Subject: Re: [PATCH] bpf, sockmap: fix deadlocks in the sockhash and sockmap

From: Martin KaFai Lau <[email-masked]>
Date: Wed, 20 Sep 2023 18:31:42 -0700

On 9/20/23 11:07 AM, John Fastabend wrote:
>>> pay much attention to their deletion. Compared with hash
>>> maps, sockhash only provides spin_lock_bh protection.
>>> This causes it to appear to have self-locking behavior
>>> in the interrupt context, as CVE-2023-0160 points out.
> 
> CVE is a bit exagerrated in my opinion. I'm not sure why
> anyone would delete an element from interrupt context. But,
> OK if someone wrote such a thing we shouldn't lock up.

This should only happen in tracing program?
not sure if it will be too drastic to disallow tracing program to use 
bpf_map_delete_elem during load time now.

A followup question, if sockmap can be accessed from tracing program, does it 
need an in_nmi() check?

>>>    	hash = sock_hash_bucket_hash(key, key_size);
>>>    	bucket = sock_hash_select_bucket(htab, hash);
>>>    
>>> -	spin_lock_bh(&bucket->lock);
>>> +	spin_lock_irqsave(&bucket->lock, flags);
> 
> The hashtab code htab_lock_bucket also does a preempt_disable()
> followed by raw_spin_lock_irqsave(). Do we need this as well
> to handle the PREEMPT_CONFIG cases.

iirc, preempt_disable in htab is for the CONFIG_PREEMPT but it is for the 
__this_cpu_inc_return to avoid unnecessary lock failure due to preemption, so 
probably it is not needed here. The commit 2775da216287 ("bpf: Disable 
preemption when increasing per-cpu map_locked")

If map_delete can be called from any tracing context, the raw_spin_lock_xxx 
version is probably needed though. Otherwise, splat (e.g. 
PROVE_RAW_LOCK_NESTING) could be triggered.