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From: Maxime Devos <maximedevos@telenet.be>
To: "Ludovic Courtès" <ludo@gnu.org>
Cc: guix-devel@gnu.org
Subject: Re: Potential security weakness in Guix services
Date: Tue, 02 Feb 2021 14:38:23 +0100	[thread overview]
Message-ID: <69968b3a01d872cabdf55a94b6c82d5057e010c9.camel@telenet.be> (raw)
In-Reply-To: <87o8h2ehy7.fsf@gnu.org>

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On Tue, 2021-02-02 at 14:07 +0100, Ludovic Courtès wrote:
> OK, I see.  Roughly, this symlink chown story would be a local exploit
> that the attacker can take advantage of after exploiting the RCE to
> potentially get root access.
> 
> ‘mkdir-p/perms’ could check that the directory is not a symlink, to
> begin with.  Is this what you had in mind, Maxime?

Yes!  Though the parent- and grandparent and etc. should be checked as well.
If e.g. (I don't have a real example at hand) knot's activation has
a (mkdir-p/perms "/var/lib/knot/e/t/c/e/t/e/r/a" uid/gid-stuff) line, then
mkdir-p/perms has to take care that the "e", "t", "c", "e", "t", "e, "r"
and "a" directories aren't symlinks.

I don't know how I should implement this properly in Guile, though.
In C, I would use loop using openat with O_NOFOLLOW, in combination
with stat, but Guile doesn't have openat or O_NOFOLLOW.

I've proposed adding the O_NOFOLLOW to guile [1].  I don't have a
proposal for openat in guile yet.  If I do propose an interface
to openat(2); I should probably make a proposal for fchownat
and other *at variants at the same time.  I don't have a concrete
proposal for how a nice Scheme interface would look like.

In the mean time, I suppose it should be possible to use openat
via the FFI and define O_NOFOLLOW manually in Guix.

I'll look into writing a concrete proposal for *at in guile.
I'll post a link to the guile mailing list message when it has
been composed and sent.

Greetings, Maxime.

[1] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=46220

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  reply	other threads:[~2021-02-02 13:38 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-28 21:53 Potential security weakness in Guix services Leo Famulari
2021-01-29 13:33 ` Maxime Devos
2021-01-29 15:25   ` Maxime Devos
2021-02-01 15:35 ` Ludovic Courtès
2021-02-01 15:47   ` Julien Lepiller
2021-02-01 16:19     ` Maxime Devos
2021-02-02 13:07       ` Ludovic Courtès
2021-02-02 13:38         ` Maxime Devos [this message]
2021-02-02 15:30           ` Maxime Devos
2021-02-05  9:57           ` Ludovic Courtès
2021-02-05 12:20             ` Maxime Devos
2021-02-05 14:16               ` Maxime Devos
2021-02-06 21:28                 ` Ludovic Courtès
2021-02-06 22:01                   ` Maxime Devos
2021-02-10 20:45                     ` Ludovic Courtès
2021-02-06 21:26               ` Ludovic Courtès
2021-02-14 12:29                 ` TOCTTOU race (was: Potential security weakness in Guix services) Maxime Devos
2021-02-14 17:19                   ` Bengt Richter
2021-02-18 17:54                   ` TOCTTOU race Ludovic Courtès
2021-02-19 18:01                     ` Maxime Devos
2021-02-22  8:54                       ` Ludovic Courtès
2021-02-22 19:13                         ` Maxime Devos
2021-02-23 15:30                           ` Ludovic Courtès
2021-02-27  7:41                             ` Maxime Devos
2021-03-10 10:07                               ` Ludovic Courtès
2021-02-10 20:54             ` Potential security weakness in Guix services Christopher Lemmer Webber

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