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From: Maxime Devos <maximedevos@telenet.be>
To: "Ludovic Courtès" <ludo@gnu.org>
Cc: 47584@debbugs.gnu.org
Subject: bug#47584: Race condition in ‘copy-account-skeletons’: possible privilege escalation.
Date: Sun, 04 Apr 2021 09:36:05 +0200	[thread overview]
Message-ID: <7ab30aad812e5de1216c95b3becb784e3363e615.camel@telenet.be> (raw)
In-Reply-To: <87mtufw1kh.fsf@gnu.org>

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On Sat, 2021-04-03 at 22:33 +0200, Ludovic Courtès wrote:
> Maxime Devos <maximedevos@telenet.be> skribis:
> 
> > The attack consists of the user being logged in after the account
> > skeletons have been copied to the home directory, but before the
> > owner of the account skeletons have been set.  The user then deletes
> > a copied account skeleton (e.g. @file{$HOME/.gdbinit}) and replaces
> > it with a symbolic link to a file not owned by the user, such as
> > @file{/etc/shadow}.
> > 
> > The activation code then changes the ownership
> > of the file the symbolic link points to instead of the symbolic
> > link itself.  At that point, the user has read-write access
> > to the target file.
> 
> In the draft blog post, you mention that the attack cannot be carried
> out when protected symlinks are enabled.

In the blog post, I thought I wrote the attack can be carried out
*even if* protected symlinks are enabled.  Looking at

https://sysctl-explorer.net/fs/protected_symlinks/,

I don't think the Linux protected symlink feature helps, as home
directories are never sticky and word-writable.

Perhaps I should have written ‘possible’ instead of ‘not impossible’
in the blog post.

> Please mention that protected symlinks are enabled by default on Guix
> System since a March 16th commit, with a link to [...]

See my response above.

I agree with all other comments on this bug report.

Greetings,
Maxime.

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  reply	other threads:[~2021-04-04  7:37 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-03 16:09 bug#47584: Race condition in ‘copy-account-skeletons’: possible privilege escalation Maxime Devos
2021-04-03 16:22 ` Maxime Devos
2021-04-03 16:32   ` Maxime Devos
2021-04-03 20:15   ` Ludovic Courtès
2021-04-03 16:26 ` Maxime Devos
2021-04-03 20:45   ` Ludovic Courtès
2021-04-03 20:49   ` Ludovic Courtès
2021-04-04 13:29   ` Maxime Devos
2021-04-03 20:27 ` Ludovic Courtès
2021-04-03 20:33 ` Ludovic Courtès
2021-04-04  7:36   ` Maxime Devos [this message]
2021-04-05 19:54     ` Ludovic Courtès
2021-04-06  9:56       ` Maxime Devos
2021-04-06 11:57         ` Ludovic Courtès
2021-04-07 18:28           ` Maxime Devos
2022-10-21  9:31 ` Maxime Devos
2022-10-28 16:03 ` bug#47584: [DRAFT PATCH v2 0/4] Fix race condition in mkdir-p/perms Maxime Devos
2022-10-28 16:04 ` bug#47584: [PATCH 1/3] guile-next: Update to 3.0.8-793fb46 Maxime Devos
2022-10-28 16:04   ` bug#47584: [PATCH 2/3] WIP gnu: Change the Guile used for activation to one that has 'openat' Maxime Devos
2022-10-28 16:04   ` bug#47584: [PATCH 3/3] activation: Fix TOCTTOU in mkdir-p/perms Maxime Devos
2022-10-28 16:05   ` bug#47584: [PATCH 1/3] guile-next: Update to 3.0.8-793fb46 Maxime Devos

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