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    SubjectRe: [PATCH] move RLIMIT_NPROC check from set_user() to do_execve_common()
    On Thu, Jul 14, 2011 at 11:27:51AM +1000, NeilBrown wrote:
    > I'm still trying to understand the full consequences, but I agree that there
    > is no rush - the code has been this way for quite a while and their is no
    > obvious threat that would expedite things (as far as I know).

    I don't insist on getting this in sooner than in the next merge window,
    although I would have liked that. Relevant userspace vulnerabilities
    are being found quite often - I'll include some examples below.

    > However I'm not convinced that testing will help all that much - if there are
    > problems they will be is rare corner cases that testing is unlikely to find.

    This makes sense.

    > The only case where this change will improve safety is where:
    > 1/ a process has RLIMIT_NPROC set
    > 2/ the process is running with root privileges
    > 3/ the process calls setuid() and doesn't handle errors.

    Yes, and this is a pretty common case.

    > I think the only times that a root process would have RLIMIT_NPROC set are:
    > 1/ if it explicitly set up rlimits before calling setuid. In this case
    > we should be able to expect that the process checks setuid .. maybe
    > this is naive(?)

    RLIMIT_NPROC could be set by the parent process or by pam_limits. The
    machine I am typing this on has:

    * hard nproc 200

    (as well as other limits) in /etc/security/limits.conf, so if this
    machine's kernel let setuid() fail on RLIMIT_NPROC, I would be taking
    extra risk of a security compromise by reducing the risk of system
    crashes from inadvertent excessive resource consumption by runaway
    processes - a tradeoff I'd rather avoid.

    > 2/ if the process was setuid-root and inherited rlimits from before, and
    > never re-set them. In this case it is easy to imagine that a setuid()
    > would not be checked.

    Right. (In practice, all kinds of programs tend to forget to check
    setuid() return value, though.)

    Actually, for the problem to apply to setuid-root programs, they need to
    switch their real uid first (more fully become root), then try to switch
    to a user - but this is common.

    Here are some examples for 2011-2010:

    "... missing setuid() retval check in opielogin which leads to easy root

    "The /usr/lib/libgnomesu/gnomesu-pam-backend suid binary which belongs
    to the libgnomesu package is not checking setuid() return values.

    As a result, two cooperating users, or users with access to guest,
    cgi or web accounts can run arbitrary commands as root very easily."

    pam_xauth (exploitable if pam_limits is also used):

    A collection of examples from 2006:

    > So maybe an alternate 'fix' would be to reset all rlimits to match init_task
    > when a setuid-root happens. There are other corner cases where inappropriate
    > rlimits can cause setuid programs to behave in ways they don't expect.
    > Obviously such programs are buggy, but so are programs that don't check
    > 'setuid'. (There is a CVE about mount potentially corrupting mtab.)

    Right, but to me possibly resetting rlimits is not an "alternative" to
    moving the RLIMIT_NPROC check. setuid-root exec is not the only case
    where having setuid() fail on RLIMIT_NPROC is undesirable. We also
    don't want such failures with pam_limits, nor on daemon startup:

    As to resetting rlimits on SUID/SGID exec, I think this would make
    sense for RLIMIT_FSIZE, which would mitigate the mount mtab issue
    (thank you for bringing it up!) But it's to be discussed separately.


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