1,71 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * code tagging framework
> + */
> +#ifndef _LINUX_CODETAG_H
> +#define _LINUX_CODETAG_H
> +
> +#include <linux/types.h>
> +
> +struct codetag_iterator;
> +struct codetag_type;
> +struct seq_buf;
> +struct module;
> +
> +/*
> + * An instance of this structure is created in a special ELF section at every
> + * code location being tagged.  At runtime, the special section is treated as
> + * an array of these.
> + */
> +struct codetag {
> +	unsigned int flags; /* used in later patches */
> +	unsigned int lineno;
> +	const char *modname;
> +	const char *function;
> +	const char *filename;
> +} __aligned(8);
> +
> +union codetag_ref {
> +	struct codetag *ct;
> +};
> +
> +struct codetag_range {
> +	struct codetag *start;
> +	struct codetag *stop;
> +};
> +
> +struct codetag_module {
> +	struct module *mod;
> +	struct codetag_range range;
> +};
> +
> +struct codetag_type_desc {
> +	const char *section;
> +	size_t tag_size;
> +};
> +
> +struct codetag_iterator {
> +	struct codetag_type *cttype;
> +	struct codetag_module *cmod;
> +	unsigned long mod_id;
> +	struct codetag *ct;
> +};
> +
> +#define CODE_TAG_INIT {					\
> +	.modname	= KBUILD_MODNAME,		\
> +	.function	= __func__,			\
> +	.filename	= __FILE__,			\
> +	.lineno		= __LINE__,			\
> +	.flags		= 0,				\
> +}
> +
> +void codetag_lock_module_list(struct codetag_type *cttype, bool lock);
> +struct codetag_iterator codetag_get_ct_iter(struct codetag_type *cttype);
> +struct codetag *codetag_next_ct(struct codetag_iterator *iter);
> +
> +void codetag_to_text(struct seq_buf *out, struct codetag *ct);
> +
> +struct codetag_type *
> +codetag_register_type(const struct codetag_type_desc *desc);
> +
> +#endif /* _LINUX_CODETAG_H */
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 975a07f9f1cc..0be2d00c3696 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -968,6 +968,10 @@ config DEBUG_STACKOVERFLOW
>  
>  	  If in doubt, say "N".
>  
> +config CODE_TAGGING
> +	bool
> +	select KALLSYMS
> +
>  source "lib/Kconfig.kasan"
>  source "lib/Kconfig.kfence"
>  source "lib/Kconfig.kmsan"
> diff --git a/lib/Makefile b/lib/Makefile
> index 6b09731d8e61..6b48b22fdfac 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -235,6 +235,7 @@ obj-$(CONFIG_OF_RECONFIG_NOTIFIER_ERROR_INJECT) += \
>  	of-reconfig-notifier-error-inject.o
>  obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
>  
> +obj-$(CONFIG_CODE_TAGGING) += codetag.o
>  lib-$(CONFIG_GENERIC_BUG) += bug.o
>  
>  obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o
> diff --git a/lib/codetag.c b/lib/codetag.c
> new file mode 100644
> index 000000000000..7708f8388e55
> --- /dev/null
> +++ b/lib/codetag.c
> @@ -0,0 +1,199 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <linux/codetag.h>
> +#include <linux/idr.h>
> +#include <linux/kallsyms.h>
> +#include <linux/module.h>
> +#include <linux/seq_buf.h>
> +#include <linux/slab.h>
> +
> +struct codetag_type {
> +	struct list_head link;
> +	unsigned int count;
> +	struct idr mod_idr;
> +	struct rw_semaphore mod_lock; /* protects mod_idr */
> +	struct codetag_type_desc desc;
> +};
> +
> +static DEFINE_MUTEX(codetag_lock);
> +static LIST_HEAD(codetag_types);
> +
> +void codetag_lock_module_list(struct codetag_type *cttype, bool lock)
> +{
> +	if (lock)
> +		down_read(&cttype->mod_lock);
> +	else
> +		up_read(&cttype->mod_lock);
> +}
> +
> +struct codetag_iterator codetag_get_ct_iter(struct codetag_type *cttype)
> +{
> +	struct codetag_iterator iter = {
> +		.cttype = cttype,
> +		.cmod = NULL,
> +		.mod_id = 0,
> +		.ct = NULL,
> +	};
> +
> +	return iter;
> +}
> +
> +static inline struct codetag *get_first_module_ct(struct codetag_module *cmod)
> +{
> +	return cmod->range.start < cmod->range.stop ? cmod->range.start : NULL;
> +}
> +
> +static inline
> +struct codetag *get_next_module_ct(struct codetag_iterator *iter)
> +{
> +	struct codetag *res = (struct codetag *)
> +			((char *)iter->ct + iter->cttype->desc.tag_size);
> +
> +	return res < iter->cmod->range.stop ? res : NULL;
> +}
> +
> +struct codetag *codetag_next_ct(struct codetag_iterator *iter)
> +{
> +	struct codetag_type *cttype = iter->cttype;
> +	struct codetag_module *cmod;
> +	struct codetag *ct;
> +
> +	lockdep_assert_held(&cttype->mod_lock);
> +
> +	if (unlikely(idr_is_empty(&cttype->mod_idr)))
> +		return NULL;
> +
> +	ct = NULL;
> +	while (true) {
> +		cmod = idr_find(&cttype->mod_idr, iter->mod_id);
> +
> +		/* If module was removed move to the next one */
> +		if (!cmod)
> +			cmod = idr_get_next_ul(&cttype->mod_idr,
> +					       &iter->mod_id);
> +
> +		/* Exit if no more modules */
> +		if (!cmod)
> +			break;
> +
> +		if (cmod != iter->cmod) {
> +			iter->cmod = cmod;
> +			ct = get_first_module_ct(cmod);
> +		} else
> +			ct = get_next_module_ct(iter);
> +
> +		if (ct)
> +			break;
> +
> +		iter->mod_id++;
> +	}
> +
> +	iter->ct = ct;
> +	return ct;
> +}
> +
> +void codetag_to_text(struct seq_buf *out, struct codetag *ct)
> +{
> +	seq_buf_printf(out, "%s:%u module:%s func:%s",
> +		       ct->filename, ct->lineno,
> +		       ct->modname, ct->function);
> +}

