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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]
    On Tue, Jan 17, 2012 at 4:56 PM, Indan Zupancic <> wrote:
    > On Tue, January 17, 2012 18:45, Andrew Lutomirski wrote:
    >> On Tue, Jan 17, 2012 at 9:05 AM, Oleg Nesterov <> wrote:
    >>> On 01/17, Andrew Lutomirski wrote:
    >>>> (is_compat_task says whether the executable was marked as 32-bit. �The
    >>>> actual execution mode is determined by the cs register, which the user
    >>>> can control.
    >>> Confused... Afaics, TIF_IA32 says that the binary is 32-bit (this comes
    >>> along with TS_COMPAT).
    >>> TS_COMPAT says that, say, the task did "int 80" to enters the kernel.
    >>> 64-bit or not, we should treat is as 32-bit in this case.
    >> I think you're right, and checking which entry was used is better than
    >> checking the cs register (since 64-bit code can use int80).  That's
    >> what I get for insufficiently careful reading of the assembly.  (And
    >> for going from memory from when I wrote the vsyscall emulation code --
    >> that code is entered from a page fault, so the entry point used is
    >> irrelevant.)
    > Wait: If a tasks is set to 64 bit mode, but calls into the kernel via
    > int 0x80 it's changed to 32 bit mode for that system call and back to
    > 64 bit mode when the system call is finished!?
    > Our ptrace jailer is checking cs to figure out if a task is a compat task
    > or not, if the kernel can change that behind our back it means our jailer
    > isn't secure for x86_64 with compat enabled. Or is cs changed before the
    > ptrace stuff and ptrace sees the "right" cs value? If not, we have to add
    > an expensive PTRACE_PEEKTEXT to check if it's an int 0x80 or not. Or is
    > there another way?

    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 being said, is_compat_task is apparently a good indication of
    whether the current *syscall* entry is a 64-bit syscall or a 32-bit
    syscall. Perhaps the function should be renamed to in_compat_syscall,
    because that's what it does.

    > I think this behaviour is so unexpected that it can only cause security
    > problems in the long run. Is anyone counting on this? Where is this
    > behaviour documented?

    Nowhere, I think.

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