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

[sashiko-bot@kernel.org]

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 <jiayuan.chen@linux.dev>

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?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260728060517.95183-1-jiayuan.chen@linux.dev?part=1