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    SubjectRe: [RFC] prevention of syscalls from writable segments, breaking bug exploits
    On Wed, Jan 03, 2001 at 04:54:38PM -0500, Alexander Viro wrote:
    > On Wed, 3 Jan 2001, Dan Aloni wrote:
    > > It is known that most remote exploits use the fact that stacks are
    > > executable (in i386, at least).
    > >
    > > On Linux, they use INT 80 system calls to execute functions in the kernel
    > > as root, when the stack is smashed as a result of a buffer overflow bug in
    > > various server software.
    > >
    > > This preliminary, small patch prevents execution of system calls which
    > > were executed from a writable segment. It was tested and seems to work,
    > > without breaking anything. It also reports of such calls by using printk.
    > Get real. Attacker can set whatever registers he needs and jump to one
    > of the many instances of int 0x80 in libc. There goes your protection.
    > Win: 0
    > Loss: cost of find_vma() (and down(&mm->mmap_sem), BTW) on every system
    > call.
    > And the reason to apply that patch would be...?

    Should be a moot point, anyway, as x86 has a seperate stack for each
    priviledge level. Even if the kernel somehow tried to execute code in a
    lower priviledge segment (stack or otherwise) shouldn't a GPF get
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