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Re: /proc filesystem allows bypassing directory permissions on Linux
From: "Kinzel, David" <dakinzel () suncor com>
Date: Mon, 26 Oct 2009 15:27:15 -0600

-----Original Message-----
From: nomail () nomail com [mailto:nomail () nomail com]
Sent: Monday, October 26, 2009 9:15 AM
To: bugtraq () securityfocus com
Subject: Re: Re: /proc filesystem allows bypassing directory
permissions on Linux

I do not think mounting /proc should change access control

It didn't in fact change anything. If the guest created
hardlink to that file in a unrestricted location, what would you say?

Do your homework and test it. You can't create the hardlink -
the link(oldpath, newpath) call will fail with EACCES if
search permission is denied for any directory in oldpath or
newpath. Documented in the manpage, and I just tested and verified it.

It's creating the hardlink (or setting the fd in /proc) before the directory is locked down to user-only permissions. 
You will be allowed access to whatever the file allows, despite what the directory permissions are. This seems expected 
to me, since the files will be the same inode. If I'm following this correctly it comes down to some distributions 
treating /proc/*/fd/* as hardlinks, for whatever reason.

Fact is, directory permissions are relevant in Unix. Despite
it's permissions, under the Unix access permission semantics
the file is unwriteable for anyone but the owner, and this
bug pokes a hole into that.

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