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Re: S16: chown, chmod
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From:
Brandon S. Allbery KF8NH
Date:
November 24, 2008 22:20
Subject:
Re: S16: chown, chmod
Message ID:
C0BF845A-B2EF-4319-8494-8009D51EBA50@ece.cmu.edu
On 2008 Nov 24, at 10:36, dpuu wrote:
> On Nov 23, 3:56 pm, allb...@ece.cmu.edu (Brandon S. Allbery KF8NH)
> wrote:
>> I think you're seeing something other than what we are. Checking any
>> external resource before operating on it introduces a race condition
>> which can allow an attacker to swap resources on you, so the item you
>> (in this case) chown() isn't the one you tested.
>
> If the "chown" is restricted then it's going to fail anyway, assuming
> that the underlying Unix function fails. If "chown" can succeed
> incorrectly then there's nothing that P6 can do to prevent that. My
Still misunderstanding, I think. Yes, it will fail anyway, but in the
general case you're checking to see if as a privileged process it is
safe to operate on a given file. In such case the correct thing to do
is relinquish privilege and then simply do the operation, trapping any
error --- not testing and then doing it.
(I grant this isn't quite the same thing --- unless you're trying to
decide if your root process should chown() a file on behalf of an
unprivileged process.)
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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