Re: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure

On 7/28/26 2:23 PM, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>
> Pre-existing issues:
> - [High] BPF arena page faults can spuriously crash the user process with SIGSEGV due to transient lockless allocation failures.
> --
>
> --- Patch [1]: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure ---
> commit c57623f4077e743e8c901306fcec7c67d2aafc17
> Author: Jiayuan Chen <[email protected]>
>
> bpf: arena: fix mmap_lock deadlock on arena lock failure
>
> This commit fixes a potential deadlock in the arena page fault handler by
> returning VM_FAULT_SIGBUS instead of VM_FAULT_RETRY when the arena spinlock
> cannot be acquired. Returning retry would cause the fault path to attempt
> retaking the mmap_lock, which was never dropped.
>
>> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
>> --- a/kernel/bpf/arena.c
>> +++ b/kernel/bpf/arena.c
>> @@ -490,8 +490,12 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
>>   	kaddr = kbase + (u32)(vmf->address);
>>   
>>   	if (raw_res_spin_lock_irqsave(&arena->spinlock, flags))
>> -		/* Make a reasonable effort to address impossible case */
>> -		return VM_FAULT_RETRY;
>> +		/*
>> +		 * A failed lock means a possible deadlock was detected. Don't
>> +		 * return VM_FAULT_RETRY: this handler never took mmap_lock, but
>> +		 * the fault path would re-take it on retry and deadlock. Fail.
>> +		 */
>> +		return VM_FAULT_SIGBUS;
> [Severity: High]
> This is a pre-existing issue, but looking further down in arena_vm_fault(),
> does a transient allocation failure unexpectedly cause a SIGSEGV?
>
> Since the raw spinlock is held, disabling IRQs, bpf_map_alloc_pages()
> falls back to an opportunistic lockless allocation which can fail under
> memory pressure:
>
> 	ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page);
> 	if (ret) {
> 		range_tree_set(&arena->rt, vmf->pgoff, 1);
> 		goto out_sigsegv_memcg;
> 	}
>
> When this lockless allocation fails, the error path jumps to
> out_sigsegv_memcg and returns VM_FAULT_SIGSEGV instead of VM_FAULT_OOM or
> safely dropping the lock to retry.
>
> Could this cause user processes mapping a BPF arena to experience spurious
> crashes during memory contention?


Yes I'm on it 
https://lore.kernel.org/bpf/[email protected]/

And current patch is from your review result of that patch...