kernel BUG at kernel/cred.c:434!
cj.chengjian at huawei.com
Tue Apr 16 14:46:01 UTC 2019
On 2019/4/16 11:40, Kees Cook wrote:
> On Mon, Apr 15, 2019 at 11:20 AM Paul Moore <paul at paul-moore.com> wrote:
>> On Mon, Apr 15, 2019 at 11:05 AM Oleg Nesterov <oleg at redhat.com> wrote:
>>> On 04/15, Paul Moore wrote:
>>>> On Mon, Apr 15, 2019 at 9:43 AM Oleg Nesterov <oleg at redhat.com> wrote:
>>>>> Well, acct("/proc/self/attr/current") doesn't look like a good idea, but I do
>>>>> not know where should we put the additional check... And probably
>>>>> "echo /proc/self/attr/current > /proc/sys/kernel/core_pattern" can hit the
>>>>> same problem, do_coredump() does override_creds() too.
>>>>> May be just add
>>>>> if (current->cred != current->real_cred)
>>>>> return -EACCES;
>>>>> into proc_pid_attr_write(), I dunno.
>>>> Is the problem that do_acct_process() is calling override_creds() and
>>>> the returned/old credentials are being freed before do_acct_process()
>>>> can reinstall the creds via revert_creds()? Presumably because the
>>>> process accounting is causing the credentials to be replaced?
>>> Afaics, the problem is that do_acct_process() does override_creds() and
>>> then __kernel_write(). Which calls proc_pid_attr_write(), which in turn calls
>>> selinux_setprocattr(), which does another prepare_creds() + commit_creds();
>>> and commit_creds() hits
>>> BUG_ON(task->cred != old);
>> Gotcha. In the process of looking at the backtrace I forgot about the
>> BUG_ON() at the top of the oops message.
>> I wonder what terrible things would happen if we changed the BUG_ON()
>> in commit_creds to simple returning an error an error code to the
>> caller. There is a warning/requirement in commit_creds() function
>> header comment that it should always return 0.
> Would callers be expected to call abort_creds() on failure? There are
> a number of places where it'd need fixing up. And would likely be best
> with a __must_check marking.
> It seems like avoiding the pathological case might be simpler?
Yeah, Avoiding this pathological case is a better solution.
It seems like that we can't commit_creds() during
override_creds() and revert_creds().
So how about just put commit_creds outside !
override_creds() // cred -=> new
// may BUG_ON if commit_creds done.
revert_creds() // cred -=> old
commit_creds // cred = real_cred = new |
--Before we call commit_creds in selinux_setprocattr(),
if we find that cred != real_cred, it may have been overridden
before, we should revert it.
--The same to revert_creds, when we found someone have committed,
orig_cred != current->real_cred may hits, this means that
we have reverted before(see ).
--Sometimes new and old are the same, then we need to consider this
The code just like:
From: Yang Yingliang <yangyingliang at huawei.com>
Date: Sat, 13 Apr 2019 21:56:01 +0800
Subject: [PATCH] fix cred bug_on
Signed-off-by: Yang Yingliang <yangyingliang at huawei.com>
kernel/acct.c | 3 ++-
kernel/cred.c | 6 ++++++
security/selinux/hooks.c | 2 ++
3 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/acct.c b/kernel/acct.c
index addf7732fb56..f2065f899eee 100644
@@ -522,7 +522,8 @@ static void do_acct_process(struct bsd_acct_struct
current->signal->rlim[RLIMIT_FSIZE].rlim_cur = flim;
+ if (orig_cred == current->real_cred) // 
diff --git a/kernel/cred.c b/kernel/cred.c
index ecf03657e71c..c4d5ba92fb9b 100644
@@ -522,6 +522,9 @@ const struct cred *override_creds(const struct cred
const struct cred *old = current->cred;
+ if (old == new) // 
+ return old;
@@ -551,6 +554,9 @@ void revert_creds(const struct cred *old)
const struct cred *override = current->cred;
+ if (override == old) // 
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index b5017beb4ef7..bc8108e4e90f 100644
@@ -6590,6 +6590,8 @@ static int selinux_setprocattr(const char *name,
void *value, size_t size)
+ if (current->cred != current->real_cred) // 
We have tested this patch for 3 days and it works well.
Are there any cases that are not covered here ?
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