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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

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.

>  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) {