Re: [PATCH v2 1/3] module: verify address is read-only

[Jessica Yu]

+++ Eddie Kovsky [17/02/17 22:58 -0700]:
> Implement a mechanism to check if a module's address is in
> the rodata or ro_after_init sections. It mimics the exsiting functions
> that test if an address is inside a module's text section.

It would be helpful to explain in the changelog the motivation or
reason we're adding to the current api, and why this is needed.

> Signed-off-by: Eddie Kovsky <ewk@xxxxxxxxxxxx>
> ---
> include/linux/module.h |  7 +++++++
> kernel/module.c        | 44 ++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 51 insertions(+)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 0297c5cd7cdf..1608d3570ee2 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -492,7 +492,9 @@ static inline int module_is_live(struct module *mod)
>
> struct module *__module_text_address(unsigned long addr);
> struct module *__module_address(unsigned long addr);
> +struct module *__module_ro_address(unsigned long addr);
> bool is_module_address(unsigned long addr);
> +bool is_module_ro_address(unsigned long addr);
> bool is_module_percpu_address(unsigned long addr);
> bool is_module_text_address(unsigned long addr);
>
> @@ -645,6 +647,11 @@ static inline struct module *__module_address(unsigned long addr)
> 	return NULL;
> }
>
> +static inline struct module *__module_ro_address(unsigned long addr)
> +{
> +	return NULL;
> +}
> +

There needs to be a corresponding !CONFIG_MODULES stub for
is_module_ro_address() as well, see is_module_address(), etc.

> static inline struct module *__module_text_address(unsigned long addr)
> {
> 	return NULL;
> diff --git a/kernel/module.c b/kernel/module.c
> index 7eba6dea4f41..298cfe4645b1 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -4275,6 +4275,50 @@ struct module *__module_text_address(unsigned long addr)
> }
> EXPORT_SYMBOL_GPL(__module_text_address);
>
> +/**
> + * is_module_text_ro_address - is this address inside read-only module code?
> + * @addr: the address to check.

s/is_module_text_ro_address/is_module_ro_address/

Although I would slightly prefer the name is_module_rodata_address, or
something similar, because it's unclear to me (without looking at the
code) whether the function covers text addresses. And we already have
is_module_text_address() for that case.

> + *
> + */
> +bool is_module_ro_address(unsigned long addr)
> +{
> +	bool ret;
> +
> +	preempt_disable();
> +	ret = __module_ro_address(addr) != NULL;
> +	preempt_enable();
> +
> +	return ret;
> +}
> +
> +/*
> + * __module_ro_address - get the module whose code contains a read-only address.

Maybe we should be more specific in the comment and clarify that we're
getting the module whose rodata/ro_after_init sections contain the
given address.

> + * @addr: the address.
> + *
> + * Must be called with preempt disabled or module mutex held so that
> + * module doesn't get freed during this.
> + */
> +struct module *__module_ro_address(unsigned long addr)
> +{
> +	struct module *mod = __module_address(addr);
> +
> +	if (mod) {
> +		/* Make sure it's within the read-only section. */
> +		if (!within(addr, mod->init_layout.base,
> +			    mod->init_layout.ro_size)
> +		    && !within(addr, mod->core_layout.base,
> +			       mod->core_layout.ro_size))
> +			mod = NULL;

If a ro_after_init address gets passed in here, mod will be prematurely
set to NULL here, because it is not within [base, base + ro_size).
See comment above frob_text() in module.c for a module layout "diagram".

Also, starting from layout.base will give us the text region as well,
but we just want to check the rodata/ro_after_init regions, same as
what the kernel counterpart in patch 2/3 does. The rodata region
starts at base + text_size.

So we can just simplify this to just one conditional:
(ugly, but shouldn't look bad when cleaned up with some variables)

       if (!within(addr, mod->init_layout.base + mod->init_layout.text_size,
                   mod->init_layout.ro_after_init_size - mod->init_layout.text_size)
        && !within(addr, mod->core_layout.base + mod->core_layout.text_size,
                   mod->core_layout.ro_after_init_size - mod->core_layout.text_size))

This is because range [base + text_size, base + ro_after_init_size)
encompasses both the rodata and ro_after_init regions.

Jessica

> +		if (!within(addr, mod->init_layout.base,
> +			    mod->init_layout.ro_after_init_size)
> +		    && !within(addr, mod->core_layout.base,
> +			       mod->core_layout.ro_after_init_size))
> +			mod = NULL;
> +	}
> +	return mod;
> +}
> +EXPORT_SYMBOL_GPL(__module_ro_address);
> +
> /* Don't grab lock, we're oopsing. */
> void print_modules(void)
> {
> --
> 2.11.1