linux-kernel - Re: [PATCH] livepatch: Fix leak on klp_init_patch

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Date:   Mon, 13 Dec 2021 12:10:22 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Vernet <void@...ifault.com>
Cc:     pmladek@...e.com, linux-doc@...r.kernel.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        jikos@...nel.org, mbenes@...e.cz, joe.lawrence@...hat.com,
        corbet@....net, yhs@...com, songliubraving@...com
Subject: Re: [PATCH] livepatch: Fix leak on klp_init_patch_early failure path

On Mon, Dec 13, 2021 at 11:17:35AM -0800, David Vernet wrote:
> When enabling a KLP patch with `klp_enable_patch`, we invoke
> `klp_init_patch_early` to initialize the kobjects for the patch itself, as
> well as the `struct klp_object*`'s and `struct klp_func*`'s that comprise
> it. However, there are some paths where we may fail to do an
> early-initialization of an object or its functions if certain conditions
> are not met, such as an object having a `NULL` funcs pointer. In these
> paths, we may currently leak the `struct klp_patch*`'s kobject, as well as
> any of its objects or functions, as we don't free the patch in
> `klp_enable_patch` if `klp_init_patch_early` returns an error code. For
> example, if we added the following object entry to the sample livepatch
> code, it would cause us to leak the vmlinux `klp_object`, and its `struct
> klp_func` which updates `cmdline_proc_show`:
> 
> ```
> static struct klp_object objs[] = {
>         {
>                 .name = "kvm",
>         }, { }
> };
> ```
> 
> Without this change, if we enable `CONFIG_DEBUG_KOBJECT` and try to `kpatch
> load livepatch-sample.ko`, we don't observe the kobjects being released
> (though of course we do observe `insmod` failing to insert the module).
> With the change, we do observe that the `kobject` for the patch and its
> `vmlinux` object are released.
> 
> Signed-off-by: David Vernet <void@...ifault.com>

Thanks for reporting the issue and submitting the patch!

The patch description needs a few tweaks.  In the kernel we don't use
Markdown for patch descriptions.

A function can be postfixed with a trailing pair of parentheses, like
klp_enable_patch().

Other symbols can be enclosed with single quotes, like 'struct
klp_object'.

I'd also recommend avoiding the excessive use of "we", in favor of more
imperative-type language.

See Documentation/process/submitting-patches.rst for more details.  It's
also a good idea to look at some kernel commit logs to get a general
idea of the kernel patch description style.

> @@ -1052,10 +1052,7 @@ int klp_enable_patch(struct klp_patch *patch)
>  	}
>  
>  	ret = klp_init_patch_early(patch);
> -	if (ret) {
> -		mutex_unlock(&klp_mutex);
> -		return ret;
> -	}
> +		goto err;
>  
>  	ret = klp_init_patch(patch);
>  	if (ret)

I don't think the fix will be quite that simple.  For example, if
klp_init_patch_early() fails, that means try_module_get() hasn't been
done, so klp_free_patch_finish() will wrongly do a module_put().

-- 
Josh