Re: [PATCH v2012.1] fs: symlink restrictions on sticky directories

[Kees Cook]

On Fri, Jan 6, 2012 at 1:58 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 6 Jan 2012 10:43:40 +0100 Ingo Molnar <mingo@xxxxxxx> wrote:
>> * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> > > +config PROTECTED_STICKY_SYMLINKS
>> > > + bool "Protect symlink following in sticky world-writable directories"
>> > > + default y
>> > > + help
>> > > +   A long-standing class of security issues is the symlink-based
>> > > +   time-of-check-time-of-use race, most commonly seen in
>> > > +   world-writable directories like /tmp. The common method of
>> > > +   exploitation of this flaw is to cross privilege boundaries
>> > > +   when following a given symlink (i.e. a root process follows
>> > > +   a malicious symlink belonging to another user).
>> > > +
>> > > +   Enabling this solves the problem by permitting symlinks to be
>> > > +   followed only when outside a sticky world-writable directory,
>> > > +   or when the uid of the symlink and follower match, or when
>> > > +   the directory and symlink owners match.
>> >
>> > This is all quite misleading.  One would expect that
>> > CONFIG_PROTECTED_STICKY_SYMLINKS turns the entire feature on
>> > or off permanently.  ie, it controls whether the code is
>> > generated into vmlinux in the usual fashion.  But it's not
>> > that at all - the user gets the feature whether or not he
>> > wants it, and this variable only sets the initial value.
>> >
>> > Why are we forcing the user to have the feature if he doesn't
>> > want it, btw?
>>
>> Basing on the (not yet fully confirmed) assertion that no apps
>> are broken by this change but exploits, I'd argue that this is
>> actually the sane and correct semantics for symlinks - i.e. we
>> want this to be the default Linux behavior - not just a
>> 'feature'.
>>
>> That way the configuration knobs are compat settings in essence
>> - in case some app cares.
>>
>> If people disagree and want it default off and as a separate
>> feature then it has to be modularized out some more. There's
>> notable silence from VFS folks on all this so Kees made an
>> educated guess. It might be wrong.
>
> Maybe true for a general purpose computer, but someone who is making a
> single-purpose device such as a digital TV or a wifi router won't want
> it.
> [...]
> I'd have thought the way to configure this feature would be to have
> CONFIG_PROTECTED_STICKY_SYMLINKS to control the code generation then a
> 0 or 1 CONFIG_PROTECTED_STICKY_SYMLINKS_ENABLED to control the initial
> setting.

This seems like probably the best approach, though I dislike the
silliness required in Kconfig to get a boolean into 1/0 form instead
of set/unset in way that doesn't require the user to type "1" or "0".
I'm happy to do it, though.

-Kees

-- 
Kees Cook
ChromeOS Security
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