linux-kernel - Re: fs: use-after-free in path

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Date:   Sun, 5 Mar 2017 18:38:25 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: fs: use-after-free in path_lookupat

On Sun, Mar 5, 2017 at 6:33 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Sun, Mar 5, 2017 at 5:33 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> On Sun, Mar 05, 2017 at 05:14:23PM +0100, Dmitry Vyukov wrote:
>>> On Sun, Mar 5, 2017 at 4:57 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>>> > On Sun, Mar 05, 2017 at 12:37:13PM +0100, Dmitry Vyukov wrote:
>>> >
>>> >> I am pretty sure it is that one.
>>> >> I don't think I ever used name_to_handle_at syscall in my life and I
>>> >> definitely didn't make it lookup a memfd :)
>>> >
>>> > So what does it normally return?  On the runs where we do not hit that
>>> > use-after-free, that is.
>>> >
>>> > What gets triggered there is nd->path.dentry pointing to already freed
>>> > dentry.  We are in RCU mode, so we are not pinning the dentry and it
>>> > might have reached dentry_free().  However, anything with DCACHE_RCUACCESS
>>> > set would have freeing RCU-delayed, making that impossible.
>>> >
>>> > memfd stuff does *not* have DCACHE_RCUACCESS, which would've made it
>>> > plausible, but... there we really should've been stopped cold by
>>> > the d_can_lookup() check - that is done while we are still holding
>>> > a reference to struct file, which should've prevented freeing and
>>> > reuse.  So at the time of that check we have dentry still not reused
>>> > by anything, and d_can_lookup() should've failed.
>>> >
>>> > There is a race that could bugger the things up in that area, but it needs
>>> > empty name, so this one is something else...
>>>
>>> You can see from the log above that s always empty somehow, so the
>>> d_can_lookup check is simply not done. I have not looked at the code,
>>> but it's not racy, so should follow from the arguments passed to
>>> name_to_handle_at.
>>
>> Umm...  name_to_handle_at() in your log:
>> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300", &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0, 0x1000)
>> and unless I'm misreading what you are printing there, you have "./bus0"
>> passed as the second argument.  Right?  That's pretty much why I asked about
>> other possible calls triggering it...
>>
>> If you are somehow getting there with empty name and if there's another
>> thread closing these memfd descriptors, I understand what's going on there.
>> It's how we are getting that empty name on your syscall arguments that
>> looks very odd...
>
>
> Added more debug output.
>
> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300",
> &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0,
> 0x1000)
>
> actually passes name="" because of the overlapping addresses. Flags
> contain AT_EMPTY_PATH.


The problem can be more general as a bunch of xxxat calls support AT_EMPTY_PATH.