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    SubjectRe: [PATCH v2012.1] fs: symlink restrictions on sticky directories
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    On Thu, Jan 5, 2012 at 12:08 PM, Nick Bowler <nbowler@elliptictech.com> wrote:
    > On 2012-01-05 11:34 -0800, Kees Cook wrote:
    >> On Thu, Jan 5, 2012 at 6:30 AM, Nick Bowler <nbowler@elliptictech.com> wrote:
    >> > On 2012-01-04 12:18 -0800, Kees Cook wrote:
    >> >> diff --git a/fs/Kconfig b/fs/Kconfig
    >> >> index 5f4c45d..26ede24 100644
    >> >> --- a/fs/Kconfig
    >> >> +++ b/fs/Kconfig
    >> >> @@ -278,3 +278,19 @@ source "fs/nls/Kconfig"
    >> >>  source "fs/dlm/Kconfig"
    >> >>
    >> >>  endmenu
    >> >> +
    >> >> +config PROTECTED_STICKY_SYMLINKS
    >> >> +     bool "Protect symlink following in sticky world-writable directories"
    >> >> +     default y
    >> > [...]
    >> >
    >> > Why do we need a config option for this?  What's wrong with just using
    >> > the sysctl?
    >>
    >> This way the sysctl can configured directly without needing to have a
    >> distro add a new item to sysctl.conf.
    >
    > This seems totally pointless to me.  There are tons of sysctls that
    > don't have Kconfig options: what makes this one special?

    Most are system tuning; this is directly related to vulnerability
    mitigation. Besides, I like having CONFIGs for sysctls because then I
    can build my kernel the way I want it without having to worry about
    tweaking my userspace sysctl.conf file, or run newer kernels on older
    userspaces, etc etc.

    >> > Why have you made this option "default y", when enabling it clearly
    >> > makes user-visible changes to kernel behaviour?
    >>
    >> Ingo specifically asked me to make it "default y".
    >
    > But this is a brand new feature that changes longstanding behaviour of
    > various syscalls.  Making it default to enabled is rather mean to users
    > (since it will tend to get enabled by "oldconfig") and seems almost
    > guaranteed to cause regressions.

    I couldn't disagree more. There has been zero evidence of this change
    causing anything but regressions in _attacks_. :P If anything, I think
    there should be no CONFIG and no sysctl, and it should be entirely
    non-optional. But since this patch needs consensus, I have provided
    knobs to control it. This is the way of security features. For
    example, years back I added a knob for /proc/$pid/maps protection
    being optional (and defaulted it to insecure because of people's fear
    of regression), and eventually it changed to secure-by-default, and
    then the knob went away completely because it didn't actually cause
    problems.

    -Kees

    --
    Kees Cook
    ChromeOS Security
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