Messages in this thread | | | Subject | Re: kernel BUG at kernel/cred.c:434! | From | Yang Yingliang <> | Date | Sat, 20 Apr 2019 15:38:55 +0800 |
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On 2019/4/20 0:13, Paul Moore wrote: > On Fri, Apr 19, 2019 at 10:34 AM Yang Yingliang > <yangyingliang@huawei.com> wrote: >> On 2019/4/19 21:24, Paul Moore wrote: >>> On Thu, Apr 18, 2019 at 10:42 PM Yang Yingliang >>> <yangyingliang@huawei.com> wrote: >>>> On 2019/4/19 10:04, Paul Moore wrote: >>>>> On Wed, Apr 17, 2019 at 10:50 PM Yang Yingliang >>>>> <yangyingliang@huawei.com> wrote: >>>>>> On 2019/4/18 8:24, Casey Schaufler wrote: >>>>>>> On 4/17/2019 4:39 PM, Paul Moore wrote: >>>>>>>> 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 >>>>> I'm sorry, you've lost me. After override_creds() returns >>>>> current->cred and current->real_cred are not going to be the same, >>>>> yes? >>>> It's possible the new cred is equal to current->real_cred and >>>> current->cred, >>>> so after overrides_creds(), they have the same value. >>> Both task_struct.cred and task_struct.real_cred are pointer values, >>> assuming that one uses prepare_creds() to allocate/initialize a new >>> cred struct for use with override_creds() then the newly created cred >>> should never be equal to task_struct.real_cred. Am I missing >>> something, or are you thinking of something else? >> In do_acct_process(), file->f_cred may equal to current->real_cred, I >> confirm >> it by adding some debug message in do_acct_process() like this: > I would expect that; real_cred is the task's objective DAC > credentials, so using it for f_cred makes sense. > > What we are now talking about is the task's subjective credentials, > which can be overridden via override_creds(), and are what the LSMs > change via proc_pid_attr_write(). I'm not sure you got my point.
I was saying cred != real_cred check is not quite right, because the cred can be overridden by a same pointer as my print messages showing.
"cred != real_cred" means override_creds() is called, but "cred == real_cred" doesn't mean override_creds() is not called.
When we use "cred != real_cred" check, we may lost the situation that cred is overridden by a same pointer. In this case, we will do override_creds() => commit_creds() => revert_creds(), this make cred != real_cred, when a new commit_creds() is called, it also will trigger a BUG_ON(). > >> --- a/kernel/acct.c >> +++ b/kernel/acct.c >> @@ -481,6 +481,7 @@ static void do_acct_process(struct bsd_acct_struct >> *acct) >> flim = current->signal->rlim[RLIMIT_FSIZE].rlim_cur; >> current->signal->rlim[RLIMIT_FSIZE].rlim_cur = RLIM_INFINITY; >> /* Perform file operations on behalf of whoever enabled >> accounting */ >> + pr_info("task:%px new cred:%px real cred:%px cred:%px\n", >> current, file->f_cred, current->real_cred, current->cred); >> orig_cred = override_creds(file->f_cred);
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