Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755562Ab3JCVKW (ORCPT ); Thu, 3 Oct 2013 17:10:22 -0400 Received: from mail-lb0-f171.google.com ([209.85.217.171]:51875 "EHLO mail-lb0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754917Ab3JCVKQ (ORCPT ); Thu, 3 Oct 2013 17:10:16 -0400 MIME-Version: 1.0 In-Reply-To: <20131003201332.GA3500@dztty> References: <1380659178-28605-1-git-send-email-tixxdz@opendz.org> <1380659178-28605-3-git-send-email-tixxdz@opendz.org> <524B78A2.40007@amacapital.net> <20131002145506.GA2669@dztty> <20131003143653.GA32445@dztty> <20131003192926.GB2390@dztty> <20131003201332.GA3500@dztty> From: Andy Lutomirski Date: Thu, 3 Oct 2013 14:09:55 -0700 Message-ID: Subject: Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task To: Djalal Harouni Cc: "Eric W. Biederman" , Kees Cook , Al Viro , Andrew Morton , Linus Torvalds , Ingo Molnar , "Serge E. Hallyn" , Cyrill Gorcunov , David Rientjes , LKML , Linux FS Devel , "kernel-hardening@lists.openwall.com" , Djalal Harouni Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6238 Lines: 120 On Thu, Oct 3, 2013 at 1:13 PM, Djalal Harouni wrote: > On Thu, Oct 03, 2013 at 12:37:49PM -0700, Andy Lutomirski wrote: >> On Thu, Oct 3, 2013 at 12:29 PM, Djalal Harouni wrote: >> > On Thu, Oct 03, 2013 at 04:12:37PM +0100, Andy Lutomirski wrote: >> >> On Thu, Oct 3, 2013 at 3:36 PM, Djalal Harouni wrote: >> >> > On Wed, Oct 02, 2013 at 05:44:17PM +0100, Andy Lutomirski wrote: >> >> >> On Wed, Oct 2, 2013 at 3:55 PM, Djalal Harouni wrote: >> >> >> > On Tue, Oct 01, 2013 at 06:36:34PM -0700, Andy Lutomirski wrote: >> >> >> >> On 10/01/2013 01:26 PM, Djalal Harouni wrote: >> >> >> >> > Since /proc entries varies at runtime, permission checks need to happen >> >> >> >> > during each system call. >> >> >> >> > >> >> >> >> > However even with that /proc file descriptors can be passed to a more >> >> >> >> > privileged process (e.g. a suid-exec) which will pass the classic >> >> >> >> > ptrace_may_access() permission check. The open() call will be issued in >> >> >> >> > general by an unprivileged process while the disclosure of sensitive >> >> >> >> > /proc information will happen using a more privileged process at >> >> >> >> > read(),write()... >> >> >> >> > >> >> >> >> > Therfore we need a more sophisticated check to detect if the cred of the >> >> >> >> > process have changed, and if the cred of the original opener that are >> >> >> >> > stored in the file->f_cred have enough permission to access the task's >> >> >> >> > /proc entries during read(), write()... >> >> >> >> > >> >> >> >> > Add the proc_allow_access() function that will receive the file->f_cred >> >> >> >> > as an argument, and tries to check if the opener had enough permission >> >> >> >> > to access the task's /proc entries. >> >> >> >> > >> >> >> >> > This function should be used with the ptrace_may_access() check. >> >> >> >> > >> >> >> >> > Cc: Kees Cook >> >> >> >> > Suggested-by: Eric W. Biederman >> >> >> >> > Signed-off-by: Djalal Harouni >> >> >> >> > --- >> >> >> >> > fs/proc/base.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> >> >> >> > fs/proc/internal.h | 2 ++ >> >> >> >> > 2 files changed, 58 insertions(+) >> >> >> >> > >> >> >> >> > diff --git a/fs/proc/base.c b/fs/proc/base.c >> >> >> >> > index e834946..c29eeae 100644 >> >> >> >> > --- a/fs/proc/base.c >> >> >> >> > +++ b/fs/proc/base.c >> >> >> >> > @@ -168,6 +168,62 @@ int proc_same_open_cred(const struct cred *fcred) >> >> >> >> > cap_issubset(cred->cap_permitted, fcred->cap_permitted)); >> >> >> >> > } >> >> >> >> > >> >> >> >> > +/* Returns 0 on success, -errno on denial. */ >> >> >> >> > +static int __proc_allow_access(const struct cred *cred, >> >> >> >> > + struct task_struct *task, unsigned int mode) >> >> >> >> > +{ >> >> >> >> > + int ret = 0; >> >> >> >> > + const struct cred *tcred; >> >> >> >> > + const struct cred *fcred = cred; >> >> >> >> > + >> >> >> >> > + rcu_read_lock(); >> >> >> >> > + tcred = __task_cred(task); >> >> >> >> > + if (uid_eq(fcred->uid, tcred->euid) && >> >> >> >> > + uid_eq(fcred->uid, tcred->suid) && >> >> >> >> > + uid_eq(fcred->uid, tcred->uid) && >> >> >> >> > + gid_eq(fcred->gid, tcred->egid) && >> >> >> >> > + gid_eq(fcred->gid, tcred->sgid) && >> >> >> >> > + gid_eq(fcred->gid, tcred->gid)) >> >> >> >> > + goto out; >> >> >> >> > + >> >> >> >> >> >> >> >> What's this for? Is it supposed to be an optimization? If so, it looks >> >> >> >> potentially exploitable, although I don't really understand what you're >> >> >> >> trying to do. >> >> >> > This function should be used in addition to the ptrace_may_access() one. >> >> >> >> >> >> Sorry, I was unclear. I meant: what are the uid and gid checks for? >> >> > The uid/gid are checks of the current (reader) on the target task, like >> >> > the ptrace checks. fcred here is the cred of current at open time. >> >> > >> >> >> >> This isn't a faithful copy of __ptrace_may_access -- the real function >> >> gives LSMs a chance to veto ptracing. That's critical even without >> >> LSMs because cap_ptrace_access_check needs to get called. (Think >> >> about setcap'd programs instead of setuid programs.) >> > Yes, I already did this, not only setuid, capabilities also are handled >> > See the whole patch, please! >> > >> > >> > Yes, and speaking about LSMs I've mentioned in my patches and doc, that >> > the proposed function proc_allow_access() should be used after >> > ptrace_may_access(). proc_allow_access() is not a replacement for >> > ptrace_may_access(), it should be used *after* it. >> > >> > So cap_ptrace_access_check() is called, and before the file->f_cred >> > checks. The LSM veto is already there. >> >> It's possible that I've misunderstood your patches, but I really don't >> see where you're calling into LSMs to give them a chance to veto >> access by *f_cred*. > Ahh ok, I see, but why you want absolutly to put *f_cred* in this ? > > That's not its job, LSM veto is handled during read() correctly before > proc_allow_access() and f_cred check. And if you want to do it correctly > then f_cred should be handled during its time, during ->open(). > The correct way to handle it: ptrace_may_access() during ->open() and > each syscall for sensitive files. > > Why add and speak about all this complexity where the correct check is > just add ptrace_may_access() during ->open() ? using *f_cred* in this > context and bring it here is not a valid argument IMO. I don't want to put f_cred into this. I'd rather the patches just check everything at open() time. Doing that will require some form of revocation, I think. Your current patches use f_cred, and they seem to do it wrong. So please either fix it, stop using f_cred, or explain why it it's okay despite not invoking LSM in the expected way. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/