Date: Wed, 8 Dec 2010 07:51:18 +0300 From: Solar Designer <solar@...nwall.com> To: oss-security@...ts.openwall.com Subject: Re: kernel: Dangerous interaction between clear_child_tid, set_fs(), and kernel oopses Nelson, Dan, Steve - It's been a few days, so I'll over-quote a little bit. Please see below: On Thu, Dec 02, 2010 at 12:21:14AM -0500, Nelson Elhage wrote: > I've discovered an interesting interaction in the Linux kernel between the > clear_child_tid feature of clone(2), and the set_fs() function used internally > in the kernel to temporarily disable access_ok() checking of userspace pointers. > > Under some (not totally uncommon) circumstances, it is possible for a user to > leverage this interaction to turn a kernel oops or BUG() into a write of an > integer 0 to a user-controlled address in kernel memory. > > I'm not sure if this merits a CVE or not; It is (as far as I can tell) only a > problem in the presence of another security bug, but it potentially makes a > large class of bugs significantly more dangerous (DoS -> privesc). > > Reference: > https://lkml.org/lkml/2010/12/1/543 To me, things like this are more important than individual NULL pointer dereference bugs or the like. So if those get CVEs, this one definitely should as well. Nelson - why are you proposing adding set_fs(USER_DS); not to the very beginning of do_exit(), but below a few calls/checks? I don't think there's any performance improvement from that, and it feels "theoretically safer" to return to the sane/safe state as soon as possible. I am currently looking at do_exit() in OpenVZ's RHEL5-based 2.6.18-194.26.1.el5.028stab079.1 - it does a bit more work before reaching the place you patch. So I am tempted to introduce set_fs(USER_DS); as the very first statement in do_exit() instead. Did you check whether 2.4 kernels are affected as well? Thanks, Alexander
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