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    SubjectRe: FUSE merging?
    > > 
    > > - Frank points out that a user can send a sigstop to his own setuid(0)
    > > task and he intimates that this could cause DoS problems with FUSE. More
    > > details needed please?
    > It's the other way around:
    > Apparently it is not a security problem to SIGSTOP or even SIGKILL a
    > setuid program. So why is it a security problem when such a program is
    > delayed by a supposedly malicious behaving FUSE mount?

    Perfectly valid argument. My question: is it not a security problem
    to allow signals to reach a suid program?

    > I think that setuid programs take too many things for granted, especially
    > "time". I also think the ptrace equivalence principle (item C2 in the
    > FUSE doc) is too harsh for FUSE.

    It's obviously not equivalence. FUSE filesystem gets a subset of
    ptrace's capabilities (and rather a small one).

    > Suppose the process changes id to full root and we can no longer send
    > signals to it. Are there any other ways we could affect its scheduling
    > without FUSE? I think "yes", clearly not that easy as when it accesses a
    > FUSE mount but "yes". Think about typing ^S (XOFF), or by letting it read
    > from a pipe or from a file on a very very slow device. Or by renicing
    > the parent in advance. Regarding the pipe: yes the setuid program could
    > check that with fstat() but is such a check fundamentally the right
    > approach? I have doubt because unified I/O is a good thing and there is
    > no guarantee whatsoever about completion of any FS operation within a
    > certain amount of time. Suppose another malicious process does a lookup
    > in a huge directory without hashed names? What about a process consuming
    > lots of memory, pushing everything else into swap? What about deleting
    > a _huge_ file or do other things which might(?) take a considerable
    > amount of kernel time? [id]notify might even help using this to delay
    > a root process at a crucial point to exploit a race. So, I think there
    > are many ways to affect the execution speed of [setuid] programs. I
    > have never heard of a setuid root program which renices itself, such,
    > that it successfully avoids a race or DoS exploit.

    There's a huge difference between slowing down, and stopping a
    process. I wouldn't consider the first a true DoS.

    > And then the DoS thing using simulated endless files within FUSE. It is
    > already possible to create terabyte sized [sparse] files. Can the fstat()
    > size/blocks info be trusted from FUSE? no more than fstat() outside FUSE
    > because the file may still be growing!
    > > - I don't recall seeing an exhaustive investigation of how an
    > > unprivileged user could use a FUSE mount to implement DoS attacks against
    > > other users or against root.
    > In general I think it is _hard_ to protect against a local DoS for many
    > reasons and I don't see any new fundamental problem here with FUSE:
    > it is just making it more obvious that it's hard to write secure setuid
    > programs. Those programs should _know_ that input data and anything else
    > from the user is "tainted" and that they must be _very_ careful with it,
    > in every detail.

    Yes. The extra problem with FUSE, is that they are not _able_ to be
    careful. They can't even check if a file is in fact on a FUSE mount
    or not without the FUSE daemon's intervention (lookup on a file will
    be passed to userspace).

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