Messages in this thread |  | | Subject | Re: kernel BUG at kernel/cred.c:434! | From | Yang Yingliang <> | Date | Thu, 18 Apr 2019 10:49:56 +0800 |
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Hi, Casey
On 2019/4/18 8:24, Casey Schaufler wrote: > On 4/17/2019 4:39 PM, Paul Moore wrote: >> On Wed, Apr 17, 2019 at 12:27 PM Oleg Nesterov <oleg@redhat.com> wrote: >>> On 04/17, Paul Moore wrote: >>>> On Wed, Apr 17, 2019 at 10:57 AM Oleg Nesterov <oleg@redhat.com> >>>> wrote: >>>>> On 04/17, Paul Moore wrote: >>>>>> I'm tempted to simply return an error in selinux_setprocattr() if >>>>>> the task's credentials are not the same as its real_cred; >>>>> What about other modules? I have no idea what smack_setprocattr() is, >>>>> but it too does prepare_creds/commit creds. >>>>> >>>>> it seems that the simplest workaround should simply add the >>>>> additional >>>>> cred == real_cred into proc_pid_attr_write(). >>>> Yes, that is simple, but I worry about what other LSMs might want to >>>> do. While I believe failing if the effective creds are not the same >>>> as the real_creds is okay for SELinux (possibly Smack too), I worry >>>> about what other LSMs may want to do. After all, >>>> proc_pid_attr_write() doesn't change the the creds itself, that is >>>> something the specific LSMs do. >>> Yes, but if proc_pid_attr_write() is called with cred != real_cred then >>> something is already wrong? >> True, or at least I would think so. >> >> Looking at the current tree there are three LSMs which implement >> setprocattr hooks: SELinux, Smack, and AppArmor. I know Casey has >> already mentioned that he wasn't able to trigger the problem in Smack, >> but looking at smack_setprocattr() I see the similar commit_creds() >> usage so I would expect the same problem in Smack; what say you Casey? > > I say that my test program runs without ill effect. I call acct() > with "/proc/self/attr/current", which succeeds and enables accounting > just like it is supposed to. I then have the program open > "/proc/self/attr/current" and read it, all of which goes swimmingly. > When Smack frees a cred it usually does not free any memory of its > own, so it is conceivable that I'm just getting lucky. Or, I may not > have sufficient debug enabled. > >> Looking at apparmor_setprocattr(), it appears that it too could end >> up calling commit_creds() via aa_set_current_hat(). >> >> Since it looks like all three LSMs which implement the setprocattr >> hook are vulnerable I'm open to the idea that proc_pid_attr_write() is >> a better choice for the cred != read_cred check, but I would want to >> make sure John and Casey are okay with that. >> >> John? >> >> Casey? > > I'm fine with the change going into proc_pid_attr_write(). The cred != real_cred checking is not enough.
Consider this situation, when doing override, cred, real_cred and new_cred are all same:
after override_creds() cred == real_cred == new1_cred after prepare_creds() new2_cred after commit_creds() becasue the check is false, so cred == real_cred == new2_cred after revert_creds() cred == new1_cred, real_cred == new2_cred
It will cause cred != real_cred finally.
Regards, Yang
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