On Tue, Sep 06, 2011 at 00:36 +0400, Cyrill Gorcunov wrote:
> > 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?
The security model of procfs is: /proc/PID/fd/ is available to users
that may ptrace PID only. Particularly, the number of opened file
descriptors is a private information. If other task that may not ptrace
PID is able to get this information, this is an issue. Keeping opened
file descriptor of /proc/PID/fd/ and exec'ing some setxid binary as PID
might lead to the infoleak. It can be used in certain rare cases when
the knowledge of whether specific fd is opened/closed gains some
important information, e.g. whether some security check has
failed/succeeded (which is indirectly signaled by the kept fd). As for
map_files/ it may reveal ASLR offsets (but only some bits, not all of
them, I guess).
Without dropping denries it can be identified by calling stat() or
link() against dentries existing in the cache. In more details:
1) an attacker has a task with pid=PID with many opened fds.
2) Other task (PID2) opens /proc/PID/fd/ and fills the dentry cache.
Now dcache contains procfs entries for file descriptors of PID.
3) PID execve's setxid binary. (From this point PID2 should not get
_any_ information about /proc/PID/fd/, but this rule is violated in (4).)
4) PID2 does something to learn whether any fd of PID is opened/closed.
a) before "proc: fix races against execve() of /proc/PID/fd**" patch
PID2 could simply do getdents() against kept file descriptor of
/proc/PID/fd and get the list of opened fds.
b) Without dentry dropping on each access PID2 could use link(2) to
read /proc/PID/fd/* dentries from dcache. As they are in the
dcache since (2), ptrace check from ->lookup() is not applied.
c) If dentry is lazily dropped on each access attempt (or each illegal
access) then PID2 can:
i) read dentry line of /proc/slabinfo
ii) call link(2) against /proc/PID/fd, which invalidates the
specific dentry
iii) re-read dentry line of /proc/slabinfo. If it has decreased by
one, the dentry existed before (ii).
Is it possible to either allocate already dropped dentry or to force
->lookup() without invalidating dentry? The latter would potentially
pollute the dchache, though.
Thanks,
--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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