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

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Message-ID: <565A3228.5080908@digikod.net>
Date:	Sun, 29 Nov 2015 00:00:56 +0100
From:	Mickaël Salaün <mic@...ikod.net>
To:	Richard Weinberger <richard@....at>, 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



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 :)

 Mickaël


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