Re: [PATCH] set*uid() must not fail-and-return on OOM/rlimits

[Peter Williams <pwil3058@bigpond.net.au>]

Alan Cox wrote:
> Ar Llu, 2006-08-21 am 02:12 +0400, ysgrifennodd Solar Designer:
> > Are you referring to killing of processes on OOM?  That was in Linux
> > already, this patch does not introduce it.
>
> (pedantic) Only if you have overcommit disabled.
>
> > As it relates to setuid() in particular, POSIX.1-2001 says:
> >
> >      The setuid() function shall fail, return -1, and set errno to the
> >      corresponding value if one or more of the following are true:
> >
> >      [EINVAL]
> >              The value of the uid argument is invalid and not supported by
> >              the implementation.
> >      [EPERM]                                                                                 The process does not have appropriate privileges and uid does
> >              not match the real user ID or the saved set-user-ID.
> >
> > No other error conditions are defined.  
>
> > I'd say that the behavior of returning EAGAIN is non-compliant.
>
> You are allowed to return other errors. What you must not do is return a
> different error for the description described in the text as I
> understand it.
>
> > But the kills are needed.  They are more correct and safer than
> > returning EAGAIN.  An alternative would be to not allocate memory on
> > set*uid() at all - like we did not in older kernels - but that would
> > be an inappropriate behavior change for 2.4.
>
> It is certainly an awkward case to get right when setuid code is not
> being audited but I still think you are chasing the symptom, and its not
> symptom of crap code, so you are not likely to "fix" security. A lot of
> BSD code for example doesn't check malloc returns but you don't want an
> auto-kill if mmap fails ?
>
> The kill has the advantage that it stops the situation but it may also
> be that you kill a program which can handle the case and you create a
> new DoS attack (eg against a daemon switching to your uid). The current
> situation is not good, the updated situation could be far worse.
>
> The message is important, we want to know it happened in the memory
> shortage case anyway.

How about going ahead with the uid change (if the current user is root) 
BUT still return -EAGAIN.  That way programs that ignore the return 
value will at least no longer have root privileges.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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