Re: [PATCH 1/1] NFSD: fix use-after-free in __nfs42_ssc_open()

Chuck Lever III
2022-12-12T19:48:34Z

> On Dec 12, 2022, at 2:46 PM, Jeff Layton <jlayton@kernel.org> wrote:
>
> On Mon, 2022-12-12 at 19:28 +0000, Chuck Lever III wrote:
>>
>>> On Dec 12, 2022, at 2:16 PM, Dai Ngo <dai.ngo@oracle.com> wrote:
>>>
>>>
>>> On 12/12/22 10:38 AM, Jeff Layton wrote:
>>>> On Mon, 2022-12-12 at 18:16 +0000, Chuck Lever III wrote:
>>>>>> On Dec 12, 2022, at 12:44 PM, Jeff Layton <jlayton@kernel.org> wrote:
>>>>>>
>>>>>> On Mon, 2022-12-12 at 18:14 +0100, Greg KH wrote:
>>>>>>> On Mon, Dec 12, 2022 at 09:31:19AM -0500, Jeff Layton wrote:
>>>>>>>> On Mon, 2022-12-12 at 14:59 +0100, Greg KH wrote:
>>>>>>>>> On Mon, Dec 12, 2022 at 08:40:31AM -0500, Jeff Layton wrote:
>>>>>>>>>> On Mon, 2022-12-12 at 05:34 -0800, dai.ngo@oracle.com wrote:
>>>>>>>>>>> On 12/12/22 4:22 AM, Jeff Layton wrote:
>>>>>>>>>>>> On Sun, 2022-12-11 at 11:22 -0800, Dai Ngo wrote:
>>>>>>>>>>>>> Problem caused by source's vfsmount being unmounted but remains
>>>>>>>>>>>>> on the delayed unmount list. This happens when nfs42_ssc_open()
>>>>>>>>>>>>> return errors.
>>>>>>>>>>>>> Fixed by removing nfsd4_interssc_connect(), leave the vfsmount
>>>>>>>>>>>>> for the laundromat to unmount when idle time expires.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Reported-by: Xingyuan Mo <hdthky0@gmail.com>
>>>>>>>>>>>>> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
>>>>>>>>>>>>> ---
>>>>>>>>>>>>> fs/nfsd/nfs4proc.c | 23 +++++++----------------
>>>>>>>>>>>>> 1 file changed, 7 insertions(+), 16 deletions(-)
>>>>>>>>>>>>>
>>>>>>>>>>>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
>>>>>>>>>>>>> index 8beb2bc4c328..756e42cf0d01 100644
>>>>>>>>>>>>> --- a/fs/nfsd/nfs4proc.c
>>>>>>>>>>>>> +++ b/fs/nfsd/nfs4proc.c
>>>>>>>>>>>>> @@ -1463,13 +1463,6 @@ nfsd4_interssc_connect(struct nl4_server *nss, struct svc_rqst *rqstp,
>>>>>>>>>>>>> return status;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> -static void
>>>>>>>>>>>>> -nfsd4_interssc_disconnect(struct vfsmount *ss_mnt)
>>>>>>>>>>>>> -{
>>>>>>>>>>>>> - nfs_do_sb_deactive(ss_mnt->mnt_sb);
>>>>>>>>>>>>> - mntput(ss_mnt);
>>>>>>>>>>>>> -}
>>>>>>>>>>>>> -
>>>>>>>>>>>>> /*
>>>>>>>>>>>>> * Verify COPY destination stateid.
>>>>>>>>>>>>> *
>>>>>>>>>>>>> @@ -1572,11 +1565,6 @@ nfsd4_cleanup_inter_ssc(struct vfsmount *ss_mnt, struct file *filp,
>>>>>>>>>>>>> {
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> -static void
>>>>>>>>>>>>> -nfsd4_interssc_disconnect(struct vfsmount *ss_mnt)
>>>>>>>>>>>>> -{
>>>>>>>>>>>>> -}
>>>>>>>>>>>>> -
>>>>>>>>>>>>> static struct file *nfs42_ssc_open(struct vfsmount *ss_mnt,
>>>>>>>>>>>>> struct nfs_fh *src_fh,
>>>>>>>>>>>>> nfs4_stateid *stateid)
>>>>>>>>>>>>> @@ -1762,7 +1750,8 @@ static int nfsd4_do_async_copy(void *data)
>>>>>>>>>>>>> struct file *filp;
>>>>>>>>>>>>>
>>>>>>>>>>>>> filp = nfs42_ssc_open(copy->ss_mnt, &copy->c_fh,
>>>>>>>>>>>>> - &copy->stateid);
>>>>>>>>>>>>> + &copy->stateid);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> if (IS_ERR(filp)) {
>>>>>>>>>>>>> switch (PTR_ERR(filp)) {
>>>>>>>>>>>>> case -EBADF:
>>>>>>>>>>>>> @@ -1771,7 +1760,7 @@ static int nfsd4_do_async_copy(void *data)
>>>>>>>>>>>>> default:
>>>>>>>>>>>>> nfserr = nfserr_offload_denied;
>>>>>>>>>>>>> }
>>>>>>>>>>>>> - nfsd4_interssc_disconnect(copy->ss_mnt);
>>>>>>>>>>>>> + /* ss_mnt will be unmounted by the laundromat */
>>>>>>>>>>>>> goto do_callback;
>>>>>>>>>>>>> }
