linux-kernel - Re: Linux 3.18.111

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Message-Id: <20180810064258epcas1p3eb68d29dbd60b705d0473b3c645496ff~Jcs48Q55D0165001650epcas1p3e@epcas1p3.samsung.com>
Date:   Fri, 10 Aug 2018 15:43:02 +0900
From:   Seung-Woo Kim <sw0312.kim@...sung.com>
To:     sw0312.kim@...sung.com
CC:     Al Viro <viro@...IV.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>, lwn@....net,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: Linux 3.18.111

On 2018년 08월 08일 19:06, Seung-Woo Kim wrote:
> On 2018년 07월 05일 09:52, Al Viro wrote:
>> On Mon, Jul 02, 2018 at 10:01:25PM -0700, Linus Torvalds wrote:
>>> On Mon, Jul 2, 2018 at 9:43 PM Seung-Woo Kim <sw0312.kim@...sung.com> wrote:
>>>>
>>>> I think the commit itself is required. Simple, but not reliable,
>>>> workaround fix is like below:
>>>>
>>>> diff --git a/fs/dcache.c b/fs/dcache.c
>>>> index a34d401..7c751f2 100644
>>>> --- a/fs/dcache.c
>>>> +++ b/fs/dcache.c
>>>> @@ -1879,6 +1879,8 @@ void d_instantiate_new(struct dentry *entry,
>>>> struct inode *inode)
>>>>         BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
>>>>         BUG_ON(!inode);
>>>>         lockdep_annotate_inode_mutex_key(inode);
>>>> +       /* WORKAROUND for calling security_d_instantiate() */
>>>> +       entry->d_inode = inode;
>>>>         security_d_instantiate(entry, inode);
>>>>         spin_lock(&inode->i_lock);
>>>>         __d_instantiate(entry, inode);
>>>
>>> Ugh. That looks horrible even if it might avoid the oops.
>>>
>>> I think a much better solution is to back-port commit b296821a7c42
>>> ("xattr_handler: pass dentry and inode as separate arguments of
>>> ->get()") to older kernels. Then the inode is passed down all the way,
>>> and you don't have people try to get it from the (not yet initialized)
>>> dentry.
>>>
>>> But there might be other parts missing too, and I didn't look at how
>>> easy/painful that backport would be.
>>>
>>> Al - comments? This is all because of commit 1e2e547a93a0 ("do
>>> d_instantiate/unlock_new_inode combinations safely") being marked for
>>> stable, and various cases of security_d_instantiate() calling down to
>>> getxattr. Which used to not get the inode at all, so those older
>>> kernels use d_inode(dentry), which doesn't work in this path since
>>> dentry->d_inode hasn't been instantiated yet..
>>
>> You also want b96809173e94 and ce23e6401334 there...
> 
> For above two commits, also b296821a7c42 is required. And after
> backport, smack still crashed because setxattr. To fix it, 5930122683df
> and 3767e255b390 are also required.
> 
> By the way, does no one have met this kind getxattr crash issue with
> selinux from 3.18.y?
> 

I have checked with selinux, and it is confirmed that there is no crash
because selinux_d_instantiate() has null check for inode. So, it is only
security smack issue.

-- 
Seung-Woo Kim
Samsung Research
--