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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Vincent Zweije <vzweije () zweije nl>
Date: Wed, 28 Oct 2009 21:28:35 +0100

On Tue, Oct 27, 2009 at 08:09:57PM +0300, Dan Yefimov wrote:

||  On 27.10.2009 14:04, Vincent Zweije wrote:

||  > After chmodding the directory to 0700, *first*
||  >check the link count, *then* chmod the file to 0666:
||  >
||  >     User1 creates file with permissions 0644
||  >                     User2 opens file for read access on file descriptor 4
||  >     User1 chmod's directory to 0700
||  >     User1 verifies no hard links to file
||
||  Here's a window, during which User2 is able to create a hardlink
||  and that will remain unnoticed by User1. There's no way to perform
||  link check and conditionally do chmod in an atomic manner.

User2 cannot create a hard link on the file any more now because it
requires search access on the directory, and that has been revoked by
chmod 0700 on the directory.

||  >     User1 chmod's file to 0666
||  >                     User2 can not open the file for read or write access
||  >                     User2 can not write to file descriptor 4
||  >                     User2 _can_ write to /proc/$$/fd/4

However, as has been pointed out elsewhere in this thread, openat()
will at this point allow User2 to open the file for writing, provided
that he has a open file descriptor on the directory, opened with O_SEARCH.

This is a valid but different attack from the race above.

Ciao.                                                             Vincent.
-- 
Vincent Zweije <zweije () xs4all nl>    | "If you're flamed in a group you
<http://www.xs4all.nl/~zweije/>      | don't read, does anybody get burnt?"
[Xhost should be taken out and shot] |            -- Paul Tomblin on a.s.r.

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