>>>>>>>>>>>>> nfserr = nfsd4_do_copy(copy, filp, copy->nf_dst->nf_file,
>>>>>>>>>>>>> @@ -1852,8 +1841,10 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>>>>>>>>>>>>> if (async_copy)
>>>>>>>>>>>>> cleanup_async_copy(async_copy);
>>>>>>>>>>>>> status = nfserrno(-ENOMEM);
>>>>>>>>>>>>> - if (nfsd4_ssc_is_inter(copy))
>>>>>>>>>>>>> - nfsd4_interssc_disconnect(copy->ss_mnt);
>>>>>>>>>>>>> + /*
>>>>>>>>>>>>> + * source's vfsmount of inter-copy will be unmounted
>>>>>>>>>>>>> + * by the laundromat
>>>>>>>>>>>>> + */
>>>>>>>>>>>>> goto out;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>> This looks reasonable at first glance, but I have some concerns with the
>>>>>>>>>>>> refcounting around ss_mnt elsewhere in this code. nfsd4_ssc_setup_dul
>>>>>>>>>>>> looks for an existing connection and bumps the ni->nsui_refcnt if it
>>>>>>>>>>>> finds one.
>>>>>>>>>>>>
>>>>>>>>>>>> But then later, nfsd4_cleanup_inter_ssc has a couple of cases where it
>>>>>>>>>>>> just does a bare mntput:
>>>>>>>>>>>>
>>>>>>>>>>>> if (!nn) {
>>>>>>>>>>>> mntput(ss_mnt);
>>>>>>>>>>>> return;
>>>>>>>>>>>> }
>>>>>>>>>>>> ...
>>>>>>>>>>>> if (!found) {
>>>>>>>>>>>> mntput(ss_mnt);
>>>>>>>>>>>> return;
>>>>>>>>>>>> }
>>>>>>>>>>>>
>>>>>>>>>>>> The first one looks bogus. Can net_generic return NULL? If so how, and
>>>>>>>>>>>> why is it not a problem elsewhere in the kernel?
>>>>>>>>>>> it looks like net_generic can not fail, no where else check for NULL
>>>>>>>>>>> so I will remove this check.
>>>>>>>>>>>
>>>>>>>>>>>> For the second case, if the ni is no longer on the list, where did the
>>>>>>>>>>>> extra ss_mnt reference come from? Maybe that should be a WARN_ON or
>>>>>>>>>>>> BUG_ON?
>>>>>>>>>>> if ni is not found on the list then it's a bug somewhere so I will add
>>>>>>>>>>> a BUG_ON on this.
>>>>>>>>>>>
>>>>>>>>>> Probably better to just WARN_ON and let any references leak in that
>>>>>>>>>> case. A BUG_ON implies a panic in some environments, and it's best to
>>>>>>>>>> avoid that unless there really is no choice.
>>>>>>>>> WARN_ON also causes machines to boot that have panic_on_warn enabled.
>>>>>>>>> Why not just handle the error and keep going? Why panic at all?
>>>>>>>>>
>>>>>>>> Who the hell sets panic_on_warn (outside of testing environments)?
>>>>>>> All cloud providers and anyone else that wants to "kill the system that
>>>>>>> had a problem and have it reboot fast" in order to keep things working
>>>>>>> overall.
>>>>>>>
>>>>>> If that's the case, then this situation would probably be one where a
>>>>>> cloud provider would want to crash it and come back. NFS grace periods
>>>>>> can suck though.
>>>>>>
>>>>>>>> I'm
>>>>>>>> suggesting a WARN_ON because not finding an entry at this point
>>>>>>>> represents a bug that we'd want reported.
>>>>>>> Your call, but we are generally discouraging adding new WARN_ON() for
>>>>>>> anything that userspace could ever trigger. And if userspace can't
>>>>>>> trigger it, then it's a normal type of error that you need to handle
>>>>>>> anyway, right?
>>>>>>>
>>>>>>> Anyway, your call, just letting you know.
>>>>>>>
>>>>>> Understood.
>>>>>>
>>>>>>>> The caller should hold a reference to the object that holds a vfsmount
>>>>>>>> reference. It relies on that vfsmount to do a copy. If it's gone at this
>>>>>>>> point where we're releasing that reference, then we're looking at a
>>>>>>>> refcounting bug of some sort.
>>>>>>> refcounting in the nfsd code, or outside of that?
>>>>>>>
>>>>>> It'd be in the nfsd code, but might affect the vfsmount refcount. Inter-
