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

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

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.
---
 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) {
-- 
2.43.0