Re: [Bug-tar] opening files with O_NONBLOCK causes problems

[Eric Blake]

On 01/06/2012 02:47 AM, Joerg Schilling wrote:
> Paul Eggert <address@hidden> wrote:
> 
>> On 01/05/12 14:59, Kamil Dudka wrote:
>>> Is there a known attack on tar that the use of O_NONBLOCK can prevent?  
>>
>> Yes, of course.  For example, the attacker can create a
>> hard link to a fifo while tar is running, which means that
>> root doing a backup will hang indefinitely.
> 
> star does not open FIFO files.....

Yes, it probably does.  From your description, it sounds like star is
using a stat() before open() to avoid FIFOs; but this is a classic
TOCTTOU race where an attacker can replace a regular file with a FIFO,
meaning that star will open FIFO files.

> 
> Why should gtar open FIFO files?

The question is not why an archiver opens a FIFO file, but what it does
after opening a file O_NONBLOCK (the TOCTTOU race is eliminated by
switching stat()/open() to open()/fstat() filtering, and once we have
ascertained that an open fd is not a FIFO, if we can then use fcntl() to
remove the O_NONBLOCK, hopefully that will resolve the situation with DMF).

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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