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    SubjectRe: [kernel-hardening] [RFC V2 4/6] lib: vsprintf: default kptr_restrict to the maximum value
    On Wed, Oct 4, 2017 at 10:08 AM, Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    >
    > So I honestly doubt the value of kptr_restrict. Any *sane* policy
    > pretty much has to be in the caller, and by thinking about what you
    > print out. IOW, things like proc_pid_wchan().

    Looking at /proc/kallsyms is actually a prime example of this.

    IOW, the old "open /proc/kallsyms as a normal user, then make it stdin
    for some suid-root program that can be fooled to output it probably
    works on it.

    So kptr_restrict ends up being entirely the wrong thing to do there.

    The only value in kptr_restrict ends up being as a complete hack,
    where you say "I trust nobody" and make %p almost entirely useless.

    And as mentioned, that will just make people use %x instead, or
    randomly sprinkle the new "I didn't really mean this" modifiers like
    the already discussed pr_debug() case.

    So even when kptr_restrict "works", it ends up just fighting itself.
    And most of the time it just doesn't work.

    Linus

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