Re: procfs: open("/proc/self/fd/...") allows bypassing O_RDONLY

From: Pavel Machek <pavel@ucw.cz>
To: Simon Ser <contact@emersion.fr>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: procfs: open("/proc/self/fd/...") allows bypassing O_RDONLY
Date: Thu, 26 May 2022 13:08:11 +0200	[thread overview]
Message-ID: <20220526110811.GB5138@localhost> (raw)
In-Reply-To: <lGo7a4qQABKb-u_xsz6p-QtLIy2bzciBLTUJ7-ksv7ppK3mRrJhXqFmCFU4AtQf6EyrZUrYuSLDMBHEUMe5st_iT9VcRuyYPMU_jVpSzoWg=@emersion.fr>

Hi!

> I'm a user-space developer working on Wayland. Recently we've been
> discussing about security considerations related to FD passing between
> processes [1].
> 
> A Wayland compositor often needs to share read-only data with its
> clients. Examples include a keyboard keymap, or a pixel format table.
> The clients might be untrusted. The data sharing can happen by having
> the compositor send a read-only FD (ie, a FD opened with O_RDONLY) to
> clients.
> 
> It was assumed that passing such a FD wouldn't allow Wayland clients to
> write to the file. However, it was recently discovered that procfs
> allows to bypass this restriction. A process can open(2)
> "/proc/self/fd/<fd>" with O_RDWR, and that will return a FD suitable for
> writing. This also works when running the client inside a user namespace.
> A PoC is available at [2] and can be tested inside a compositor which
> uses this O_RDONLY strategy (e.g. wlroots compositors).
> 
> Question: is this intended behavior, or is this an oversight? If this is
> intended behavior, what would be a good way to share a FD to another
> process without allowing it to write to the underlying file?

Sounds like a bug. Not all world is Linux, and 'mount /proc' changing
security characteristics of fd passing is nasty and surprising.

We should not surprise people when it has security implications.

Best regards,
							Pavel

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