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

Chuck Lever III
2022-12-12T19:28:39Z

> 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?

Is there a reason not fix it correctly now?

I'd rather not merge a fix that leaves the possibility of a leak.

--
Chuck Lever