Re: [PATCH v8 06/12] kernel/module: add documentation for try_module_get()

[Kees Cook]

On Mon, Sep 27, 2021 at 09:37:59AM -0700, Luis Chamberlain wrote:
> There is quite a bit of tribal knowledge around proper use of
> try_module_get() and that it must be used only in a context which
> can ensure the module won't be gone during the operation. Document
> this little bit of tribal knowledge.
> 
> I'm extending this tribal knowledge with new developments which it
> seems some folks do not yet believe to be true: we can be sure a
> module will exist during the lifetime of a sysfs file operation.
> For proof, refer to test_sysfs test #32:
> 
> ./tools/testing/selftests/sysfs/sysfs.sh -t 0032
> 
> Without this being true, the write would fail or worse,
> a crash would happen, in this test. It does not.
> 
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>  include/linux/module.h | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index c9f1200b2312..22eacd5e1e85 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -609,10 +609,40 @@ void symbol_put_addr(void *addr);
>     to handle the error case (which only happens with rmmod --wait). */
>  extern void __module_get(struct module *module);
>  
> -/* This is the Right Way to get a module: if it fails, it's being removed,
> - * so pretend it's not there. */
> +/**
> + * try_module_get() - yields to module removal and bumps refcnt otherwise

I find this hard to parse. How about:
	"Take module refcount unless module is being removed"

> + * @module: the module we should check for
> + *
> + * This can be used to try to bump the reference count of a module, so to
> + * prevent module removal. The reference count of a module is not allowed
> + * to be incremented if the module is already being removed.

This I understand.

> + *
> + * Care must be taken to ensure the module cannot be removed during the call to
> + * try_module_get(). This can be done by having another entity other than the
> + * module itself increment the module reference count, or through some other
> + * means which guarantees the module could not be removed during an operation.
> + * An example of this later case is using try_module_get() in a sysfs file
> + * which the module created. The sysfs store / read file operations are
> + * gauranteed to exist through the use of kernfs's active reference (see
> + * kernfs_active()). If a sysfs file operation is being run, the module which
> + * created it must still exist as the module is in charge of removing the same
> + * sysfs file being read. Also, a sysfs / kernfs file removal cannot happen
> + * unless the same file is not active.

I can't understand this paragraph at all. "Care must be taken ..."? Why?
Shouldn't callers of try_module_get() be satisfied with the results? I
don't follow the example at all. It seems to just say "sysfs store/read
functions don't need try_module_get() because whatever opened the sysfs
file is already keeping the module referenced." ?

> + *
> + * One of the real values to try_module_get() is the module_is_live() check
> + * which ensures this the caller of try_module_get() can yield to userspace
> + * module removal requests and fail whatever it was about to process.

Please document the return value explicitly.

> + */
>  extern bool try_module_get(struct module *module);
>  
> +/**
> + * module_put() - release a reference count to a module
> + * @module: the module we should release a reference count for
> + *
> + * If you successfully bump a reference count to a module with try_module_get(),
> + * when you are finished you must call module_put() to release that reference
> + * count.
> + */
>  extern void module_put(struct module *module);
>  
>  #else /*!CONFIG_MODULE_UNLOAD*/
> -- 
> 2.30.2
> 

-- 
Kees Cook