linux-kernel - Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

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Message-ID: <hbtc9t$629$1@taverner.cs.berkeley.edu>
Date:	Fri, 23 Oct 2009 22:55:57 +0000 (UTC)
From:	daw@...berkeley.edu (David Wagner)
To:	linux-kernel@...r.kernel.org
Subject: Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

NiTRo  wrote:
>    Just discovered this security problem [...]
>If a FD is opened on a allowed file and then the permission is changed
>the file is still redeable starting from the already read position to
>the EOF.

This is not a security problem; this is Unix working as designed.
That's how file descriptors work.  File permissions are checked when
the file is opened, not on every read/write to the file descriptor.
That's a pretty fundamental aspect of how Unix works, and it is
well-documented and well-understood.

On the other hand, the /proc attack that Pavel explains *is* a
security problem.  But that's different.
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