Thank you for using seq_buf here!

Also, will this need an EXPORT_SYMBOL_GPL()?

> +
> +static inline size_t range_size(const struct codetag_type *cttype,
> +				const struct codetag_range *range)
> +{
> +	return ((char *)range->stop - (char *)range->start) /
> +			cttype->desc.tag_size;
> +}
> +
> +static void *get_symbol(struct module *mod, const char *prefix, const char *name)
> +{
> +	char buf[64];

Why is 64 enough? I was expecting KSYM_NAME_LEN here, but perhaps this
is specialized enough to section names that it will not be a problem?
If so, please document it clearly with a comment.

> +	int res;
> +
> +	res = snprintf(buf, sizeof(buf), "%s%s", prefix, name);
> +	if (WARN_ON(res < 1 || res > sizeof(buf)))
> +		return NULL;

Please use a seq_buf here instead of snprintf, which we're trying to get
rid of.

	DECLARE_SEQ_BUF(sb, KSYM_NAME_LEN);
	char *buf;

	seq_buf_printf(sb, "%s%s", prefix, name);
	if (seq_buf_has_overflowed(sb))
		return NULL;

	buf = seq_buf_str(sb);

> +
> +	return mod ?
> +		(void *)find_kallsyms_symbol_value(mod, buf) :
> +		(void *)kallsyms_lookup_name(buf);
> +}
> +
> +static struct codetag_range get_section_range(struct module *mod,
> +					      const char *section)
> +{
> +	return (struct codetag_range) {
> +		get_symbol(mod, "__start_", section),
> +		get_symbol(mod, "__stop_", section),
> +	};
> +}
> +
> +static int codetag_module_init(struct codetag_type *cttype, struct module *mod)
> +{
> +	struct codetag_range range;
> +	struct codetag_module *cmod;
> +	int err;
> +
> +	range = get_section_range(mod, cttype->desc.section);
> +	if (!range.start || !range.stop) {
> +		pr_warn("Failed to load code tags of type %s from the module %s\n",
> +			cttype->desc.section,
> +			mod ? mod->name : "(built-in)");
> +		return -EINVAL;
> +	}
> +
> +	/* Ignore empty ranges */
> +	if (range.start == range.stop)
> +		return 0;
> +
> +	BUG_ON(range.start > range.stop);
> +
> +	cmod = kmalloc(sizeof(*cmod), GFP_KERNEL);
> +	if (unlikely(!cmod))
> +		return -ENOMEM;
> +
> +	cmod->mod = mod;
> +	cmod->range = range;
> +
> +	down_write(&cttype->mod_lock);
> +	err = idr_alloc(&cttype->mod_idr, cmod, 0, 0, GFP_KERNEL);
> +	if (err >= 0)
> +		cttype->count += range_size(cttype, &range);
> +	up_write(&cttype->mod_lock);
> +
> +	if (err < 0) {
> +		kfree(cmod);
> +		return err;
> +	}
> +
> +	return 0;
> +}
> +
> +struct codetag_type *
> +codetag_register_type(const struct codetag_type_desc *desc)
> +{
> +	struct codetag_type *cttype;
> +	int err;
> +
> +	BUG_ON(desc->tag_size <= 0);
> +
> +	cttype = kzalloc(sizeof(*cttype), GFP_KERNEL);
> +	if (unlikely(!cttype))
> +		return ERR_PTR(-ENOMEM);
> +
> +	cttype->desc = *desc;
> +	idr_init(&cttype->mod_idr);
> +	init_rwsem(&cttype->mod_lock);
> +
> +	err = codetag_module_init(cttype, NULL);
> +	if (unlikely(err)) {
> +		kfree(cttype);
> +		return ERR_PTR(err);
> +	}
> +
> +	mutex_lock(&codetag_lock);
> +	list_add_tail(&cttype->link, &codetag_types);
> +	mutex_unlock(&codetag_lock);
> +
> +	return cttype;
> +}
> -- 
> 2.43.0.687.g38aa6559b0-goog
> 

-- 
Kees Cook