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    SubjectRe: [PATCH] alloc_tag: Tighten file permissions on /proc/allocinfo
    On Thu, Apr 25, 2024 at 05:43:33PM -0700, Kees Cook wrote:
    > On Thu, Apr 25, 2024 at 08:27:05PM -0400, Kent Overstreet wrote:
    > > On Thu, Apr 25, 2024 at 04:47:18PM -0700, Andrew Morton wrote:
    > > > On Thu, 25 Apr 2024 15:42:30 -0700 Kees Cook <keescook@chromium.org> wrote:
    > > >
    > > > > > The concern about leaking image layout could be addressed by sorting the
    > > > > > output before returning to userspace.
    > > > >
    > > > > It's trivial to change permissions from the default 0400 at boot time.
    > > > > It can even have groups and ownership changed, etc. This is why we have
    > > > > per-mount-namespace /proc instances:
    > > > >
    > > > > # chgrp sysmonitor /proc/allocinfo
    > > > > # chmod 0440 /proc/allocinfo
    > > > >
    > > > > Poof, instant role-based access control. :)
    > > >
    > > > Conversely, the paranoid could set it to 0400 at boot also.
    > > >
    > > > > I'm just trying to make the _default_ safe.
    > > >
    > > > Agree with this.
    > > >
    > > > Semi-seriously, how about we set the permissions to 0000 and force
    > > > distributors/users to make a decision.
    > >
    > > I'm ok with 0400 for now since it's consistent with slabinfo, but I'd
    > > really like to see a sysctl for debug info paranoia. We shouldn't be
    > > leaving this to the distros; we're the ones with the expertise to say
    > > what would be covered by that sysctl.
    >
    > We've not had great luck with sysctls (see userns sysctl discussions)
    > since they don't provide sufficient granularity.
    >
    > All this said, I'm still not excited about any of these files living
    > in /proc at all -- we were supposed to use /sys for this kind of thing,
    > but its interface wasn't great for this kind of more "free-form" data,
    > and debugfs isn't good for production interfaces. /proc really should
    > only have pid information -- we end up exposing these top-level files to
    > every mount namespace with a /proc mount. :( But that's a yet-to-be-solved
    > problem...

    It really wouldn't be that hard to relax the 4k file limit in sysfs.

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