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    SubjectRe: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree
    On Wed, Feb 15, 2012 at 05:22:22PM +0100, Oleg Nesterov wrote:
    > > So I don't understand how it's different from what
    > > is provided in this patch. What I'm missing?
    > environ_read() does
    > mm = mm_access(task);
    > if (mm)
    > do_something(mm);
    > even if it races with, say, execve(setuid_app) we can't read the
    > new ->mm.

    Wait, I'm confused

    process 1 (reader) process 2 ("task" itself)
    mm = mm_access(task);
    task changes own credentials
    so reader can't access on next
    read if it would try, but since
    access already granted... it
    continues do_something(mm)
    if (mm)

    So in the patch I tried the same, once access is granted it
    belongs to a caller.

    > while your code (very roughly) does something like
    > mm = mm_access(task);
    > if (mm)
    > do_something(task->mm);
    > while it is quite possible that mm != task->mm.

    Oleg, could you please explain me where it happens
    that task->mm (I've got access to) will be changed
    to some new -mm while I'm inspecting it.

    If permission changed while the caller inside syscall,
    it's the same situation as with mm_access above. No?

    > My only point is: this check is obviously racy, and thus it looks
    > confusing. Whether this is fine or not, I do not know. Personally
    > I see no reason for ptrace_may_access(), but I am not security
    > expert.

    The idea was -- non-privilege caller should not have access
    to privileged tasks.


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