[kernel-hardening] Re: [patch 2/2] fs, proc: Introduce the /proc/<pid>/map_files/ directory v6

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Mon, Sep 05, 2011 at 11:49:08PM +0400, Vasiliy Kulikov wrote:
...
> 
> Actually, it can be speed up by introducing the same ptrace check.  If
> ptrace check fails, then just drop the dentry, otherwise continue to use
> it.  Then each revalidate would trigger ptrace check instead of full
> drop-lookup-alloc cycle.  If one process actively looks into
> map_files/ or fd/, it will not become significantly slower.  However, it
> will trigger 2 capable() fail alerts in ptrace_may_access() instead of
> one :)

Hmm, at least it's better than trashing dcache I think.

> 
> But I still see one very nasty issue - one may trigger this ptrace check,
> trigger d_drop() and then look at /proc/slabinfo at "dentry" row.  If
> the number has changed, then the interested dentry existed before the
> revalidate call.  This infoleak is tricky to fix without any race.
> 
> Probably it's time to close /proc/slabinfo infoleak? 
> 

Actually I miss to see how exactly this infoleak can be used by attacker
or whoever. So, Vasiliy, what the security issue there?

	Cyrill