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Re: Where to report: root fails to edit other users file in sticky bit directory

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Am 08.12.2020 um 22:03 schrieb Greg Wooledge:
> Unexpected clobberin' by root is the actual *worst* case that this
> security feature is trying to protect against.  Imagine this hypothetical
> scenario:
> 
> 1) Process A running as regular user alice creates a temp file in /tmp,
>     owned by alice.
> 
> 2) Dumb process B running as root decides to use the exact same temp file
>     name that A is using.  It opens the file naively, and writes some
>     stuff to it, intending to read it back later.
> 
> 3) Process A rewinds the file and writes new stuff to it.
> 
> 4) Process B reads process A's data from the file, thinking it was going
>     to read its own data.
> 
> 5) Process B calculates the wrong thing, and turns off the life support
>     system.
> 
> I don't know how realistic this is, or how closely it mirrors the thought
> processes of the people who came up with the modification.  But if you're
> going to make this modification at all, it needs to affect root, or else
> it's pointless.

Such a (custom) process should simply not run as root, should create an 
own temporary sub directory with limited permissions and write it's own 
files there. If really something is written to the world-writeable /tmp 
directory root, it must be expected that it can be read and written to 
or created by other users. Btw. the way the security feature is 
implemented, explicitly allows anyone writing to the file (as long as 
UNIX permissions are given), so it does not totally rule out your 
scenario, but only when process B uses a O_CREAT access method. But it 
could instead check if the file exists first, and otherwise write to it 
without O_CREAT.

But I got the point, as especially root processes are likely more 
sensitive and create larger damage when manipulated (accidentally or 
intentionally) such a way.

> Your unique situation, where user A is putting a file in /tmp for some
> reason, and then user R is supposed to edit it in place, is really weird.
> I don't know why you're putting this important shared file in /tmp, but
> you might want to move it to a more appropriate place.

There is no reason to use the /tmp root (without sub directory) aside of 
expecting that the file is potentially shared, IMO. If it must only be 
accessed by the same process/user, it should be placed elsewhere, at 
least into a sub directory or a private /tmp.

Sharing files through /tmp is probably rare, but /tmp it is for 
temporary files, it's by default a tmpfs (relevant when one wants to 
minimize file system writes, e.g. drive spin up or SD cards with short 
life cycle, or runs a R/O system), and it's the only FHS directory that 
is world-writeable by default. So I cannot imagine any better place to 
share temporary information through files cross-process/user then /tmp :D.

So yeah, basically the feature prevents users, who do not know what they 
are doing, from rare but possible errors and attacks, but it limits 
users, who know that they are doing, from rarely relevant possibilities 
xD. I think it is all fine as long as it's well documented, i.e. part of 
docs and man pages that deal with the sticky bit, like man chmod(1).

Best regards,

Micha

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Thread

Where to report: root fails to edit other users file in sticky bit  directory MichaIng <micha@dietpi.com> - 2020-12-04 15:00 +0100
  Re: Where to report: root fails to edit other users file in sticky  bit directory <tomas@tuxteam.de> - 2020-12-04 15:20 +0100
  Re: Where to report: root fails to edit other users file in sticky  bit directory Greg Wooledge <wooledg@eeg.ccf.org> - 2020-12-04 16:10 +0100
  Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-08 16:00 +0100
    Re: Where to report: root fails to edit other users file in sticky  bit directory Greg Wooledge <wooledg@eeg.ccf.org> - 2020-12-08 16:20 +0100
      Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-08 16:50 +0100
        Re: Where to report: root fails to edit other users file in sticky  bit directory Andrei POPESCU <andreimpopescu@gmail.com> - 2020-12-09 10:30 +0100
    Re: Where to report: root fails to edit other users file in sticky  bit directory <tomas@tuxteam.de> - 2020-12-08 17:00 +0100
      Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-08 17:10 +0100
        Re: Where to report: root fails to edit other users file in sticky  bit directory <tomas@tuxteam.de> - 2020-12-08 17:50 +0100
          Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-08 18:10 +0100
            Re: Where to report: root fails to edit other users file in sticky  bit directory <tomas@tuxteam.de> - 2020-12-08 18:50 +0100
              Re: Where to report: root fails to edit other users file in sticky  bit directory Greg Wooledge <wooledg@eeg.ccf.org> - 2020-12-08 19:30 +0100
                Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-08 22:00 +0100
                Re: Where to report: root fails to edit other users file in sticky  bit directory Greg Wooledge <wooledg@eeg.ccf.org> - 2020-12-08 22:10 +0100
                Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-09 00:10 +0100
                Re: Where to report: root fails to edit other users file in sticky  bit directory Greg Wooledge <wooledg@eeg.ccf.org> - 2020-12-09 04:00 +0100
                Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-09 17:40 +0100
    Re: Where to report: root fails to edit other users file in sticky  bit directory Andrei POPESCU <andreimpopescu@gmail.com> - 2020-12-09 10:40 +0100
      Re: Where to report: root fails to edit other users file in sticky  bit directory MichaIng <micha@dietpi.com> - 2020-12-09 16:50 +0100
        Re: Where to report: root fails to edit other users file in sticky  bit directory Andrei POPESCU <andreimpopescu@gmail.com> - 2020-12-09 16:50 +0100

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