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    SubjectRe: [patch 7/8] fdmap v2 - implement sys_socket2
    Date
    On Jun 09, 2007, at 13:24:29, Al Viro wrote:
    > On Sat, Jun 09, 2007 at 10:08:59AM -0700, Ulrich Drepper wrote:
    >> - - there are two interface to use: open + fcntl. This is racy.
    >> And don't tell me this doesn't matter.
    > Racy with respect to what? Return-to-libc exploits from another
    > thread?

    How about racy with respect to normal open or fork+exec from another
    thread? Specifically there are cases where libc or other libraries
    want to create a backend thread dealing with file descriptors in
    response to the program's straightforward calls into that library
    (Examples include using syslets or event-based polling threads).


    SCENARIO 1:

    Program Thread: Library Thread:
    fd = socket(AF_*, SOCK_*, 0);
    fork();
    int x = FD_CLOEXEC;
    fcntl(fd, F_SETFD, &x);

    New Process:
    setgroups(...);
    seteuid(...);
    exec(....);

    Whoops!!! Suddenly the user process executed by the (theoretically)
    single-threaded program got a handle to a netlink socket affecting
    some system resource!!!


    SCENARIO 2:

    Program Thread: Async libc getpwent()-cache syslet
    close(0);
    fd = open("/etc/shadow");
    open("/dev/null");
    code_which_insecurely_reads_from_stdin();

    Here we were trying to safely call into code which reads from stdin
    and shouldn't be given privileged data, but the syslet makes the
    common paradigm 'close(0); open("/dev/null");' horribly insecure.

    If you extend all the FD syscalls to all take a "flags" parameter and
    add the appropriate flags, then you can pass O_CLOEXEC|O_RANDFD to
    whatever syscall you are using and both problems vanish.

    Cheers,
    Kyle Moffett

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