>>>>>> server copy is quite the tenuous house of cards. ;)
>>>>>>
>>>>>>>> I would expect anyone who sets panic_on_warn to _desire_ a panic in this
>>>>>>>> situation. After all, they asked for it. Presumably they want it to do
>>>>>>>> some coredump analysis or something?
>>>>>>>>
>>>>>>>> It is debatable whether the stack trace at this point would be helpful
>>>>>>>> though, so you might consider a pr_warn or something less log-spammy.
>>>>>>> If you can recover from it, then yeah, pr_warn() is usually best.
>>>>>>>
>>>>>> It does look like Dai went with pr_warn on his v2 patch.
>>>>>>
>>>>>> We'd "recover" by leaking a vfsmount reference. The immediate crash
>>>>>> would be avoided, but it might make for interesting "fun" later when you
>>>>>> went to try and unmount the thing.
>>>>> This is a red flag for me. If the leak prevents the system from
>>>>> shutting down reliably, then we need to do something more than
>>>>> a pr_warn(), I would think.
>>>>>
>>>> Sorry, I should correct myself.
>>>>
>>>> We wouldn't (necessarily) leak a vfsmount reference. If the entry was no
>>>> longer on the list, then presumably it has already been cleaned up and
>>>> the vfsmount reference put.
>>>
>>> I think the issue here is not vfsmount reference count. The issue is that
>>> we could not find a nfsd4_ssc_umount_item on the list that matches the
>>> vfsmount ss_mnt. So the question is what should we do in this case?
>>>
>>> Prior to this patch, when we hit this scenario we just go ahead and
>>> unmount the ss_mnt there since it won't be unmounted by the laundromat
>>> (it's not on the delayed unmount list).
>>>
>>> With this patch, we don't even unmount the ss_mnt, we just do a pr_warn.
>>>
>>> I'd prefer to go back to the previous code to do the unmount and also
>>> do a pr_warn.
>>>
>>>> It's still a bug though since we _should_ still have a reference to the
>>>> nfsd4_ssc_umount_item at this point. So this is really just a potential
>>>> use-after-free.
>>>
>>> The ss_mnt still might have a reference on the nfsd4_ssc_umount_item
>>> but we just can't find it on the list. Even though the possibility for
>>> this to happen is from slim to none, we still have to check for it.
>>>
>>>> FWIW, the object handling here is somewhat weird as the copy operation
>>>> holds a reference to the nfsd4_ssc_umount_item but passes around a
>>>> pointer to the vfsmount
>>>>
>>>> I have to wonder if it'd be cleaner to have nfsd4_setup_inter_ssc pass
>>>> back a pointer to the nfsd4_ssc_umount_item, so you could pass that to
>>>> nfsd4_cleanup_inter_ssc and skip searching for it again at cleanup time.
>>>
>>> Yes, I think returning a pointer to the nfsd4_ssc_umount_item approach
>>> would be better. We won't have to deal with the situation where we can't
>>> find an item on the list (even though it almost never happen).
>>>
>>> Can we do this enhancement after fixing this use-after-free problem, in
>>> a separate patch series?
>
> ^^^
> I think that'd be best.
>
>> Is there a reason not fix it correctly now?
>>
>> I'd rather not merge a fix that leaves the possibility of a leak.
>
> We're going to need to backport this to earlier kernels and it'll need
> to go in soon. I think it'd be to take a minimal fix for the reported
> crash, and then Dai can have some time to do a larger cleanup.

Backporting is important, of course.

What I was hearing was that the simple fix couldn't be done without
introducing a leak or UAF.

--
Chuck Lever