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    SubjectRe: Really good idea to allow mmap(0, FIXED)?
    On Thursday 05 October 2006 21:50, linux-os (Dick Johnson) wrote:
    > Of course you must be able to remap the physical address 0 (offset
    > zero in the whole machine), and if your 'hint' to mmap() in
    > user code is a 0, it can (it's allowed) to return a pointer
    > initialized to zero --and it's your fault if it's incompatible
    > with some 'C' runtime libraries.
    > It is a perfectly good address and the fact that malloc() returns
    > (void *)0 upon failure, does not qualify it as king or some other
    > ruler. In fact, mmap() returns (void *)-1 upon failure.

    You are explainint something that is _completely_ unrelated to the
    issue I am describing.

    > > I say no, because this can potentially be used to turn rather harmless
    > > kernel bugs into a security vulnerability.
    > >
    > Can't. The kernel doesn't check for NULL for user access, it
    > simply traps if the address is bad. That's why we have copy/to/from_user()
    > for user-mode access.

    See my example.

    > > Let's say we have some kernel NULL pointer dereference bug somewhere,
    > > that's rather harmless, if it happens in process context and
    > > does not leak any resources on segfaulting the triggering app.
    > > So the worst thing that happens is a crashing app. Yeah, this bug must
    > > be fixed. But my point is that this bug can probably be used to
    > > manipulate the way the kernel works or even to inject code into
    > > the kernel from userspace.
    > >
    > Can't.

    See my example. It _does_ inject a userspace controlled value into
    the kernel.

    > > Attached to this mail is an example. The kernel module represents
    > > the actual "kernel-bug". Its whole purpose in this example is to
    > > introduce a user-triggerable NULL pointer dereference.
    > > Please stop typing now, if you are typing something like
    > > "If you can load a kernel module, you have access to the kernel anyway".
    > > This is different. We always _had_ and most likely _have_ NULL pointer
    > > dereference bugs in the kernel.
    > >
    > > The example programm injects a magic value 0xB15B00B2 into the
    > > kernel, which is printk'ed on success.
    > Well this shows nothing interesting.

    It _does_. What if the pointer was a function pointer and the
    kernel executed it? Eh? It would continue to execute userspace
    controlled code.

    > >
    > > In my opinion, this should be forbidden by disallowing mmapping
    > > to address 0. A NULL pointer dereference is such a common bug, that
    > > it is worth protecting against.
    > > Besides that, I currently don't see a valid reason to mmap address 0.
    > >
    > That's where the real-mode BIOS table is. Who says that I can't
    > look at any piece of physical memory I want. It's my machine.

    Doing it in the kernel and not from unprivileged user processes
    would be a good idea for this anyway...

    > The whole concept of a NULL pointer is simply an artifact of
    > incorrect engineering.

    I do _NOT_ complain about the NULL pointer. You simply did not get
    my point.

    Greetings Michael.
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      Last update: 2006-10-06 16:43    [W:0.024 / U:0.068 seconds]
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