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    SubjectRe: [PATCH] futex: do not leak robust list to unprivileged process
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    On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
    > On Tue, 20 Mar 2012, Serge Hallyn wrote:
    >
    >> Quoting Kees Cook (keescook@chromium.org):
    >> > It was possible to extract the robust list head address from a setuid
    >> > process if it had used set_robust_list(), allowing an ASLR info leak. This
    >> > changes the permission checks to be the same as those used for similar
    >> > info that comes out of /proc.
    >> >
    >> > Running a setuid program that uses robust futexes would have had:
    >> >   cred->euid != pcred->euid
    >> >   cred->euid == pcred->uid
    >> > so the old permissions check would allow it. I'm not aware of any setuid
    >> > programs that use robust futexes, so this is just a preventative measure.
    >> >
    >> > (This patch is based on changes from grsecurity.)
    >> >
    >> > Signed-off-by: Kees Cook <keescook@chromium.org>
    >>
    >> I like the change.  Much cleaner.  I'm not 100% sure though that
    >> there are no legitimate cases of robust futexes use which would now
    >> be forbidden.  (Explicitly cc:ing Ingo)
    >
    > get_robust_list is not necessary for robust futexes. There is no
    > reference to get_robust_list in glibc.
    >
    > I really wonder why we have this syscall at all.

    The documentation I found yesterday while looking at this was:
    http://linux.die.net/man/2/get_robust_list

    Which says "The system call is only available for debugging purposes
    and is not needed for normal operations. Both system calls are not
    available to application programs as functions; they can be called
    using the syscall(3) function."

    Dropping the syscall entirely would certainly make it secure. ;)

    -Kees

    --
    Kees Cook
    ChromeOS Security
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