From 0b10b945479c954393d62ee3229d2f224a4ca91c Mon Sep 17 00:00:00 2001 From: Jiayuan Chen Date: Tue, 28 Jul 2026 14:05:16 +0800 Subject: [PATCH] bpf: Fix mmap_lock deadlock on arena lock failure 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 Reviewed-by: Emil Tsalapatis Link: https://lore.kernel.org/bpf/20260728060517.95183-1-jiayuan.chen@linux.dev Signed-off-by: Kumar Kartikeya Dwivedi --- 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) {