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Re: [kernel-hardening] Re: RLIMIT_NPROC check in set_user()

From:  Vasiliy Kulikov <segoon-AT-openwall.com>
To:  kernel-hardening-AT-lists.openwall.com
Subject:  Re: [kernel-hardening] Re: RLIMIT_NPROC check in set_user()
Date:  Wed, 6 Jul 2011 22:59:32 +0400
Message-ID:  <20110706185932.GB3299@albatros>
Cc:  linux-kernel-AT-vger.kernel.org, Greg Kroah-Hartman <gregkh-AT-suse.de>, Andrew Morton <akpm-AT-linux-foundation.org>, "David S. Miller" <davem-AT-davemloft.net>, Jiri Slaby <jslaby-AT-suse.cz>, James Morris <jmorris-AT-namei.org>, Neil Brown <neilb-AT-suse.de>
Archive-link:  Article

On Wed, Jul 06, 2011 at 11:01 -0700, Linus Torvalds wrote:
> My reaction is: "let's just remote the crazy check from set_user()
> entirely".

Honestly, I didn't expect such a positive reaction from you in the first
reply :)


> The whole point of RLIMIT_NPROC is to avoid fork-bombs.

It is also used in cases where there is implicit or explicit limit on
some other resource per process leading to the global limit of
RLIMIT_NPROC*X.  The most obvious case of X is RLIMIT_AS.

Purely pragmatic approach is introducing the check in execve() to
heuristically limit the number of user processes.  If the program uses
PAM to register a user session, maxlogins from pam_limits is the Right
Way.  But many programs simply don't use PAM because of the performance
issues.  E.g. apache doesn't use PAM.  On a shared web hosting this is a
real issue.

In -ow patch execve() checked for the exceeded RLIMIT_NPROC, which
effectively solved Apache's problem.

...and execve() error handling is hard to miss ;-)


> So let's keep it in kernel/fork.c where we actually create a *new*
> process (and where everybody knows exactly what the limit means, and
> people who don't check for error cases are just broken). And remove it
> from everywhere else.

There are checks only in copy_process() and set_user().

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments


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