linux-kernel - Re: [PATCH next] audit: fix potential double free on error path from fsnotify_add_inode

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Date:   Tue, 23 Aug 2022 10:06:24 +0800
From:   cuigaosheng <cuigaosheng1@...wei.com>
To:     Paul Moore <paul@...l-moore.com>, Jan Kara <jack@...e.cz>,
        <cuigaosheng1@...wei.com>
CC:     <eparis@...hat.com>, <mszeredi@...hat.com>, <amir73il@...il.com>,
        <linux-audit@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH next] audit: fix potential double free on error path from
 fsnotify_add_inode_mark


在 2022/8/23 6:49, Paul Moore 写道:
> On Mon, Aug 22, 2022 at 1:59 PM Jan Kara <jack@...e.cz> wrote:
>> On Mon 22-08-22 13:39:23, Paul Moore wrote:
>>> On Mon, Aug 22, 2022 at 11:20 AM Jan Kara <jack@...e.cz> wrote:
>>>> On Mon 22-08-22 10:34:15, Paul Moore wrote:
>>>>> On Mon, Aug 22, 2022 at 4:50 AM Jan Kara <jack@...e.cz> wrote:
>>>>>> On Mon 22-08-22 10:29:05, Gaosheng Cui wrote:
>>>>>>> Audit_alloc_mark() assign pathname to audit_mark->path, on error path
>>>>>>> from fsnotify_add_inode_mark(), fsnotify_put_mark will free memory
>>>>>>> of audit_mark->path, but the caller of audit_alloc_mark will free
>>>>>>> the pathname again, so there will be double free problem.
>>>>>>>
>>>>>>> Fix this by resetting audit_mark->path to NULL pointer on error path
>>>>>>> from fsnotify_add_inode_mark().
>>>>>>>
>>>>>>> Fixes: 7b1293234084d ("fsnotify: Add group pointer in fsnotify_init_mark()")
>>>>>>> Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
>>>>>> Good spotting! The patch looks good to me. Feel free to add:
>>>>>>
>>>>>> Reviewed-by: Jan Kara <jack@...e.cz>
>>>>>>
>>>>>>> ---
>>>>>>>   kernel/audit_fsnotify.c | 1 +
>>>>>>>   1 file changed, 1 insertion(+)
>>>>>>>
>>>>>>> diff --git a/kernel/audit_fsnotify.c b/kernel/audit_fsnotify.c
>>>>>>> index 6432a37ac1c9..c565fbf66ac8 100644
>>>>>>> --- a/kernel/audit_fsnotify.c
>>>>>>> +++ b/kernel/audit_fsnotify.c
>>>>>>> @@ -102,6 +102,7 @@ struct audit_fsnotify_mark *audit_alloc_mark(struct audit_krule *krule, char *pa
>>>>>>>
>>>>>>>        ret = fsnotify_add_inode_mark(&audit_mark->mark, inode, 0);
>>>>>>>        if (ret < 0) {
>>>>>>> +             audit_mark->path = NULL;
>>>>>>>                fsnotify_put_mark(&audit_mark->mark);
>>>>> As I'm tracing the code path from audit through fsnotify, and back
>>>>> into audit, I'm wondering if we still have a problem.  When
>>>>> fsnotify_add_inode_mark() fails it will end up freeing not just
>>>>> audit_mark->path, but audit_mark itself via audit_fsnotify_mark_free()
>>>>> (via a call into fsnotify_put_mark()), yes?
>>>> I don't think so. fsnotify_add_mark_locked() will call fsnotify_put_mark()
>>>> but that is just a counter part to fsnotify_get_mark() a few lines above.
>>>> The caller of fsnotify_add_inode_mark() still holds its own mark reference
>>>> which prevents mark from being freed.
>>> Okay, that sounds reasonable, but I'm still looking for a code path
>>> that only frees audit_mark:path and not the audit_mark itself.  What
>>> am I not seeing?
>> The callers of audit_alloc_mark() call kfree(path) if audit_alloc_mark()
>> returns error (which is a sensible thing because in some cases path indeed
>> needs freeing).
> Of course!  Thanks for that, I think I got a bit of tunnel vision on
> this for some reason.
>
> I'll merge this into audit/stable-6.0 now and once testing is complete
> I'll send it up to Linus.  Thanks everyone!

Thanks for taking the time to review this patch. Thanks everyone!