linux-kernel - Re: [PATCH v2] smackfs: restrict bytes count in smackfs write functions

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Date:   Thu, 28 Jan 2021 18:10:21 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sabyrzhan Tasbolatov <snovitoll@...il.com>
Cc:     andreyknvl@...gle.com, jmorris@...ei.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, mhocko@...e.com,
        serge@...lyn.com,
        syzbot+a71a442385a0b2815497@...kaller.appspotmail.com,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v2] smackfs: restrict bytes count in smackfs write
 functions

On 1/28/2021 6:24 AM, Tetsuo Handa wrote:
> On 2021/01/28 22:27, Sabyrzhan Tasbolatov wrote:
>>> Doesn't this change break legitimate requests like
>>>
>>>   char buffer[20000];
>>>
>>>   memset(buffer, ' ', sizeof(buffer));
>>>   memcpy(buffer + sizeof(buffer) - 10, "foo", 3);
>>>   write(fd, buffer, sizeof(buffer));
>>>
>>> ?
>> It does, in this case. Then I need to patch another version with
>> whitespace stripping before, after label. I just followed the same thing
>> that I see in security/selinux/selinuxfs.c sel_write_enforce() etc.
>>
>> It has the same memdup_user_nul() and count >= PAGE_SIZE check prior to that.
> Since sel_write_enforce() accepts string representation of an integer value, PAGE_SIZE is sufficient.
> But since smk_write_onlycap() and smk_write_relabel_self() accept list of space-delimited words,
> you need to prove why PAGE_SIZE does not break userspace in your patch.

if PAGE_SIZE >= SMK_LOADSIZE all legitimate requests can be made
using PAGE_SIZE as a limit. Your example with 19990 spaces before
the data demonstrates that the interface is inadequately documented.
Tizen and Automotive Grade Linux are going to be fine with a PAGE_SIZE
limit. The best way to address this concern is to go ahead with the
PAGE_SIZE limit and create ABI documents for the smackfs interfaces.
I will take your patch for the former and create a patch for the latter.


>
> Also, due to the "too small to fail" memory-allocation rule, memdup_user_nul() for
> count < PAGE_SIZE * 8 bytes is "never fails with -ENOMEM unless SIGKILLed by the OOM
> killer". Also, memdup_user_nul() for count >= PAGE_SIZE * (1 << MAX_ORDER) - 1 bytes is
> "never succeeds". Thus, you can safely add
>
> 	if (count >= PAGE_SIZE * (1 << MAX_ORDER) - 1)
> 		return -EINVAL; // or -ENOMEM if you want compatibility
>
> to smackfs write functions. But it is a strange requirement that the caller of
> memdup_user_nul() has to be aware of upper limit in a way that we won't hit
>
> 	/*
> 	 * There are several places where we assume that the order value is sane
> 	 * so bail out early if the request is out of bound.
> 	 */
> 	if (unlikely(order >= MAX_ORDER)) {
> 		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
> 		return NULL;
> 	}
>
> path. memdup_user_nul() side should do
>
> 	if (count >= PAGE_SIZE * (1 << MAX_ORDER) - 1)
> 		return -ENOMEM;
>
> check and return -ENOMEM if memdup_user_nul() does not want to use __GFP_NOWARN.
> I still believe that memdup_user_nul() side should be fixed.
>