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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 8/3/26 9:21 AM, Kumar Kartikeya Dwivedi wrote:
> On Tue Jul 28, 2026 at 8:05 AM CEST, Jiayuan Chen wrote:
>> Reported by the Sashiko AI review.
>>
>> arena_vm_fault() returns VM_FAULT_RETRY when it can't take
>> arena->spinlock, but it never took mmap_lock. The fault path assumes a
>> VM_FAULT_RETRY handler already dropped mmap_lock and re-takes it on the
>> retry, so mmap_lock gets taken twice and can deadlock:
>>
>> 	do_user_addr_fault()
>> 	{
>> 		fault = handle_mm_fault(...);   // calls arena_vm_fault()
>> 		if (fault & VM_FAULT_RETRY)
>> 			goto retry;   // re-locks mmap_lock
>> 		mmap_read_unlock(mm);
>> 	}
>>
>> Return VM_FAULT_SIGBUS instead, for two reasons:
>>
>> 1. We could keep VM_FAULT_RETRY, but then we'd have to drop the fault
>>     lock first and cap the retry ourselves, the way __folio_lock_or_retry()
>>     does.
>>
>> 2. A failed raw_res_spin_lock_irqsave() already means a possible deadlock
>>     was detected, so retrying just hits the same lock again.
>>
>> So returning VM_FAULT_RETRY here is overkill.
>>
>> Fixes: b8467290edab ("bpf: arena: make arena kfuncs any context safe")
>> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
>>
>> ---
>> target to bpf-next since I think it is moderate.
>> ---
> This looks ok to me, but after staring at this function for longer, I think all
> kinds of non-recoverable errors should be using VM_FAULT_SIGBUS vs SIGSEGV.
>
> The only case with SIGSEGV should be the flag based request to disable
> allocation on lazy faults. That should include the scratch page case, since it
> represents a hole.
>
> SIGSEGV: BPF_F_SEGV_ON_FAULT, scratch page.
> SIGBUS: lock acquisition failure, range-tree failures, page allocation failure,
> kernel PTE install failure.
>
> All of these SIGBUS are almost improbable, but I think it would be cleaner to
> keep semantics clear.
>
> Could you follow up with this change? I applied this one for now.


Sure. I'm happy to do it.

>>   kernel/bpf/arena.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
>> index 34f023a537fe..555ee2531ef9 100644
>> --- 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;
>>
>>   	page = vmalloc_to_page((void *)kaddr);
>>   	if (page) {