linux-kernel - Re: [PATCH -next] selinux: Fix potential memory leak in selinux_add

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Date:   Wed, 15 Jun 2022 17:34:35 +0800
From:   xiujianfeng <xiujianfeng@...wei.com>
To:     Paul Moore <paul@...l-moore.com>
CC:     <stephen.smalley.work@...il.com>, <eparis@...isplace.org>,
        <omosnace@...hat.com>, <selinux@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] selinux: Fix potential memory leak in
 selinux_add_opt


在 2022/6/15 9:17, Paul Moore 写道:
> On Mon, Jun 13, 2022 at 9:18 PM xiujianfeng <xiujianfeng@...wei.com> wrote:
>> 在 2022/6/14 4:22, Paul Moore 写道:
>>> On Sat, Jun 11, 2022 at 5:07 AM Xiu Jianfeng <xiujianfeng@...wei.com> wrote:
>>>> In the entry of selinux_add_opt, *mnt_opts may be assigned to new
>>>> allocated memory, and also may be freed and reset at the end of the
>>>> function. however, if security_context_str_to_sid failed, it returns
>>>> directly and skips the procedure for free and reset, even if it may be
>>>> handled at the caller of this function, It is better to handle it
>>>> inside.
>>>>
>>>> Fixes: 70f4169ab421 ("selinux: parse contexts for mount options early")
>>>> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
>>>> ---
>>>>    security/selinux/hooks.c | 12 +++++++-----
>>>>    1 file changed, 7 insertions(+), 5 deletions(-)
>>> Have you actually observed a memory leak from the selinux_mnt_opts
>>> allocation in selinux_add_opt()?
>>>
>>> The selinux_add_opt() function has two callers:
>>> selinux_sb_eat_lsm_opts() and selinux_fs_context_parse_param().  The
>>> former cleans up the selinux_mnt_opts allocation it its error handler
>>> while the latter will end up calling
>>> security_free_mnt_opts()/selinux_free_mnt_opts() to free the
>>> fs_context:security when the fs_context is destroyed.
>>>
>>> This patch shouldn't be necessary.
>> I may not have made it clear, I said potential means may have a third
>> caller in the future.
> Let's not worry about it.  If you wanted to add a comment header to
> the function (see selinux_skb_peerlbl_sid() for an example) to make it
> clear that callers are responsible for cleaning up @mnt_opts on error
> I think that would be okay ... although even that is going to be a
> problem in the new mount API case where selinux_add_opt() is going to
> be called multiple times.
>
>> I think the error handler as following is not necessary:
>>
>> err:
>>           if (is_alloc_opts) {
>>                   kfree(opts);
>>                   *mnt_opts = NULL;
>>           }
>>
>> otherwise, some error paths goto err label while others don't, It's
>> confusing.
> That's a fair point.  Looking at the patch which added it, we should
> probably also return EINVAL when @s is NULL instead of ENOMEM.  In
> fact, in all the cases where we currently jump to @err, I think we are
> guaranteed that @is_alloc_opts is false as it requires a previously
> populated @opts.
>
> If you want to submit another patch, I would suggest doing the
> following in the patch:
>
> 1. Change the @s NULL check to return -EINVAL when @s is NULL.
> 2. Allocate @opts/@..._opts if NULL, but don't call kfree() on the
> object in case of error.  The new mount API will cleanup when it is
> done and selinux_sb_eat_lsm_opts() will cleanup on error.
>
> If you don't have time to put together a patch for this, let me know and I will.

no problem, I will do it, thanks for your suggestion.

>