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

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Date:   Mon, 13 Dec 2021 23:32:25 +0000
From:   Song Liu <songliubraving@...com>
To:     David Vernet <void@...ifault.com>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        "corbet@....net" <corbet@....net>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH] livepatch: Fix leak on klp_init_patch_early failure path



> On Dec 13, 2021, at 2:58 PM, David Vernet <void@...ifault.com> wrote:
> 
> Josh Poimboeuf <jpoimboe@...hat.com> wrote on Mon [2021-Dec-13 12:10:22 -0800]:
>> 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.
> 
> Understood, I'll take a read through and re-submit the patch to honor the
> norms for Linux kernel patches. My sincere apologies for the noise, and
> thank you for the positive and constructive suggestions.
> 
>> 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().
> 
> Ugh, good point and thank you for catching that. Another problem with the
> current patch is that we'll call kobject_put() on the patch even if we
> never call kobject_init on the patch due to patch->objs being NULL.
> 
> Perhaps we should pull try_module_get() and the NULL check for patch->objs
> out of klp_init_patch_early()? It feels a bit more intuitive to me if
> klp_init_patch_early() were only be responsible for initializing kobjects
> for the patch and its objects / funcs anyways.

Pulling those logic out of klp_init_patch_early() makes sense to me. 
Alternatively, we may also have a cleanup section in klp_init_patch_early(), 
like below. I am not sure which way will be cleaner. 

Thanks,
Song



diff --git i/kernel/livepatch/core.c w/kernel/livepatch/core.c
index 335d988bd811..20b959c82204 100644
--- i/kernel/livepatch/core.c
+++ w/kernel/livepatch/core.c
@@ -864,7 +864,7 @@ static void klp_init_object_early(struct klp_patch *patch,

 static int klp_init_patch_early(struct klp_patch *patch)
 {
-       struct klp_object *obj;
+       struct klp_object *obj, *obj2;
        struct klp_func *func;

        if (!patch->objs)
@@ -880,7 +880,7 @@ static int klp_init_patch_early(struct klp_patch *patch)

        klp_for_each_object_static(patch, obj) {
                if (!obj->funcs)
-                       return -EINVAL;
+                       goto cleanup;

                klp_init_object_early(patch, obj);

@@ -890,9 +890,15 @@ static int klp_init_patch_early(struct klp_patch *patch)
        }

        if (!try_module_get(patch->mod))
-               return -ENODEV;
+               goto cleanup;

        return 0;
+cleanup:
+       klp_for_each_func_static(patch, obj2) {
+               if (obj2 == obj)
+                       break;  // done
+               /* do clean up */
+       }
 }

 static int klp_init_patch(struct klp_patch *patch)