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Date: Tue, 12 Dec 2017 19:01:06 +0100
From: Salvatore Mesoraca <s.mesoraca16@...il.com>
To: Solar Designer <solar@...nwall.com>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 0/2] Restrict dangerous open in sticky directories
2017-12-11 17:07 GMT+01:00 Solar Designer <solar@...nwall.com>:
> On Thu, Dec 07, 2017 at 11:23:34PM +0100, Solar Designer wrote:
> > On Tue, Dec 05, 2017 at 11:13:50AM +0100, Salvatore Mesoraca wrote:
> > > 2017-11-30 20:05 GMT+01:00 Solar Designer <solar@...nwall.com>:
> > > > A new thought: a directory that has someone else as its owner is for our
> > > > purposes effectively the same as a group-writable directory. So maybe
> > > > whatever we'll implement for group-writable directories should also be
> > > > done for directories that have other than the current fsuid as their
> > > > owner. It's currently very uncommon to have directories with the sticky
> > > > bit set that are not at least group-writable, so this will rarely make a
> > > > difference (and when it does, that's just right), but also it provides a
> > > > way to explicitly include any dibrectory under this monitoring (if the
> > > > group-writable protection is on) - e.g., if a sysadmin wants this
> > > > monitoring for users' home directories, they can change permissions for
> > > > those from e.g. 700 to 1700. This could be handy for development and
> > > > auditing of software, even though in production it could be easily
> > > > circumvented by the directory owner (who can remove the sticky bit,
> > > > which we should document to avoid providing a false sense of security)
> > > > and it won't automatically apply to subdirectories. It'd also cover
> > > > part of what we intend to achieve later by possibly extending the
> > > > feature to non-sticky directories, where we might also want to treat
> > > > different owner the same as group-writable (without the circumvention).
> > >
> > > Agreed. I'll extend it to also check for the directory owner.
> > > Maybe I could use another bit and make this additional restriction
> > > independent from the "group-writable" one.
> >
> > I'd prefer not to waste a bit on this. We'll need plenty of bits later
> > on, perhaps also in pairs or (with your suggestion to separate this one)
> > in triplets, and every extra bit complicates the calculation of the
> > bitmask value.
>
> OTOH, if we were to interleave the world and group bits, then maybe
> instead of having the world and group bits as separate we could have a
> two bit number, which naturally leaves us an extra value we'd use for
> owner. So:
>
> 0 - none
> 1 - world
> 2 - group and world
> 3 - owner, group, and world
>
> This would remove the ability to enable our restrictions for group (and
> owner) while disabling them for world, but that's likely just an obscure
> misconfiguration.
>
> Unfortunately, this is not compatible with my other idea of easily
> extending a tested policy from world onto group by simply assigning the
> same value to the other sysctl.
>
> I'm not sure which of these approaches is cleaner.
I prefer the version with separate sysctls for world/group, it seems more
usable.
Salvatore
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