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    SubjectRe: Compat 32-bit syscall entry from 64-bit task!? [was: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]
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    On Thu, January 19, 2012 02:19, Andrew Lutomirski wrote:
    > On Wed, Jan 18, 2012 at 5:06 PM, Indan Zupancic <indan@nul.nu> wrote:
    >> On Wed, January 18, 2012 02:01, Andrew Lutomirski wrote:
    >>> On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <indan@nul.nu> wrote:
    >>> I don't know what your ptrace jailer does. But a task can switch
    >>> itself between 32-bit and 64-bit execution at will, and there's
    >>> nothing the kernel can do about it. (That isn't quite true -- in
    >>> theory the kernel could fiddle with the GDT, but that would be
    >>> expensive and wouldn't work on Xen.)
    >>
    >> That's why we don't cache the CS value but check it for every system call.
    >> But you said elsewhere that checking CS isn't always correct either.
    >> I grepped arch/x86 for "user_64bit_mode", but couldn't find anything,
    >> but maybe my kernel sources are too old, I haven't updated this system
    >> for almost a year. The current code only handles 0x23 and 0x33 and kills
    >> the jail if it encounters anything else.
    >
    > I think you're hosed on Xen, then. Xen regularly runs with a
    > different Xen-specific cs value.

    That's fine as long as a cs value of 0x23 or 0x33 gives reliable information.
    Not running is highly prefered above running insecurely. Security first,
    functionality second.

    Greetings,

    Indan




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