Re: [PATCH net v1] net/ipv6: mcast: fix circular locking dependency in __ipv6_dev_mc_inc()

[Paolo Abeni <pabeni@redhat.com>]

Adding Josef.

On 3/19/26 4:26 AM, Jakub Kicinski wrote:
> On Thu, 19 Mar 2026 11:04:24 +0800 Jiayuan Chen wrote:
>>>> Split mca_alloc() into mca_alloc() + mca_init(): mca_alloc() does the
>>>> GFP_KERNEL allocation before mc_lock, mca_init() initializes under
>>>> mc_lock. If the address already exists, the pre-allocated memory is
>>>> simply freed. Also move inet6_ifmcaddr_notify() outside mc_lock since
>>>> it also does GFP_KERNEL allocation.  
>>> Moving the allocation seems fine, but also having to move the
>>> notification, potentially letting the notification go out of order
>>> makes me wonder if we aren't better off adding helpers for taking this
>>> lock which also call memalloc_noio_{save,restore} ?  
>> Yeah, using memalloc_noio helpers is simpler. I checked and there
>> are about 18 places taking mc_lock, so having a common mc_lock()/mc_unlock()
>> wrapper that does the noio save/restore covers them all (if necessary).
>>
>> The only thing that feels a bit odd is using memalloc_noio in the networking
>> subsystem. It makes sense in block/fs to protect itself from recursion.
> 
> Totally agree that it feels a bit odd that we have to worry about IO,
> but unless we can figure out a way to prevent nbd sockets from getting
> here all our solutions are dealing with noio in networking code :(
> IMHO it's better to acknowledge this with the explicit memalloc_noio 
> so future developers don't break things again with a mis-placed
> allocation.

I think a problem here is that the nbd socket is still exposed to
user-space, while in use by the block device. I fear that the more
syzkaller will learn new tricks, the more we will have to had strange
noio all around the networking code.

I *think* we could prevent this kind of races with something alike the
following:
- nbd sets a DOIO sk flag on the sockets it uses.
- the socket layer prevents socketopts()/ioctl() entirely on DOIO sk

I'm not sure if that could break nbd users, but allowing the user-space
to mess with the socket used for backing a block device looks very
dangerous.

/P