linux-kernel - Re: [PATCH 1/2] um: Set secure access mode for temporary file

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Message-ID: <565A34AB.5010303@nod.at>
Date:	Sun, 29 Nov 2015 00:11:39 +0100
From:	Richard Weinberger <richard@....at>
To:	Mickaël Salaün <mic@...ikod.net>,
	linux-kernel@...r.kernel.org
Cc:	Jeff Dike <jdike@...toit.com>,
	Tristan Schmelcher <tschmelcher@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	user-mode-linux-devel@...ts.sourceforge.net,
	user-mode-linux-user@...ts.sourceforge.net
Subject: Re: [PATCH 1/2] um: Set secure access mode for temporary file

Am 29.11.2015 um 00:00 schrieb Mickaël Salaün:
> 
> 
> On 28/11/2015 23:55, Richard Weinberger wrote:
>> Am 28.11.2015 um 23:52 schrieb Mickaël Salaün:
>>>
>>> On 28/11/2015 22:40, Richard Weinberger wrote:
>>>> Am 28.11.2015 um 22:32 schrieb Mickaël Salaün:
>>>>> Replace the default insecure mode 0777 with 0700 for temporary file.
>>>>>
>>>>> Prohibit other users to change the executable mapped code.
>>>>
>>>> Hmm, isn't the tmp file already unlinked at this stage?
>>>>
>>>
>>> Yes, but if someone could open it before the unlink e.g. because of the umask (which does not seems to be the case thanks to mkstemp, but remains unspecified [1]), this user should then be able to have write access to the file descriptor/description.
>>
>> Yes, someone can open it before the unlink. But you change the file mode after that.
>> How does it improve the situation? The attacker has already the file handle.
> 
> The attacker could have the file handle only in a read-only mode, which is a bit different than being able to write and execute arbitrary code thanks to a file descriptor mapped RWX :)

Fair point. Please describe this in detail in the patch changelog. :-)

Thanks,
//richard
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