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Date: Mon, 27 Jun 2011 19:07:24 +0900
From: KOSAKI Motohiro <>
Subject: Re: [PATCH 1/2] proc: restrict access to /proc/PID/io

(2011/06/27 17:52), Vasiliy Kulikov wrote:
> (cc'ed Linus)
> On Mon, Jun 27, 2011 at 16:33 +0900, KOSAKI Motohiro wrote:
>> (2011/06/27 16:03), Vasiliy Kulikov wrote:
>>> On Mon, Jun 27, 2011 at 11:58 +0900, KOSAKI Motohiro wrote:
>>>> (2011/06/24 21:08), Vasiliy Kulikov wrote:
>>>>> /proc/PID/io may be used for gathering private information.  E.g. for
>>>>> openssh and vsftpd daemons wchars/rchars may be used to learn the
>>>>> precise password length.  Restrict it to processes being able to ptrace
>>>>> the target process.
>>>>> ptrace_may_access() is needed to prevent keeping open file descriptor of
>>>>> "io" file, executing setuid binary and gathering io information of the
>>>>> setuid'ed process.
>>>>> Signed-off-by: Vasiliy Kulikov <>
>>>> This description seems makes sense to me. But Vasilly, I have one question.
>>>> Doesn't this change break iotop command or other userland tools?
>>> I don't use iotop, but after reading the sources it looks like it uses
>>> taskstats for information gathering, which will be broken for sure by
>>> the second patch.  All other userland tools using alien io files will be
>>> broken too.
>>> I'd say the whole approach of world readable debugging/statistics
>>> information was broken from the beginning, now we are stuck with these
>>> interfaces because of acient mistakes.
>> Just idea. (perhaps it's too dumb).
>> If a user want to know throughput, usually they only need KB/s granularity.
>> If a user want to know password hints, they need to know strict bytes granularity.
>> So, adding some random bytes to this statistics may help to obscure key data,
>> or just "stat = ROUND_UP(stat, 1024)".
>> But, I hope to wait another experts response. they may know better approach. :)
> Yep, Linus has already suggested a similar thing:
> As to random bytes - if it is very predictable (e.g. rand() % 10000) one
> may restore the original value.  But it would still do much harm to good
> programs (io stats going up and down!).  Adding really random bytes
> seems somewhat too complicated for these needs to me.
> As to rounding - this is a workaround, not a fix.  What if some program
> reads one byte from tty and then do some io activity exactly of 1kb-1?
> Then you just measure kbs and get original tty activity.  (just a crazy
> example to show that it is not a full solution.)
> Without any proof/estimate, just an idea: it's possible to get not only
> password length, which needs as much granularity as possible, but
> another information, which doesn't need any strict value.  E.g.
> statistics changing, what should indicate that some significant event
> has just happened.

I'm ok any alternative way if you have. I only want to don't break iotop.
It's used very widely and frequently from performance tuning engineers.

I'm sorry. I can't answer your story is real threat or not. I'm not security
expert. I don't have enough knowledge.


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